Cross-sectional view of single wafer capacitive microphone

Backplate (silicon nitride)

Electrode (Al)

Si

Sacrificial Layer (Al)

Air Gap

Electrode (gold)

Diaphragm (silicon nitride)

• One of the earlier microphones was designed by Scheeper et al. in 1991. The design is based on using a single silicon wafer with a 1 µm-thick layer of LPCVD nitride grown on each side. The nitride was then patterned and etched in KOH. On the top side of the wafer, aluminum was evaporated as a sacrificial layer which was between 1 and 3 microns thick. Another micron-thick layer of nitride was grown on top of the aluminum layer using PECVD. Then an adhesion layer of 30 nm titanium was evaporated on the surface, followed by the addition of the gold electrode. After patterning, the acoustic holes were etched in the Ti/Au layers and in the PECVD nitride. Using the acoustic holes, the aluminum sacrificial layer was etched. Finally, the last step of the fabrication was to evaporate 100 nm of aluminum on the bottom side of the wafer.

microphone fabrication process

• (a) Growth of 1 pm LPCVD silicon nitride. patterning and anisotropic etching of V grooves and diaphragm in 33wt.% KOH solution (73'C).

• (b) Evaporation of 3pm aluminium, growth of 1 um PECVD silicon nitride, evaporation and patterning of a 30nm titanium adhesion layer and a 30nm gold electrode, etching of accesslacoustic holes in TilAu and PECVD silicon nitride.

• (c) Etching of aluminium sacrificial layer, drying and evaporation of lOOnm aluminium on siliconlsilicon nitride on the reverse side (counter electrode)

Cross-sectional view of single wafer capacitive microphone

Acoustic holes

Air Gap

AlSiN

P+-Si

SiO2

SiNAl

P-Si

• This microphone consists of a rigid 20 µm thick monocrystalline silicon back plate with acoustic holes, a 0.3 µm thick metallized low stress LPCVD silicon nitride membrane, and an air gap of 1.3 µm produced by a sacrificial layer of porous silicon and SiO2.

• This microphone was made from a double-sided polished p-doped silicon wafer. The first fabrication step was to p+ doped both sides of the wafer. A thin nitride layer was deposited by LPCVD for use as a mask for the porous silicon formation. The porous silicon was created in a portion of the heavily p-doped silicon by etching in 25% HF and 50% ethanol in water. An electrical current was then sent through the porous silicon formation. The next step was to remove the masking layer and then sputter a 0.8 micron oxide layer onto the top side of the wafer. The oxide and the porous silicon would later be used as the sacrificial layer to create the air gap. Then a 300 nm layer of nitride was deposited with LPCVD on both sides. The nitride was used as a masking layer to create contact holes on the front side and windows on the back side of the wafer. After patterning the nitride by a mask, the holes and backside were etched in KOH. Aluminum was deposited on the front and back of the wafer to act as an RIE etch mask and a conductive layer (Linder et al. 1996). Acoustic holes were etched into the backplate to reach the porous silicon by RIE in SF6 plasma. The porous silicon was etched with KOH, and then the oxide layer was etched with HF. This last etching step releases the nitride diaphragm.

Cross-sectional view of a condenser microphone with polyimide

Acoustic holesAir gap

Polyimide backplate

Polyimide diaphragm

Cr/Au/Cr diaphragm electrode

Si

Cr/Au/Cr backplate electrode

SiO2

• In this microphone, the diaphragm and back plate are made of a polymer (polyimide). In this microphone, the silicon substrates used were 3”, <100>, p-type, 5 Ω cm wafers. The thickness of the substrates was 400 μm. First, a 1.5 μm thick layer of SiO2 was grown by wet oxidation at 1150 oC. The SiO2 was subsequently etched in BHF to define the active sensor area on the backside of the substrate. During the etching, the front was protected with photoresist. The substrate was then etched in a KOH solusion (33 wt.%, 73 oC) until the thickness in the sensor area was reduced to approximately 50 μm. After rinsing, a thin diaphragm electrode was evaporated and patterned on the front, using a standard lift-off process with photoresist. The electrode was a Cr/Au/Cr multilayer, with a thickness of 4/20/4 nm.

• •

• The first layer of polyimide, which is later to be the diaphragm, was then spun on the front. The polyimide was then cured for 1 h at 300 oC in an N2 atmosphere, and the thickness of the polyimide diaphragm was measured to be 0.8-0.9 μm. An Al sacrificial layer was then deposited and patterned, using electron-beem evaporation and a standard Al etch solution. The thickness of the sacrificial layer was 1.5 μm. Subsequently, the backplate electrode was deposited and patterned using resistive evaporation and lift-off. The thickness of the CR/Au/Cr multilayer electrode was 10/200/10 nm. After rinsing the substrate, the polyimide backplate layer (~18 μm) was spun on to the front. The sacrificial Al layer was then etched for 3 h using the standard Al etch solution. The final step was to release the polyimide diaphragm by etching the remaining silicon under the diaphragm

Cross section of the polysilicon diaphragm condenser microphone

Air gap

Acoustic holes

P+ backplate electrode

Metal n+ popysilicon diaphragm

Silicon nitride

Silicon oxide

n Si

• A condenser microphone using a polysilicon diaphragm has been presented by Hsu et al. in 1998. The device consists of an n-type silicon substrate, a thin low-stress phosphorus doped polysilicon diaphragm, a P+ perforated back plate, and the metal contacts. To fabrication of this microphone, on (100) n-type silicon wafers, a 1 μm thick wet oxide is first grown at 1100 oC for 3 h. This oxide layer is patterned and etched in the buffered HF (5:1 BHF) for 12 minutes serving as a mask for the deep boron diffusion. A deep P+ boron diffusion is next introduced into the silicon from a solid source at 1175 oC for 15 h. followed by 20 minute wet oxidation at 1000 oC. The thick boron diffusion forms the stationary back electrode and the measured thickness is about 13 μm. The oxide was then stripped in a 1:1 HF: H2O solution for 4 minutes.

• A 2 μm thick layer of LPCVD low-temperature oxide (LTO) is deposited at 420 oC for 4 h and patterned in 5:1 BHF for 23 minutes. This oxide provides isolation for the two electrodes. A 0.3 μm thick layer of low stress LPCVD SiN is deposited at 875 oC. This layer is patterned and etched in hot phosphoric acid for 3 h using a 0.5 μm layer of LTO as a mask. This nitride layer protects the passivation oxide from a subsequent the sacrificial etch. A 4 μm thick LTO sacrificial layer is next deposited defining the air gap electrode spacing. This oxide is patterned and etched 5:1 BHF for about 20 minutes.

• Next, a 2 μm thick layer of LPCVD low-stress polysilicon is deposited at 588 oC. This material showed an unannealed tensile residual stress of about 100 MPa. The deposition is followed by a phosphorus ion implantation of 7x1015 cm-2 at 50 KeV. The remaining 1 μm thick layer of polysilicon is next deposited. The polysilicon is next annealed at 1050 oC for 1 h to redistribute the diaphragm dopants and remove as much residual stress as possible. The poly layer is next patterned and etched first using RIE with 20:5 SF6:O2 sccm, at 40 mT, and 60 W for 15 minutes, followed by a wet etch in 950:500:50 NHO3:H2O: NH4F for 25 minutes.

• • A 0.6 μm thick LTO mask is deposited and patterned in BHF for 7 minutes to

define the contact area of the back plate. The nitride over the contact area is then etched in hot phosphoric acid for 3 h. A second 0.5 um thick LTO layer is deposited followed by a 0.2 um Al evaporation. The LTO protects the front side of the wafer during the backside etch and the metal is used to pattern the back to front alignment key. The backside oxide is patterned and etched in 5:1 BHF for about 8 minutes. The wafer is then anisotropically etched in EDP for 8 h at 110 oC. After striping the protective LTO in 5:1 BHF for 20 min., the wafers are dried. Cr and Au are next evaporated forming the contact pads with thickness of 50 and 400 nm. The metal is next patterned and wet Au and Cr etchants for 4 and 1 min, respectively. Finally, the device is released in concentrated HF for 1 h. in this operation, the HF removes the sacrificial LTO from the backside while the wafer front is protected by the SiN layer

Simplified fabrication process of the microphone

Cross-section of the microphone chip

P-Si

Polysilicon diaphragm

Polysilicon backplate Acoustic holesSilicon nitride

SiO2

Air gap

• The component consists of a low-stress polysilocon bending diaphragm and a high-stress polysilcon perforated back plate that are separated by an air gap. The lower membrane bends due to a pressure difference, while the perforated membrane remains steady and the capacitance between the membranes changes.

• The components were fabricated by combining the bulk and surface-micromachining techniques. First, double-sided polished 4-in. p-type 100 silicon wafers were thermally oxidised. The oxide serves as an electrical insulator for the electrodes and as a masking and stopper layer in TMAH etching. A polysilicon layer for the bending membrane was then deposited using the standard LPCVD and patterned with RIE plasma etching. A sacrificial TEOS oxide layer was deposited and small holes for anti-sticking spikes were etched. The idea behind the anti-sticking spikes is to reduce the contact area between the membranes. Next, a second polysilicon layer for the perforated membrane was deposited and patterned. An LPCVD silicon nitride layer was deposited for top insulation, and contact holes for the electrodes and the substrate were etched through the nitride and oxide layers. Before the etching in TMAH, the silicon substrate, a protective LTO oxide layer was deposited on the front side of the wafers, and the thermal oxide at the backside was patterned using a double-sided lithography. After the TMAH etching, the sacrificial oxide layers were etched in a PSG etch. PSG etch was used instead of HF due to its better selectivity with respect to aluminium. The wafers were rinsed in water and tertiary butanol and dried using freezing and sublimation. Finally, the wafers were diced into chips by cleaving along the v-shaped grooves formed in the TMAH etching step.

Fabrication process of the microphone

Cross-section of the micro-machined capacitive microphone

n- Si

Silicon nitride backplate

Gold

Titanium

Poly SiGe diaphragmMetal

Acoustic holesAir gap

The MEMS Structure in the IntelliSuite Design Environment

• The device used a polysilicon Germanium diaphragm, an air gap and a gold-coated nitride back plate. The process starts with a <100> oriented n-type silicon wafer. A 1.0 micron thick silicon nitride, Si3N4, layer is deposited by LPCVD process. Over the nitride layer, a 30 nm thick layer of titanium is sputtered, which is followed by the evaporation-deposition of a 30 nm thick layer of gold. Next, a four micron thick layer of thermal oxide is deposited and then patterned. This is next followed by the deposition of a 300 nm thick layer of low-stress PECVD SiN layer. This nitride layer will act as an isolator between the poly SiGe diaphragm and the backplate electrode (Chowdhury et al. 2002).

• The PECVD nitride layer is then patterned and etched in hot phosphoric acid using a 0.5 μm thick layer of low temperature oxide. In the next step a 40 nanometer thick layer of polysilicon is deposited by LPCVD process. This polysilicon layer will act as the nucleation layer for the subsequent deposition of polysilicon germanium (poly SiGe). The poly SiGe (30% Ge) is deposited by LPCVD process using SiH4 and GeH4 gases. The poly SiGe layer is then ion-implanted with phosphorous and annealed at 925 oC to reduce the residual stress. The poly SiGe layer is then patterned and etched using reactive ion etching with SF6/CH4 plasma. After that a thin layer of low-temperature oxide is deposited and patterned in Buffered Hydrofluoric acid (BHF) to define the contact hole for the backplate (gold electrode).

• The nitride over the contact area is then etched by hot phosphoric acid. Next, the backside of the wafer is patterned using a 0.5 μm thick layer of low temperature oxide (LTO). The silicon wafer is then etched using SF6/O2 plasma. Next, the silicon nitride backplate is patterned using LTO and the silicon nitride layer and titanium layer are plasma etched using CF4/O2. The gold layer is then RIE etched using Cl2. The protective LTO is then stripped. Chromium and Gold are next evaporated to form contact pads with a thickness of 50 and 400 nm, respectively. The contact pads are then patterned and wet etched. Finally, the sacrificial thermal oxide layer is etched by BOE to release the diaphragm and then the wafer is freeze-dried using cyclohexane at atmospheric pressure under a continuous flow of nitrogen

Significant Microphone Fabrication Steps

Fabrication Process of the Capacitive Structure

• The process uses five masks and starts with a double side polished p-type (100) silicon wafer as a substrate.

• • First a 4-inch p-type (100) silicon wafer is cut into several pieces to be used as

substrate. • Each of the samples should be cleaned using standard cleaning procedure to

remove organic contaminants such as dust particles, grease or silica gel and then remove any oxide layer from the wafer surface prior to processing.

– The first step in the cleaning process is to clean the wafer using ultrasonic in the acetone solution for 5 minutes.

– The second step is to put the samples into the methanol solution using ultrasonic for 5 minutes.

– Final step is to dip the samples in a 10:1 DI water-HF solution (10% HF) until hydrophobic (i.e. no water can stick to wafer). This will remove native oxide film.

Fabrication process for DRIE of silicon wafer

• A 300-μm-thick (100)-oriented silicon wafer is used for the substrate.

• In step (1), a 300 nm-thick Al, which is a masking material for DRIE, is evaporated on surface of the silicon wafer.

• In step (2), approximately 1.5 μm of thin positive photoresist (AZ-1500) was spin-coated on the Al surface.

• In step (3), it was exposed for 10 sec using a contact mask aligner with a UV light source.

• In step (4), it was developed for 60 seconds in AZ 300 MIF.

• In step (5), Al was wet etched for 17 seconds in 10% hydrofluoric acid (HF) solution.

• In step (6), the Photoresist layer was removed in acetone.

• In step (7), a 260 μm deep cavity was fabricated in the silicon wafer by the DRIE system.

• In step (8), the Al thin film is removed by 10%HF solution.

• A 0.5 µm thick wet oxide is grown on silicon wafer.

– This oxide layer is patterned and etched in the buffered HF (6:1 BHF) serving as a mask for the phosphorus diffusion.

• A n+ phosophorous diffusion is next introduced into the silicon wafer as a back plate electrode. The doped area acts as a back plate electrode.

– A constant source diffusion process was used to obtain the n+-silicon layer with a depth of 0.4 µm and surface concentration as high as 1020 atoms/cm3.

– It was performed at 1000 oC for 30 minutes in N2 environment.

– After diffusion step, the wafer was dipped in a buffered oxide etch (BOE) to remove any silicon dioxide grown during the diffusion.

• Next, a Silicon-On-Glass (SOG) of 1.5 μm thick was spun with spin coater in order to form a sacrificial and insulating layer between diaphragm and back plate electrode.

– The layer was gradually hot plate baked from 20 oC to 180 oC for 5 minutes and after 4 minutes gradually cooled down.

• The isolating layer is then patterned and etched in PAD etch solution for 3.5 minutes to define the contact area on the back plate.

• After that, a 3 m thick layer of Al is evaporated on SOG as a material of diaphragm.

• The Al layer is then patterned using photoresist mask (AZ1500) to define the geometry of the diaphragm (0.5 mm x 0.5 mm), contact pad (250 µm x 250 µm), anchors and to form the circular acoustic holes (radius = 10 µm).

• The holes in the diaphragm are etched by Al etchant for 33 minutes.

• Finally, the sacrificial oxide layer is etched through the acoustic holes in PAD etch for 70 minutes to release the diaphragm,

• The fabrication process is completed by the release of the diaphragm by immersing it in deionized water (DI) and then acetone.

• Then the whole structure is dried on hot plate at 60 oC for 90 seconds to protect the diaphragm from sticking to the back plate.

Fabrication Process

Structure under Test

Gallium arsenide (GaAs)

• Gallium arsenide (GaAs) is a compound of the elements gallium and arsenic.

• It is a III/V semiconductor, and is used in the manufacture of devices such:– infrared light-emitting diodes,– laser diodes,– solar cells– HEMT transistors

Gallium arsenide

Properties

Molecular formula GaAs

Molar mass 144.645 g/mol

Appearance Very dark red, vitreous crystals

Odor garlic-like when moistened

Density 5.3176 g/cm3

Melting point 1238 °C, 1511 K, 2260 °F

Solubility in water insoluble

Solubilitysoluble in HCL

insoluble in ethanol, methanol, acetone

Band gap 1.424 eV (at 300 K)

Electron mobility 8500 cm2/(V·s) (at 300 K)

Thermal conductivity 0.55 W/(cm·K) (at 300 K)

Refractive index (nD) 3.8

Preparation

• Gallium arsenide can be prepared by direct reaction from the elements:

• Crystal growth using a horizontal zone furnace in the Bridgman-Stockbarger technique, in which gallium and arsenic vapors react and free molecules deposit on a seed crystal at the cooler end of the furnace.

• Liquid encapsulated Czochralski (LEC) growth is used for producing high purity single crystals that exhibit semi-insulating characteristics.

Alternative methods for producing films of GaAs

• VPE reaction of gaseous gallium metal and arsenic trichloride:

2 Ga + 2 AsCl3 → 2 GaAs + 3 Cl2

• MOCVD reaction of trimethylgallium and arsine: Ga(CH3)3 + AsH3 → GaAs + 3 CH4

• Molecular beam epitaxy (MBE) of gallium and arsenic: 4 Ga + As4 → 4 GaAs

or 2 Ga + As2 → 2 GaAs

Etching

• Wet etching of GaAs industrially uses an oxidizing agent such as hydrogen peroxide or bromine water.

Comparison with siliconGaAs advantages

• It has a higher saturated electron velocity and higher electron mobility, allowing gallium arsenide transistors to function at frequencies in excess of 250 GHz.

• GaAs devices are relatively insensitive to heat owing to their wider bandgap.

• GaAs devices tend to have less noise than silicon devices, especially at high frequencies. This is a result of higher carrier mobilities and lower resistive device parasitics.

• it has a direct band gap, which means that it can be used to emit light efficiently. Silicon has an indirect bandgap and so is very poor at emitting light. recent advances may make silicon LEDs and lasers possible.

• The direct gap of GaAs results in efficient emission of infrared light at 1.424 eV (~870 nm).

• GaAs has been used to produce (near-infrared) laser diodes.

Silicon advantages

• silicon is abundant and cheap to process. • existence of a native oxide (silicon dioxide),

which is used as an insulator in electronic devices. – Silicon dioxide can easily be incorporated onto

silicon circuits, and such layers are adherent to the underlying Si.

– GaAs does not have a native oxide and does not easily support a stable adherent insulating layer.

• it possesses a much higher hole mobility. This high mobility allows the fabrication of higher-speed P-channel field effect transistors, which are required for CMOS logic.

• Silicon has a nearly perfect lattice, impurity density is very low and allows to build very small structures (currently down to 25 nm).

• GaAs in contrast has a very high impurity density, which makes it difficult to build ICs with small structures, so the 500 nm process is a common process for GaAs.

Aluminium gallium arsenide

• The crystal structure of aluminium gallium arsenide

Aluminium gallium arsenide

• Aluminium gallium arsenide (AlxGa1-xAs) is a semiconductor material with very nearly the same lattice constant as GaAs, but a larger bandgap. – The x in the formula above is a number between 0 and

1 - this indicates an arbitrary alloy between GaAs and AlAs.

• The bandgap varies between 1.42 eV (GaAs) and 2.16 eV (AlAs).

• For x < 0.4, the bandgap is direct.• It can also be used in 1064 nm (Infra-red)

laser diodes

Light-emitting diode

Red, pure green and blue LEDs of the 5mm diffused type

Type Passive, optoelectronic

Working principle Electroluminescence

First production 1968

Electronic symbol

Pin configuration anode and cathode

Parts of an LED

LED

• A light-emitting diode (LED) is a semiconductor light source.

• LEDs are used as indicator lamps in many devices and are increasingly used for other lighting.

• Early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness.

Applications

• Light-emitting diodes are used in applications as diverse as aviation lighting, automotive lighting, advertising, general lighting, and traffic signals.

• LEDs have allowed new text, video displays, and sensors to be developed, while their high switching rates are also useful in advanced communications technology.

• Infrared LEDs are also used in the remote control units of many commercial products including televisions, DVD players and other domestic appliances.

• LEDs are also used in Seven-segment display.

Applications

Red and green traffic signals Automotive applications LEDs used on a train

LED destination signs on buses LED digital display LED illumination

• When a light-emitting diode is forward-biased (switched on), electrons are able to recombine with electron holes within the device, releasing energy in the form of photons.

• This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor.

• LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching.

Technology

The inner workings of an LED

I-V diagram for a diode

An LED will begin to emit light when the on-voltage is exceeded. Typical on voltages are 2–3 volts.

Physics

• The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction.

• As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction.

• Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages.

• When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon.

• The wavelength of the light emitted, and thus its color depends on the band gap energy of the materials forming the p-n junction.

• In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition, which produces no optical emission, because these are indirect band gap materials.

• The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet light.

• LED development began with infrared and red devices made with gallium arsenide.

• Advances in materials science have enabled making devices with ever-shorter wavelengths, emitting light in a variety of colors.

• LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well.

• Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.

Colors and materials

• Conventional LEDs are made from a variety of inorganic semiconductor materials.

• The following table shows the available colors with wavelength range, voltage drop and material.

• As of 2012, the Lumiled catalog gives the following as the best efficacy for each color

ColorWavelength range

(nm)Typical efficacy (lm/W

)

Red 620 < λ < 645 72

Red-orange 610 < λ < 620 98

Green 520 < λ < 550 93

Cyan 490 < λ < 520 75

Blue 460 < λ < 490 37

• In February 2008, a luminous efficacy of 300 lumens of visible light per watt of radiation (not per electrical watt) and warm-light emission was achieved by using nanocrystals.

Types

• LEDs are produced in a variety of shapes and sizes. The color of the plastic lens is often the same as the actual color of light emitted, but not always.

• For instance, purple plastic is often used for infrared LEDs, and most blue devices have colour less housings. Modern high power LEDs such as those used for lighting and backlighting are generally found in surface-mount technology (SMT) packages, (not shown).

• The main types of LEDs are miniature, high power devices and custom designs such as alphanumeric or multi-color.

Laser diode

a packaged laser diode shown with a penny for scale.

the laser diode chip is removed from the above package and placed on the eye of a needle for scale

A laser diode with the case cut away

The actual laser diode chip is the small black chip at the front; a photodiode at the back is used to control output power

laser diode

• A laser diode is an electrically pumped semiconductor laser in which the active medium is formed by a p-n junction of a semiconductor diode similar to that found in a light-emitting diode.

• The laser diode is distinct from the optically pumped semiconductor laser, in which, while also semiconductor based, the medium is pumped by a light beam rather than electric current.

Theory of operation

• A laser diode is formed by doping a very thin layer on the surface of a crystal wafer.

• The crystal is doped to produce an n-type region and a p-type region, one above the other, resulting in a p-n junction, or diode.

• Laser diodes form a subset of the larger classification of semiconductor p-n junction diodes.

• Forward electrical bias across the laser diode causes the two species of charge carrier – holes and electrons – to be "injected" from opposite sides of the p-n junction into the depletion region.

• Holes are injected from the p-doped, and electrons from the n-doped, semiconductor. (A depletion region, devoid of any charge carriers, forms as a result of the difference in electrical potential between n- and p-type semiconductors wherever they are in physical contact.)

• Due to the use of charge injection in powering most diode lasers, this class of lasers is sometimes termed "injection lasers," or "injection laser diode" (ILD).

• As diode lasers are semiconductor devices, they may also be classified as semiconductor lasers. Either designation distinguishes diode lasers from solid-state lasers.

• When an electron and a hole are present in the same region, they may recombine or "annihilate" with the result being spontaneous emission — – i.e., the electron may re-occupy the energy state of the hole, emitting a

photon with energy equal to the difference between the electron and hole states involved. (In a conventional semiconductor junction diode, the energy released from the recombination of electrons and holes is carried away as phonons, i.e., lattice vibrations, rather than as photons.)

• Spontaneous emission gives the laser diode below lasing threshold similar properties to an LED.

• Spontaneous emission is necessary to initiate laser oscillation, but it is one among several sources of inefficiency once the laser is oscillating.

• The difference between the photon-emitting semiconductor laser and conventional phonon-emitting (non-light-emitting) semiconductor junction diodes lies in the use of a different type of semiconductor, one whose physical and atomic structure confers the possibility for photon emission. These photon-emitting semiconductors are the so-called "direct bandgap" semiconductors.

• The properties of silicon and germanium, which are single-element semiconductors, have bandgaps that do not align in the way needed to allow photon emission and are not considered "direct." Other materials, the so-called compound semiconductors, have virtually identical crystalline structures as silicon or germanium but use alternating arrangements of two different atomic species in a checkerboard-like pattern to break the symmetry. The transition between the materials in the alternating pattern creates the critical "direct bandgap" property.

• Gallium arsenide, indium phosphide, gallium antimonide, and gallium nitride are all examples of compound semiconductor materials that can be used to create junction diodes that emit light.

Diagram of a simple laser diode

Types

• The simple laser diode structure, described above, is extremely inefficient.

• Such devices require so much power that they can only achieve pulsed operation without damage.

• Although historically important and easy to explain, such devices are not practical.

Double heterostructure lasers

• In these devices, a layer of low bandgap material is sandwiched between two high bandgap layers.

• One commonly-used pair of materials is gallium arsenide (GaAs) with aluminium gallium arsenide (AlxGa(1-x)As). Each of the junctions between different bandgap materials is called a heterostructure, hence the name "double heterostructure laser" or DH laser.

• The kind of laser diode described in the first part of the article may be referred to as a homojunction laser, for contrast with these more popular devices.

• The advantage of a DH laser is that the region where free electrons and holes exist simultaneously—the active region—is confined to the thin middle layer. This means that many more of the electron-hole pairs can contribute to amplification—not so many are left out in the poorly amplifying periphery. In addition, light is reflected from the heterojunction; hence, the light is confined to the region where the amplification takes place.

Diagram of front view of a double heterostructure laser diode

Quantum well lasers

• If the middle layer is made thin enough, it acts as a quantum well. This means that the vertical variation of the electron's wavefunction, and thus a component of its energy, is quantized. The efficiency of a quantum well laser is greater than that of a bulk laser because the density of states function of electrons in the quantum well system has an abrupt edge that concentrates electrons in energy states that contribute to laser action.

• Lasers containing more than one quantum well layer are known as multiple quantum well lasers. Multiple quantum wells improve the overlap of the gain region with the optical waveguide mode.

• Further improvements in the laser efficiency have also been demonstrated by reducing the quantum well layer to a quantum wire or to a "sea" of quantum dots.

Diagram of front view of a simple quantum well laser diode

Separate confinement heterostructure lasers

• The problem with the simple quantum well diode described above is that the thin layer is simply too small to effectively confine the light. To compensate, another two layers are added on, outside the first three. These layers have a lower refractive index than the centre layers, and hence confine the light effectively. Such a design is called a separate confinement heterostructure (SCH) laser diode.

• Almost all commercial laser diodes since the 1990s have been SCH quantum well diodes.

Diagram of front view of a separate confinement heterostructure quantum well laser diode

Uses

• Laser diodes can be arrayed to produce very high power outputs, continuous wave or pulsed.

• Such arrays may be used to efficiently pump solid-state lasers for high average power drilling, burning or for inertial confinement fusion

• Laser diodes find wide use in telecommunication as easily modulated and easily coupled light sources for fiber optics communication.

• They are used in various measuring instruments, such as rangefinders. Another common use is in barcode readers.

• Visible lasers, typically red but later also green, are common as laser pointers. • Both low and high-power diodes are used extensively in the printing industry

both as light sources for scanning (input) of images and for very high-speed and high-resolution printing plate (output) manufacturing.

• Infrared and red laser diodes are common in CD players, CD-ROMs and DVD technology.

• Violet lasers are used in HD DVD and Blu-ray technology. • Diode lasers have also found many applications in

laser absorption spectrometry (LAS) for high-speed, low-cost assessment or monitoring of the concentration of various species in gas phase.

• Diode lasers can be used in a great many fields; since light has many different properties (power, wavelength, spectral and beam quality, polarization, etc.) it is useful to classify applications by these basic properties.

Common wavelengths• 375 nm – excitation of Hoechst stain, Calcium Blue, and other fluorescent dyes in fluorescence microscopy• 405 nm – InGaN blue-violet laser, in Blu-ray Disc and HD DVD drives• 445 nm – InGaN Deep blue laser multimode diode recently introduced (2010) for use in mercury free high brightness

data projectors• 473 nm – Sky blue laser pointers, still very expensive, output of DPSS systems• 485 nm – excitation of GFP and other fluorescent dyes• 510 nm – (to ~525 nm) Green diodes recently (2010) developed by Nichia and OSRAM for laser projectors.• 532 nm – DPSS Green laser, frequency doubled from 1064 nm by means of a KTP-crystal, for use in light shows and laser pointers.• 635 nm – AlGaInP better red laser pointers, same power subjectively 5 times as bright as 670 nm one• 640 nm – High brightness red DPSS laser pointers• 657 nm – AlGaInP DVD drives, laser pointers• 670 nm – AlGaInP cheap red laser pointers• 760 nm – AlGaInP gas sensing: O2

• 785 nm – GaAlAs Compact Disc drives• 808 nm – GaAlAs pumps in DPSS Nd:YAG lasers (e.g. in green laser pointers or as arrays in higher-powered lasers)• 848 nm – laser mice• 980 nm – InGaAs pump for optical amplifiers, for Yb:YAG DPSS lasers• 1064 nm – AlGaAs fiber-optic communication, DPSS laser pump frequency• 1310 nm – InGaAsP, InGaAsN fiber-optic communication• 1480 nm – InGaAsP pump for optical amplifiers• 1512 nm – InGaAsP gas sensing: NH3

• 1550 nm – InGaAsP, InGaAsNSb fiber-optic communication• 1625 nm – InGaAsP fiber-optic communication, service channel• 1654 nm – InGaAsP gas sensing: CH4

• 1877 nm – GaSbAs gas sensing: H2O

• 2004 nm – GaSbAs gas sensing: CO2

• 2330 nm – GaSbAs gas sensing: CO• 2680 nm – GaSbAs gas sensing: CO2

• 3030 nm – GaSbAs gas sensing: C2H2

• 3330 nm – GaSbAs gas sensing: CH4

Micromachined optical gas sensor

• Aim: – environmental monitoring of gases for health,

safety and air-pollution control.

• For Example:– Detection of important gases of smokes, such as

CO2, CO, NO2, NO, O3, SO2, SO3, CH4 for air pollution control.

Techniques for gas detection

• 1- optical techniques• 2- chemical techniques

– Infrared (IR) technology• Long recognized for accurate identification of gases

and reliable concentration measurements. • Infrared radiation has wavelengths ranging from 0.8

to 100 microns.

Infrared (IR) technology

• Different gas molecules have characteristic absorption wavelength bands in the infrared spectrum because of rotational and vibrational excitation modes. Fore example:– CO2 (4.26 µm) – CO (4.65 µm) – CH4 (3.3 µm)

• When exposed to a beam of infrared radiation, most gas molecules will absorb IR-radiation at their vibration/rotation frequencies.

Micromacined optical gas sensor

Operating principle of the micromachined optical gas sensor

• An infrared light beam of an IR-source passes a gas cell. Depending on the gases present, a part of the spectrum is absorbed.

• An optical tunable filter then analyzes the beam. It passes just one wavelength and blocks other wavelengths, while an IR-detector detects the transmitted intensity.

• The wavelength of the filter can be tuned to absorption bands of different gasses.

What is LASER?

• Light amplification by stimulated emission of radiation, or laser in short, is a device that creates and amplifies electromagnetic radiation of specific frequency through process of stimulated emission.

• In laser, all the light rays have the same wavelength and they are coherent; they can travel long distances without diffusing.

How an atom gives out light.• To understand how lasers work, we must understand how an atom

gives out light. • An atom is the smallest particle in the world, and it contains

electrons. • By introducing extra photon into the atom, the electrons are forced

to move into a higher energy level, and now the atom is at an excited state.

• However, the excited atom is unstable and the electrons always tries to get back to its ground state,

• therefore releasing the excess energy it originally gained, as a photon of light radiation.

• This process is called spontaneous emission, as shown below in Figure. 1.

Laser• The laser contains a chamber in which atoms of a medium are excited,

bringing their electrons into higher orbits with higher energy states. • When one of these electrons jumps down to a lower energy state, it

gives off its extra energy as a photon with a specific frequency. • By introducing more photons into the system, the photons will

eventually encounter another atom with an excited electron, which will stimulate that electron to jump back to its original state, emitting two or more photons with the same frequency as the first and in phase with it.

• This effect cascades through the chamber, constantly stimulating other atoms to emit yet more coherent photons, and this process is called stimulated emissions.

• In other words, the light has been amplified, as shown below in Figure 2

• Furthermore, mirrors at both ends of the chamber cause the light to bounce back and forth across the medium.

• One of the mirrors is partially transparent, allowing the laser beam to exit from that end of the chamber.

• By maintaining a sufficient number of atoms in the medium by external energy source in the higher energy state, the emissions are continuously stimulated, and this process is called population inversion.

• Ultimately, it creates a stream of coherent photons which is a very concentrated beam of powerful laser light.

• Lasers have many industrial, military, and scientific uses, including welding, target detection, microscopic photography, fiber optics, surgery, and etc.

A closer look at laser diodes

The laser diode converts electrical energy into energy in form of light.

Laser diode

• Laser diode is a laser where the medium is a semiconductor, formed by a p-n junction, as shown in Fig. 3, and powered by electric current.

• The semiconductor materials that are used to create p-n junction diodes that emit light today are:– Gallium arsenide, indium phosphide, gallium antimonide, and

gallium nitride. • The reason that these are being used is because of the three-

five compound properties on chemical periodic table. • The materials have to be heavily doped to create P – N

regions, which rules out others, leaving groups three-five the ideal options.

wavelengths

• Their wavelengths can be adjusted by changing the ratio of composition.

• For instance, the wavelength of the laser beam produced by InP substrate can be increased by increasing the Indium content or lowering the Phosphate content percentage.

• Longer wavelength usually indicates a longer travel distance.

Laser diodes

• Laser diodes (also called .injection lasers.) are in effect a specialised form of LED.

• Just like a LED, they.re a form of P-N junction diode with a thin depletion layer where electrons and holes collide to create light photons, when the diode is forward biased.

• The difference is that in this case the .active. part of the depletion layer (i.e., where most of the current flows) is made quite narrow, to concentrate the carriers.

• The ends of this narrow active region are also highly polished, or coated with multiple very thin reflective layers to act as mirrors, so it forms a resonant optical cavity.

• The forward current level is also increased, to the point where the current density reaches a critical level where .carrier population inversion. occurs.

• This means there are more holes than electrons in the conduction band, and more electrons than holes in the valence band . or in other words, a very large excess population of electrons and holes which can potentially combine to release photons.

• And when this happens, the creation of new photons can be triggered not just by random collisions of electrons and holes, but also by the influence of passing photons.

• Passing photons are then able to stimulate the production of more photons, without themselves being absorbed. So laser action is able to occur: Light Amplification by Stimulated Emission of Radiation.

• And the important thing to realize is that the photons that are triggered by other passing photons have the same wavelength, and are also in phase with them.

• In other words, they end up .in sync. and forming continuous-wave coherent radiation.

• Because of the resonant cavity, photons are thus able to travel back and forth from one end of the active region to the other, triggering the production of more and more photons in sync with themselves. So quite a lot of coherent light energy is generated.

• And as the ends of the cavity are not totally reflective (typically about 90-95%), some of this coherent light can leave the laser chip . to form its output beam.

• Because a laser.s light output is coherent, it is very low in noise and also more suitable for use as a .carrier. for data communications.

• The bandwidth also tends to be narrower and better defined than LEDs, making them more suitable for optical systems where light beams need to be separated or manipulated on the basis of wavelength.

• The very compact size of laser diodes makes them very suitable for use in equipment like CD, DVD and MiniDisc players and recorders.

• As their light is reasonably well collimated (although not as well as gas lasers) and easily focussed, they.re also used in optical levels, compact handheld laser pointers, barcode scanners etc.

• There are two main forms of laser diode: – the horizontal type, which emits light from the polished ends of the chip, – and the vertical or .surface emitting. type.

• They both operate in the way just described, differing mainly in terms of the way the active light generating region and resonant cavity are formed inside the chip. (See Fig.5)

• Because laser diodes have to be operated at such a high current density, and have a very low forward resistance when lasing action occurs, they are at risk of destroying themselves due to thermal runaway.

• Their operating light density can also rise to a level where the end mirrors can begin melting.

• As a result their electrical operation must be much more carefully controlled than a LED.

LEDS & LASER DIODES• Light-emitting diodes (LEDs) - In a diode formed from a direct band gap

semiconductor, such as gallium arsenide, carriers that cross the junction emit photons when they recombine with the majority carrier on the other side.

• Depending on the material, wavelengths (or colors) from the infrared to the near ultraviolet may be produced.

• All LEDs produce incoherent, narrow-spectrum light.• LEDs can also be used as low-efficiency photodiodes in signal applications. • An LED may be paired with a photodiode or phototransistor in the same

package, to form an opto-isolator.• Laser diodes - When an LED-like structure is contained in a resonant cavity

formed by polishing the parallel end faces, a laser can be formed.• Laser diodes are commonly used in optical storage devices and for high

speed optical communication.

خورشیدی سلول

خورشیدی • یا solar cell: انگلیسیبه ) سلولphotovoltaic cell یاphotoelectric cell )یک

جامد حالت الکترونیکی انرژی قطعه که استتوسط ! مستقیما را خورشید فوتوولتاییک نور اثر

.الکتریسیتهبه می‌کند تبدیل

کاربرد خورشیدی سلول

ویفر • از شده ساخته خورشیدی بسیاری سیلیکونسلول کاربرد ،دارند.

دستگاه‌های • الزم توان کردن فراهم برای تکی سلول‌هایمانند الکترونیکی کوچک‌تر حساب .ماشین می‌روند کار به

تولید • را بازیافت‌شدنی‌ای الکتریسیته; فوتوولتاییک آرایه‌هایسیستم وجود عدم موارد در ! عمدتا که می‌کنند

الکتریکی توزیع و .انتقال دارد کاربرددسترس، – از دور محل‌های به می‌توان مثال مدارگرد برای ماهواره‌های

فضایی ، ساختمان‌های کاوشگرهای دسترس مخابراتیو از دور. کرد اشاره

که – محل‌هایی در امروزه انرژی نوع این از استفاده این بر عالوه. شده‌است مرسوم است، موجود هم توزیع شبکه;

خورشیدی باتری ساختارمواد • از ! معموال خورشیدی ! نیمه‌رساناباتری‌های مخصوصا ، سیلیسیم،

. تشکیل پیوند دیگر اتم چهار با سیلیسیم اتم هر شده‌است تشکیلشکل صورت، بدین و .کریستالیمی‌دهد می‌آید پدید آن

اضافه • ناخالصی جزئی مقداری سیلیسیم به خورشیدی باتری‌های درناخالصی. اتم اگر سیلیسیم ) ۵می‌کنند اتم باشد ظرفیتی ۴ظرفیتی

پیوند( بدون آن الیه; یک سیلیسیم اتم چهار با ارتباط در آنگاه ، استتک ) یک می‌ماند منفی(. الکترونباقی نسبی بار چون دلیل همین به

نوع سیلیسیم آن به می‌کند .N) Negativeپیدا می‌گویند( ظرفیت • دارای ناخالصی اتم که حفره; ۳درصورتی یک آنگاه باشد،

. خالی جای که گفت می‌توان گونه‌ای به را حفره می‌شود ایجاد اضافی ) جرم ) با برابر جرمی و الکترون اندازه; به مثبت بار با است، الکترون

آن. به و می‌شود ماده نسبی شدن مثبت باعث هم امر این که الکتروننوع .P) Positiveسیلیسیم می‌گویند(

خورشیدی باتری عملکرد

یک • اتصال هادی با یک pنوع نیمه هادی به از الکترون‌ها، nنوع نیمهناحیه nناحیه ناحیه حفره‌هاو pبه ناحیه pاز . nبه می‌شوند منتقل

ناحیه • به الکترون هر انتقال ناحیه pبا در مثبت یون یک ،n با وناحیه به حفره هر ناحیه nانتقال در منفی یون یک ،p . می‌ماند باقی

منفی • و مثبت الکتریکی یون‌های که میدان می‌کنند ایجاد داخلیناحیه از آن ناحیه nجهت . pبه است

• ) و ) قوی‌تر ، حفره‌ها و الکترون‌ها باربرها بیشتر انتقال با میدان این . می‌رسد صفر به باربرها خالص انتقال که جایی تا شده قویتر

شرایط • این فرمی در شده‌اند ترازهای سطح هم یکدیگر با ناحیه دو. گرفته‌است شکل نیز داخلی الکتریکی میدان یک و

پیوند • به خورشید نور شرایطی، چنین در اگرشکاف فوتون‌هاییبتابد، انرژی از آنها انرژی که

است، بیشتر هادی - نیمه حفره الکترون زوجیا تهی ناحیه در که زوج‌هایی و کرده تولید

که دارند زیادی شانس شده‌اند، تولید آن حوالیاز پیوند داخلی میدان توسط بازترکیب، از قبل

. شوند جدا هم

ناحیه • به را الکترون‌ها الکتریکی، ناحیه nمیدان به را حفره‌ها pو . می‌دهد سوق

ناحیه • در منفی بار تراکم ترتیب این ناحیه nبه در مثبت بار تراکم وp . می‌شود زیاد

گیری • اندازه قابل پیوند سر دو در ولتاژی شکل به بار، تراکم ایناست.

شود، • کوتاه اتصال یکدیگر به سیم، یک با پیوند سر دو اگرناحیه اضافی ناحیه nالکترون‌های به سیم طریق از ،p و رفته

. می‌دهند شکل را کوتاهی اتصال جریاناز • جریان عبور شود، استفاده کننده مصرف یک از سیم جای به اگر

. فوتون‌های انرژی ترتیب این به می‌دهد انرژی آن به کننده، مصرفبه خورشید الکتریکی نور .انرژی می‌شود تبدیل

باز • مدار ولتاژ باشد، قوی‌تر پیوند درون الکتریکی میدان چه هر . می‌آید بدست بزرگتری

اختالف • باید بزرگ، الکتریکی میدان یک به یافتن دست برایفرمی ماده ترازهای . nو pدو باشد زیاد یکدیگر از

• . شود انتخاب بزرگ هادی نیمه شکاف انرژی باید منظور این برایآن شکاف انرژی با خورشیدی سلول یک باز مدار ولتاژ بنابراین

. می‌یابد افزایشکمتری • فوتون‌های می‌شود، سبب شکاف انرژی افزایش اما

- جریان بنابراین و باشند داشته حفره الکترون زوج تولید توانایی . شود تولید نیز کمتری کوتاه اتصال

کوتاه • اتصال جریان و باز مدار ولتاژ روی شکاف، انرژی افزایش. دارد متفاوت اثر دو سلول

از شده ساخته خورشیدی سلولmonocrystalline silicon wafer

Fundamentals of photovoltaics

• The solar spectrum in outer space resembles the theoretical radiation provided by a black

• body of 5900 K [20]. As the light passes through the atmosphere, some of the light is absorbed or

• reflected by gasses such as water vapour and the ozone. The spectrum of the sun’s light that

• reaches Earth’s upper atmosphere ranges from the ultraviolet to the near infrared radiation, with

• peak region (48%) from 400 to 700 nm, which is the visible diapason (Figure 1).

Figure 1. Terrestrial and space solar spectrum [14].

• Photovoltaic cells can be defined as p-i-n photodiodes, which are operated under forward• bias. They are designed to capture photons from the solar spectrum by exciting electrons

across the• bandgap of a semiconductor, which creates electron-hole pairs that are then charge

separated,• typically by p-n junctions introduced by doping. The space charge at the p-n junction

interface• drives electrons and holes in opposite directions, creating at the external electrodes a

potential• difference equal to the bandgap (Figure 2) [20-22]. A semiconductor can only convert

photons• with the energy of the bandgap with good efficiency. Photons with lower energy are not

absorbed• and those with higher energy are reduced to gap energy by thermalization of the

photogenerated• carriers.

• Behaviour of a solar cell is represented by current versus voltage curves on Figure 3 [23].

• The point at which a curve intersects the vertical axis is known as the short circuit condition, and it

• defines how the cell operates if a wire is connected between its terminals, shorting it out. The

• current flow here is known as short-circuit current, sc I . For an ideal solar cell at most moderate

• resistive loss mechanisms, the short-circuit current and the light-generated current are identical.

• Therefore, the short-circuit current is the largest current which may be drawn from the solar cell

• [22].

• The point at which a curve intersects the horizontal axis is known as the open circuit

• condition. The open-circuit voltage, oc V , is the maximum voltage available from a solar cell, and

• this occurs at zero current. The open-circuit voltage corresponds to the amount of forward bias on

• the solar cell due to the bias of the solar cell junction with the light-generated current. Voc depends

• on the saturation current of the solar cell and the light-generated current. Open-circuit voltage is

• then a measure of the amount of recombination in the device [22].

• For each point on the graph, the voltage and current can be multiplied to calculate power.

• Maximum power point is the point on the I-V curve of a solar cell corresponding to the

• maximum output electrical power,

• Maximizing total power is the goal of solar cell’s design. Multi-junction photovoltaics, as• compared to single-junction cells, have reduced currents, because fixed total number of

photons is• distributed over increasing number of cell layers, so that the amount available for electron• promotion in any one layer is decreased. At the same time, the electrons excited are more

energetic• and have a greater electric potential, so the reduction of currents is compensated for by

increase in• voltages, and the overall power of the cell is greater. Moreover, multi-junction design is• advantageous, because resistive losses, which are proportional to the square of the

current, can be• significantly reduced [6].• Another defining term in the overall behaviour of a solar cell is the fill factor, FF . This is• the ratio that describes how close the I-V curve of a solar cell resembles a perfect

rectangle, which• represents the ideal solar cell:

• Quantum efficiency is a term intrinsic to the light absorbing material and not the cell as a

• whole; it refers to the percentage of absorbed photons that produce electron-hole pairs. Whereas

• energy conversion efficiency, is the percentage of incident electromagnetic radiation that is

• converted to electrical power, when a solar cell is connected to an electrical circuit. This overall

• efficiency depends on many factors including the temperature, amount of incident radiation and

• the surface area of the solar cell [21].

How Solar Cells Work• You've probably seen calculators that have solar cells -- calculators that never need batteries, and in• some cases don't even have an off button. As long as you have enough light, they seem to work• forever. You may have seen larger solar panels -- on emergency road signs or call boxes, on buoys,• even in parking lots to power lights. Although these larger panels aren't as common as solar powered• calculators, they're out there, and not that hard to spot if you know where to look. There are solar cell• arrays on satellites, where they are used to power the electrical systems.• You have probably also been hearing about the "solar revolution" for the last 20 years -- the idea that• one day we will all use free electricity from the sun. This is a seductive promise: On a bright, sunny• day, the sun shines approximately 1,000 watts of energy per square meter of the planet's surface, and• if we could collect all of that energy we could easily power our homes and offices for free.• In this article, we will examine solar cells to learn how they convert the sun's energy directly into• electricity. In the process, you will learn why we are getting closer to using the sun's energy on a daily• basis, and why we still have more research to do before the process becomes cost effective.

Solar panels absorb energy to produce hydrogen at SunLineTransit Agency.

N-type Plus P-type Silicon• The interesting part starts when you put N-type silicon together with P-type silicon. Remember that• every PV cell has at least one electric field. Without an electric field, the cell wouldn't work, and this• field forms when the N-type and P-type silicon are in contact. Suddenly, the free electrons in the N• side, which have been looking all over for holes to fall into, see all the free holes on the P side, and• there's a mad rush to fill them in.• Before now, our silicon was all electrically neutral. Our extra electrons were balanced out by the extra• protons in the phosphorous. Our missing electrons (holes) were balanced out by the missing protons• in the boron. When the holes and electrons mix at the junction between N-type and P-type silicon,• however, that neutrality is disrupted. Do all the free electrons fill all the free holes? No. If they did,• then the whole arrangement wouldn't be very useful. Right at the junction, however, they do mix and• form a barrier, making it harder and harder for electrons on the N side to cross to the P side.• Eventually, equilibrium is reached, and we have an electric field separating the two sides.

• This electric field acts as a diode, allowing (and even pushing) electrons to flow from the P side to the

• N side, but not the other way around. It's like a hill -- electrons can easily go down the hill (to the N

• side), but can't climb it (to the P side).• So we've got an electric field acting as a diode in

which electrons can only move in one direction. Let's• see what happens when light hits the cell.

When Light Hits the Cell• When light, in the form of photons, hits our solar cell, its energy frees electron-hole

pairs.• Each photon with enough energy will normally free exactly one electron, and result in

a free hole aswell. If this happens close enough to the electric field, or if free electron and free hole happen to

• wander into its range of influence, the field will send the electron to the N side and the hole to the P

• side. This causes further disruption of electrical neutrality, and if we provide an external current path,

• electrons will flow through the path to their original side (the P side) to unite with holes that the electric

• field sent there, doing work for us along the way. The electron flow provides the current, and the

• cell's electric field causes a voltage. With both current and voltage, we have power, which is the

• product of the two.

• How much sunlight energy does our PV cell absorb? Unfortunately, the most that our simple cell could

• absorb is around 25 percent, and more likely is 15 percent or less. Why so little?

Powering a House

Field-effect transistor

• The field-effect transistor (FET) is a transistor that uses an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material.

• FETs are unipolar transistors as they involve single-carrier-type operation.

Basic information

• The device consists of an active channel through which charge carriers, electrons or holes, flow from the source to the drain.

• Source and drain terminal conductors are connected to the semiconductor through ohmic contacts.

• The conductivity of the channel is a function of potential applied across the gate and source terminals.

• The FET's three terminals are:[3]

• Source (S), through which the carriers enter the channel. Conventionally, current entering the channel at S is designated by IS.

• Drain (D), through which the carriers leave the channel. Conventionally, current entering the channel at D is designated by ID. Drain to Source voltage is VDS.

• Gate (G), the terminal that modulates the channel conductivity. By applying voltage to G, one can control ID

Cross section of an n-type MOSFET

FET operation

• I–V characteristics and output plot of a JFET n-channel transistor.

• The FET controls the flow of electrons (or electron holes) from the source to drain by affecting the size and shape of a "conductive channel" created and influenced by voltage (or lack of voltage) applied across the gate and source terminals.

• In an n-channel depletion-mode device, a negative gate-to-source voltage causes a depletion region to expand in width and encroach on the channel from the sides, narrowing the channel.

• If the depletion region expands to completely close the channel, the resistance of the channel from source to drain becomes large, and the FET is effectively turned off like a switch.

• This is pinch-off, and the voltage to achieve it is the pinch-off voltage. Likewise a positive gate-to-source voltage increases the channel size and allows electrons to flow easily.

• Conversely, in an n-channel enhancement-mode device, a positive gate-to-source voltage is necessary to create a conductive channel, since one does not exist naturally within the transistor.

• The positive voltage attracts free-floating electrons within the body towards the gate, forming a conductive channel. But first, enough electrons must be attracted near the gate to counter the dopant ions added to the body of the FET; this forms a region free of mobile carriers called a depletion region, and the phenomenon is referred to as the threshold voltage of the FET.

• Further gate-to-source voltage increase will attract even more electrons towards the gate which are able to create a conductive channel from source to drain; this process is called inversion.

• In a p-channel depletion-mode device, a positive voltage from gate to body creates a depletion layer by forcing the positively charged holes away from the gate-insulator/semiconductor interface, leaving exposed a carrier-free region of immobile, negatively charged acceptor ions.

• For either enhancement- or depletion-mode devices, at drain-to-source voltages much less than gate-to-source voltages, changing the gate voltage will alter the channel resistance, and drain current will be proportional to drain voltage (referenced to source voltage).

• In this mode the FET operates like a variable resistor and the FET is said to be operating in a linear mode or ohmic mode.[4][5]

• If drain-to-source voltage is increased, this creates a significant asymmetrical change in the shape of the channel due to a gradient of voltage potential from source to drain.

• The shape of the inversion region becomes "pinched-off" near the drain end of the channel. If drain-to-source voltage is increased further, the pinch-off point of the channel begins to move away from the drain towards the source.

• The FET is said to be in saturation mode;[6] some authors refer to it as active mode, for a better analogy with bipolar transistor operating regions.[7][8]

• The saturation mode, or the region between ohmic and saturation, is used when amplification is needed. The in-between region is sometimes considered to be part of the ohmic or linear region, even where drain current is not approximately linear with drain voltage.

• Even though the conductive channel formed by gate-to-source voltage no longer connects source to drain during saturation mode, carriers are not blocked from flowing.

• Considering again an n-channel device, a depletion region exists in the p-type body, surrounding the conductive channel and drain and source regions. The electrons which comprise the channel are free to move out of the channel through the depletion region if attracted to the drain by drain-to-source voltage.

• The depletion region is free of carriers and has a resistance similar to silicon. Any increase of the drain-to-source voltage will increase the distance from drain to the pinch-off point, increasing resistance due to the depletion region proportionally to the applied drain-to-source voltage. This proportional change causes the drain-to-source current to remain relatively fixed independent of changes to the drain-to-source voltage and quite unlike the linear mode operation. Thus in saturation mode, the FET behaves as a constant-current source rather than as a resistor and can be used most effectively as a voltage amplifier. In this case, the gate-to-source voltage determines the level of constant current through the channel.

Composition

• The FET can be constructed from a number of semiconductors, silicon being by far the most common.

– Most FETs are made with conventional bulk semiconductor processing techniques, using a single crystal semiconductor wafer as the active region, or channel.

• Among the more unusual body materials are amorphous silicon, polycrystalline silicon or other amorphous semiconductors in thin-film transistors or organic field effect transistors that are based on organic semiconductors; often, OFET gate insulators and electrodes are made of organic materials, as well.

• Such FETs are manufactured using variety of materials such as– silicon carbide(SiC),– gallium arsenide(GaAs), – gallium nitride(GaN), – indium gallium arsenide(InGaAs).

• In June 2011, IBM announced that it had successfully used graphene-based FETs in an integrated circuit. These transistors are capable of about 2.23 GHz cutoff frequency, much higher than standard silicon FETs.

Types of field-effect transistors

• The channel of a FET is doped to produce either an N-type semiconductor or a P-type semiconductor.

• The drain and source may be:– doped of opposite type to the channel,in the case of

depletion mode FETs, – or doped of similar type to the channel as in

enhancement mode FETs. • Field-effect transistors are also distinguished by the

method of insulation between channel and gate.

Types of FETs

• CNTFET (Carbon nanotube field-effect transistor)• The DEPFET is a FET formed in a fully depleted

substrate and acts as a sensor, amplifier and memory node at the same time. It can be used as an image (photon) sensor.

• The DGMOSFET is a MOSFET with dual gates.• The DNAFET is a specialized FET that acts as a

biosensor.• The FREDFET (Fast Reverse or Fast Recovery Epitaxial

Diode FET) is a specialized FET designed to provide a very fast recovery (turn-off) of the body diode.

• The HEMT (high electron mobility transistor ), also called a HFET (heterostructure FET), can be made using bandgap engineering in a ternary semiconductor such as AlGaAs. The fully depleted wide-band-gap material forms the isolation between gate and body.

• The HIGFET (heterostructure insulated gate field effect transisitor), is used mainly in research now.

• The IGBT (insulated-gate bipolar transistor ) is a device for power control. – It has a structure akin to a MOSFET coupled with a bipolar-like main

conduction channel. – These are commonly used for the 200-3000 V drain-to-source voltage range

of operation. – Power MOSFETs are still the device of choice for drain-to-source voltages of

1 to 200 V.

• The ISFET (ion-sensitive field-effect transistor) used to measure ion concentrations in a solution; – when the ion concentration (such as H+, see pH electrode) changes,

the current through the transistor will change accordingly.• The JFET (junction field-effect transistor) uses a reverse biased

p-n junction to separate the gate from the body.• The MESFET (Metal–Semiconductor Field-Effect Transistor)

substitutes the p-n junction of the JFET with a Schottky barrier; used in GaAs and other III-V semiconductor materials.

• The MODFET (Modulation-Doped Field Effect Transistor) uses a quantum well structure formed by graded doping of the active region.

• The MOSFET (Metal–Oxide–Semiconductor Field-Effect Transistor) utilizes an insulator (typically SiO2) between the gate and the body.

• The NOMFET is a Nanoparticle Organic Memory Field-Effect Transistor.• The OFET is an Organic Field-Effect Transistor using an organic

semiconductor in its channel.• The GNRFET is a Field-Effect Transistor that uses a graphene nanoribbon

for its channel.• The VeSFET (Vertical-Slit Field-Effect Transistor) is a square-shaped

junction-less FET with a narrow slit connecting the source and drain at opposite corners. – Two gates occupy the other corners, and control the current through the slit.

• The TFET (Tunnel Field-Effect Transistor) is based on band to band tunneling.

Advantages of FET

• The main advantage of the FET is its:– high input resistance,

• on the order of 100M ohms or more. Thus, it is a voltage-controlled device, and shows a high degree of isolation between input and output.

– It is a unipolar device, depending only upon majority current flow. – It is less noisy and is thus found in FM tuners and in

low-noise amplifiers for VHF and satellite receivers. – It is relatively immune to radiation. – It exhibits no offset voltage at zero drain current and hence makes

an excellent signal chopper. – It typically has better thermal stability than a

bipolar junction transistor (BJT).[3]

Advantages of FET over "BJT"

• Very high input impedance ( In the order of a few Mega-Ohm )

• FET is unipolar• High miniaturisation is possible• Very high switching speed• FET is less noisy• Low temperature sensitivity• FET is voltage controlled device• The characteristics will not change with time

Disadvantages of FET

• It has a relatively low gain-bandwidth product compared to a BJT.

• The MOSFET has a drawback of being very susceptible to overload voltages, thus requiring special handling during installation.

Uses

• IGBTs see application in switching internal combustion engine ignition coils, where fast switching and voltage blocking capabilities are important.

• The most commonly used FET is the MOSFET. The CMOS (complementary metal oxide semiconductor) process technology is the basis for modern digital integrated circuits. This process technology uses an arrangement where the (usually "enhancement-mode") p-channel MOSFET and n-channel MOSFET are connected in series such that when one is on, the other is off.

• The fragile insulating layer of the MOSFET between the gate and channel makes it vulnerable to electrostatic damage during handling. This is not usually a problem after the device has been installed in a properly designed circuit.

• In FETs electrons can flow in either direction through the channel when operated in the linear mode, and the naming convention of drain terminal and source terminal is somewhat arbitrary, as the devices are typically (but not always) built symmetrically from source to drain. This makes FETs suitable for switching analog signals between paths (multiplexing). With this concept, one can construct a solid-state mixing board, for example.

• A common use of the FET is as an amplifier. For example, due to its large input resistance and low output resistance, it is effective as a buffer in common-drain (source follower) configuration.

High electron mobility transistor

• Cross section of a GaAs/AlGaAs/InGaAs pHEMT

Band structure in GaAs/AlGaAs heterojunction based HEMT

• High electron mobility transistor (HEMT), also known as heterostructure FET (HFET) or modulation-doped FET (MODFET), is a field effect transistor incorporating a junction between two materials with different band gaps (i.e., a heterojunction) as the channel instead of a doped region, as is generally the case for MOSFET. A commonly used material combination is GaAs with AlGaAs, though there is wide variation, dependent on the application of the device. Devices incorporating more indium generally show better high-frequency performance, while in recent years, gallium nitride HEMTs have attracted attention due to their high-power performance. HEMT transistors are able to operate at higher frequencies than ordinary transistors, up to millimeter wave frequencies, and are used in high frequency products such as cell phones, satellite television receivers, and radar equipment.

Explanation

• To allow conduction, semiconductors are doped with impurities which donate mobile electrons (or holes). However, these electrons are slowed down through collisions with the impurities (dopants) used to generate them in the first place. HEMTs avoid this through the use of high mobility electrons generated using the heterojunction of a highly-doped wide-bandgap n-type donor-supply layer (AlGaAs in our example) and a non-doped narrow-bandgap channel layer with no dopant impurities (GaAs in this case).

• The electrons generated in the thin n-type AlGaAs layer drop completely into the GaAs layer to form a depleted AlGaAs layer, because the heterojunction created by different band-gap materials forms a quantum well (a steep canyon) in the conduction band on the GaAs side where the electrons can move quickly without colliding with any impurities because the GaAs layer is undoped, and from which they cannot escape. The effect of this is to create a very thin layer of highly mobile conducting electrons with very high concentration, giving the channel very low resistivity (or to put it another way, "high electron mobility"). This layer is called a two-dimensional electron gas. As with all the other types of FETs, a voltage applied to the gate alters the conductivity of this layer.

• Quantum mechanism• Since GaAs has higher electron affinity, free electrons in the AlGaAs layer are

transferred to the undoped GaAs layer where they form a two dimensional high mobility electron gas within 100 ångström of the interface. The n-type AlGaAs layer of the HEMT is depleted completely through two depletion mechanisms:

• Trapping of free electrons by surface states causes the surface depletion.• Transfer of electrons into the undoped GaAs layer brings about the interface

depletion.• The Fermi energy level of the gate metal is matched to the pinning point, which

is 1.2 eV below the conduction band. With the reduced AlGaAs layer thickness, the electrons supplied by donors in the AlGaAs layer are insufficient to pin the layer. As a result, band bending is moving upward and the two-dimensional electrons gas does not appear. When a positive voltage greater than the threshold voltage is applied to the gate, electrons accumulate at the interface and form a two-dimensional electron gas.

Versions of HEMTspHEMT

• Ideally, the two different materials used for a heterojunction would have the same lattice constant (spacing between the atoms). In practice, e.g. AlGaAs on GaAs, the lattice constants are typically slightly different, resulting in crystal defects. As an analogy, imagine pushing together two plastic combs with a slightly different spacing. At regular intervals, you'll see two teeth clump together. In semiconductors, these discontinuities form deep-level traps, and greatly reduce device performance. A HEMT where this rule is violated is called a pHEMT or pseudomorphic HEMT. This is achieved by using an extremely thin layer of one of the materials – so thin that the crystal lattice simply stretches to fit the other material. This technique allows the construction of transistors with larger bandgap differences than otherwise possible, giving them better performance.

mHEMT

• Another way to use materials of different lattice constants is to place a buffer layer between them. This is done in the mHEMT or metamorphic HEMT, an advancement of the pHEMT. The buffer layer is made of AlInAs, with the indium concentration graded so that it can match the lattice constant of both the GaAs substrate and the GaInAs channel. This brings the advantage that practically any Indium concentration in the channel can be realized, so the devices can be optimized for different applications (low indium concentration provides low noise; high indium concentration gives high gain).

Induced HEMT

• In contrast to a modulation-doped HEMT, an induced high electron mobility transistor provides the flexibility to tune different electron densities with a top gate, since the charge carriers are "induced" to the 2DEG plane rather than created by dopants. The absence of a doped layer enhances the electron mobility significantly when compared to their modulation-doped counterparts. This level of cleanliness provides opportunities to perform research into the field of Quantum Billiard for quantum chaos studies, or applications in ultra stable and ultra sensitive electronic devices.

Applications

• Applications are similar to those of MESFETs – microwave and millimeter wave communications, imaging, radar, and radio astronomy – any application where high gain and low noise at high frequencies are required. HEMTs have shown current gain to frequencies greater than 600 GHz and power gain to frequencies greater than 1 THz. (Heterojunction bipolar transistors were demonstrated at current gain frequencies over 600 GHz in April 2005.) Numerous companies worldwide develop and manufacture HEMT-based devices. These can be discrete transistors but are more usually in the form of a 'monolithic microwave integrated circuit' (MMIC). HEMTs are found in many types of equipment ranging from cellphones and DBS receivers to electronic warfare systems such as radar and for radio astronomy.

میدان اثر ترانزیستور

میدان • اثر از ترانزیستور دسته‌ای مبنای ترانزیستورها، که هستندیک توسط آن‌ها در جریان کنترل الکتریکی کار . میدان می‌گیرد صورت

الکترون ) بار حامل نوع یک تنها ترانزیستورها این در اینکه به توجه با ) را آن‌ها می‌توان دارند، دخالت الکتریکی جریان ایجاد در حفره یا آزاد

مقابل در که کرد محسوب تک‌قطبی ترانزیستورهای جزودر ) همزمان اقلیت و اکثریت حامل‌های که دوقطبی ترانزیستورهای

. ) می‌گیرند قرار دارند نقش ]آن‌ها دارای ۱[ میدان اثر ترانزیستورهایپایه; . گیتو درین، سورسسه خود ترانزیستورها از دسته این هستند

گروه دو . جی‌فتو ماسفتبه نوع این در می‌شوند تقسیمکنترل که پیوندی قطبی دو ترانزیستورهای برخالف ترانزیستورها،

به جریانبا کلکتورو امیترجریان کنترل بیسورودی می‌گیرد، صورتاعمال با درین و سورس .ولتاژجریان می‌گیرد صورت گیت به

کاربردها

تراشه‌های • به‌ویژه مجمتع مدارهای در ترانزیستورهای این اصلی کاربرد . شده استفاده ماسفت هزاران تراشه‌ها این بیشتر در است دیجیتال

نیز خازن و مقاوت عنوان به بلکه فعال عنصر عنوان به تنها نه که است . به نسبت ماسفت با ساخته‌شده مدارهای هرچند می‌روند کار به

با ساخته‌شده کمتری بی‌جی‌تی‌هامدارهای سرعت و هستند پیچیده‌ترو می‌کنند اشغال کمتری فضای و دارند نیز کمتری هزینه; اما دارند،

فناوری‌های در کالن‌مقیاس بنابراین کاربرد( VLSI )یکپارچه‌سازیالکترونیک ابزارهای صنعتی، کنترل فرآیندهای در و دارند گسترده‌ای

… ایفا مهمی نقش و تلفنی سوئیچینگ مدارهای نوری، الکترونیک خودکار،کم. اغتشاش و زیاد ورودی مقاومت علت به همچنین جی‌فت‌ها می‌کنند

. ) از ) دارند ویژه‌ای اهمیت دیجیتال غیر خطی الکترونیک درتقویت‌کننده‌ها، نظیر کاربردی‌ای مدارهای در میدان اثر ترانزیستورهای

. … می‌شود استفاده و فعال بار جریان، منابع ]کلیدها، ساخت ۲[ در فت‌هاباند .اف‌امفرستنده دارند فراوانی کاربرد نیز رادیو

پایانه‌ها

• ) ترانزیستور ) پایانه‌های پایه‌های از . FETیکی است نام‌های با پایه سه دارای و S سورس - D درoینفت

به G گیت درین از عبوری جریان گیت، پایه که است . نوع دو دارای فت‌ها می‌نماید کنترل را Nسورس

و . Pکانال نوع فت در هستند که Nکانال زمانی کانالبه درین از جریان باشد مثبت سورس به نسبت گیت

. می‌کند عبور حساس FETسورس بسیار ! معموال هاتحریک نیز بدن ساکن الکتریسیته با حتی و بوده

حساس. بسیار نویز به نسبت دلیل همین به می‌گردندهستند.

میدان اثر ترانزیستورهای ویژگی‌های و اهمیت

با • مقایسه پیوندی در وسیله; ترانزیستورهای یک می‌توان را میدان اثر ترانزیستورهای ،که دانست ولتاژ به حدود ) امپدانسحساس در زیاد بسیاربسیار آن و( اهم ۱۰۱۴ورودی

. با مقایسه در ماسفت‌ها است زیاد به‌نسبت نیز خروجی‌اش بسیار بی‌جی‌تی‌هاامپدانستنها که طوری به اشغال ۲۰تا ۱۰کوچکند را آن‌ها توسط در اشغال‌شده فضای از درصدیک در بنابراین و خاطر تراشهمی‌کنند همین به و داد جای می‌توان را ماسفت زیادی تعدادی

در گسترده‌ای کاربرد ماسفت‌ها که بزرگ‌مقیاس است . یکپارچه‌سازی دیگر تفاوت دارندگستره; از بازه‌ای در ماسفت‌ها که است این در پیوندی ترانزیستورهای با ماسفت‌ها

آی‌سی‌های به نسبت و می‌کنند عمل ولتاژ با کنترل‌شده مقاومت یک مانند عملکردشان . بسیار مقاومت ماسفت‌ها سوم ویژگی می‌گیرند تراشه‌ها در کمتری جای مشابه مقاومتی

قدری به ورودی مدار زمانی ثابت که بود خواهد معنا بدین ویژگی این است؛ ورودی‌شان زیادمدت برای ورودی کوچک خازن در ذخیره‌شده الکتریکی بار می‌دهد اجازه که است باال

دیجیتال مدارهای در ذخیره‌سازی ابزار یک عنوان به آن از بتوان و بماند باقی طوالنی . می‌توانند که است توان‌باال مصارف در قابلیتشان ماسفت‌ها چهارم ویژگی کرد استفادهبی‌جی‌تی‌ها در آنچه به نسبت مدت این که کنند کلیدزنی نانوثانیه چند در را زیاد جریان‌ها

. در را ماسفت‌ها از استفاده امکان چهارم ویژگی است سریع‌تر بسیار است امکان‌پذیر. می‌سازد ممکن توان‌باال oفرکانس‌باالی ]مدارهای مدارهای ۳[ در ماسفت‌ها دیگر مزیت

. است صفر ! تقریبا خاموش حالت در آن‌ها مصرفی توان که است این ]دیجیتال

.nنوع‌‌JFETخصوصیات‌و‌طرح‌های‌خروجی‌از‌I-Vنمودار‌

‌محدود‌می‌شود.گیت‌هنگام‌اعمال‌ولتاژ‌به‌جی‌فت‌با‌کانال‌پی‌در‌یک‌درین‌به‌سورس‌از‌جریان‌الکتریکی

‌یک‌جی‌فت‌با‌کانال‌اننماد‌مداری

پی کانال با جی‌فت یک مداری نماد

• میدان اثر پیوندی ج)ی‌ف)تیا ترانزیستور به) یا JUGFET یا junction gate field-effect transistor :انگلیسیJFET) از گونه‌ای به میدان اثر گفته ترانزیستورهای

شده‌اند؛ تشکیل گیت یک و عبور کانال یک از که می‌شودبا سورس و درین پایه; دو اهمی کانال اتصال طرف دو به. دارد یکسوساز اتصال گیت پایه; و هستند ]متصل به ۱[ بستهجنس از کانال اینکه ان نوع یا نیمه‌هادی باشد به پی ،

. در می‌شود گفته پی‌جی‌فت یا ان‌جی‌فت حاصل جی‌فتسرهای بین oکانال از عبوری الکتریکی جریان جی‌فتکنترل گیت سر به ولتاژ اعمال با درین، و سورس].می‌شود ]۲

کنترل • با درین و سورس سرهای بین جریان جی‌فت‌ها در . ناحیه; مقاومت تغییر برای می‌شود مهار کانال ناحیه; مقاومت ) اعمال ) با را الکتریکی حامل بدون تهی ناحیه; عرض کانال،

. می‌دهند تغییر کانال و گیت پیوند بین معکوس ]بایاس ]۳

• . می‌توان است میدان اثر ترانزیستور نوع ساده‌ترین جی‌فتبه یا و کنترل تحت الکترونیکی سوئیچ یک عنوان به را آن . کرد استفاده ولتاژ وسیله به شده کنترل مقاومت یک عنوانترمینال بین هادی نیمه کانال یک طریق از الکتریکی بار

. drainو" "sourceهای" یک" کردن اعمال با می‌یابد جریانترمینال " به معکوس بایاس کانال" gateولتاژ این ،

"pinched . ) الکتریکی") جریان که طوری به می‌شود فشرده. می‌شود قطع کامل طور به یا یافته کاهش

ساختار•جریان • است، شده اعمال گیت به ولتاژ یک که هنگامی

یک در درین به سورس از محدود pکانال JFETالکتریکیمی‌شود.

•JFET طوری که است، هادی نیمه مواد از طوالنی کانال یکیا مثبت بار شامل اکثریت حاملین که می‌شود االییده

نوع ) در ها یا( pحفره منفی بار اکثریت، حاملین یا و باشدنوع ) .. nالکترون هرسرمنبع( پایان در اهمی تماس باشد

(Source( )تخلیه . Drainو پیوند( یک می‌دهند تشکیل در pnرابه می‌شود، تشکیل کانال اطراف در ویا طرف دو در یا یک. باشد کانال ناخالصی مخالف آن ناخالصی که گونه‌ای

عملکرد•. JFETعملکرد • آن فشردن با می‌توان را می‌کند عبور شلنگ از که آبی جریان است باغ شلنگ شبیه

. یک در الکتریکی بار جریان داد کاهش آنرا مقطع سطح تا کرد کانال JFETکنترل کردن محدود با . دارد بستگی درین و سورس بین الکتریکی میدان به همچنین جریان می‌شود کنترل انتقالی جریان.) شلنگ) انتهای هر بر فشار در تفاوت به شبیه

• - : سورس گیت کردن معکوس بایاس برای می‌شود انجام میدان اثر از استفاده با هدایت کانال ساخت. pnپیوند وسیعتر را اتصال این تهی ناحیه وسیله بدین می‌شود اعمال سورس و گیت بین ولتاژ یک

. از بودن خالی بدلیل تهی ناحیه می‌کند محدود را مقطع وسطح می‌کند تجاوز هدایت کانال به و می‌کند . لحاظ از عملی کاربردهای برای بنابراین و می‌شود خوانده نام این به که است متحرک حامل‌های

" . و می‌دهد گسترش را رسانش کانال عرض تهی، ناحیه که زمانی می‌باشد نارسانا pitchالکتریکیoff" . می‌شود" متوقف سورس و درین بین رسانش و می‌آید بایاس" pinch offبدست ولتاژ یک در

. ولتاژ و می‌افتد اتفاق سورس و گیت اتصال خاص مقدار Vpمعکوس به مشابه ابزارهای در حتی . مثال عنوان به می‌کند، تغییر مالحظه‌ای قطعه VGS(off)قابل -۰٫۸از -Temic J201برای ولت ۴تا

- (. بین آن معمول مقادیر است .) ۱۰تا -۰٫۳متغییر کانال دستگاه کردن خاموش برای می‌باشد -Nولتسورس ) و گیت بین منفی ولتاژ منبع یک به کانال( VGSنیاز دستگاه کردن خاموش برای مقابل، -pدر

به .VGSنیاز است مثبتحدودی • تا را درین سورس جریان گیت، توسط یافته توسعه الکتریکی میدان معمول، عملیات در

. می‌کند متوقفدستگاه‌های • از .JFETبرخی هستند متقارن درین و سورس به نسبت ،

دیگر • ترانزیستورهای با مقایسهگیت • جریان اتاق دمای ( JFETدر جریان ) مشابه کانال به گیت پیوند از معکوس نشت

( MOSFETدر جریان ) از کمتر بسیار اما ، دارد وجود اکسید عایق کانال و گیت بین که . متقابل رسانایی است دوقطبی اتصال ترانزیستور یک در از JFETبیس بیشتر

MOSFET . باال ظاهری درونی مقاومت پایین، نویزهای بیشتر در بنابراین است . تاریخچه می‌شود استفاده عملیاتی کننده‌های JFETوتقویت

•JFET توسطJulius Lilienfed سال ۱۹۲۵درسال اواسط در و شد بینی ۱۹۳۰پیشرا آن امتیاز حق بتواند تا شد شناخته خوبی به کافی اندازه به آن عملکرد تئوری . حرارت با شده آالئیده کریستال‌های ایجاد سال چندین برای حال، این با کند تصدیق

. سال در نبود پذیر امکان اثر این دادن نشان برای پژوهشگران ۱۹۴۷کافی ،John Bardeen ، Walter Houser Brattain وWilliam Shockley نقطه ترانزیستور که زمانی

اولین ساخت برای بودند، کرده کشف را . JFETتماسی نسل اولین می‌کردند تالشJFET ساخته پیوندی ترانزیستورهای از بعد سال‌ها آنها انتظار برخالف عملی های

یک JFETشد. از ترکیبی عنوان به می‌تواند حدودی . BJTو MOSFETتا اگرچه کند رفتارهیبرید ویژگی‌های بیشتر به بیشتری شباهت شده عایق گیت با دوقطبی ترانزیستوردارد.

ریاضی • مدلدر • کوچک N-JFETجریان ولتاژ یک دلیل زیر VDSبه فرمول توسط شده

: می‌آید بدست•= - سورس درین اشباع جریان کانال = IDSSکه = wعرض 2aضخامت

الکترونی= Lطول تحرک= Q = 1.6*10^-9شارژ نئودیمیم = μnالکترون =N دوپینگ غلظت نوع

•: اشباع ناحیه در•: خطی منطقه در

یا•که • سورس – Vpجایی گیت از ولتاژ حداقل است، شده مصرف ولتاژ

. مقایسه در که هنگامی می‌کند عبور کردن، خاموش کامل طور به برایعمل - باولتاژ شده کنترل مقاومت یک مانند دستگاه است، کوچک بامی‌کند.

ماسفت(B(‌و‌بدنه)S(،سورس)D(،درین)Gماسفت‌شامل‌پایه‌های‌گیت)

-یا ماسفت• - فلز اکسید نیمه‌رسانا اثرمیدانی metal–oxide–semiconductor: انگلیسیبه ) ترانزیستورfield-effect transistor ٫ MOSFET . گونه ( این است دیجیتال و آنالوگ مدارهای در اثرمیدان ترانزیستور معروف‌ترین

میدان از اثر سال ترانزیستور در بار . ۱۹۲۵نخستین این کارگیری به و ساخت هنگام، آن در شد معرفی میالدیفراموش دهه پنج برای روی، همین از و بود همراه دشواری با امکان، و ابزار و علم نبود سبب به ترانزیستورها،

. دهه; oآغاز در ماندند کنار بر الکترونیک پیش‌رفت‌های oمیدان از و به ۱۹۷۰شدند نگاه‌ها دیگر oبار و MOSFETم، افتاد ها oساختن مجتمع برای .مدارهای شدند گرفته کار به

میدان ) • oاثر ترانزیستور با ( FETدر تنها و نمی‌کند مصرف جریانی کنترلی، پایه; است، پیدا اش نام از که چناناز عبوری جریان رسانا، نیمه درون میدان ایجاد و ولتاژ . FETاعمال مدار این ورودی روی همین از می‌شود کنترل

. با ماسفت تفاوت عمده دارد باالیی بسیار امپدانس و نمی‌گذارد قبلی تقویت طبقات روی بر بارگذاری اثر هیچاز JFETترانزیستور الیه‌ای توسط ماسفت ترانزیستورهای گیت که است این سیلیسیم در کانال( SiO2 )اکسید از

( . به مجزا گیت با فoت ماسفتها به دلیل این به است ، IGFET: انگلیسیمجزاشده Insulated Gate FET )گفته نیز]میشود. ]۱

مجتمع • میدانo مدارهای oاثر ترانزیستورهای فناوری پایه; مدارهای MOSبر از ساده‌تر و ریزتر بسیار می‌توان را ،بزرگ ) ( مقیاس‌های و پیچیده تابع‌های و مدارها در حتی که آن بی ساخت؛ دوقطبی ترانزیستورهای پایه; بر مجتمع

به .دیود، مقاومتنیازی باشند داشته الکترونیکی قطعه‌های دیگر یا ،[ آسان ۲[ را آن‌ها oانبوه oتولید ویژگی، همیناز بیش‌تر اکنون هم که چندان فناوریo ۸۵می‌کند، پایه; بر مجتمع، مدارهای oدرصدMOS. می‌شوند ساخته و طراحی

می‌گیرد، MOSترانزیستورهای • شکل آن‌ها در که کانالی به بسته ،NMOS یاPMOS . کار، oآغاز در می‌شوند نامیدهPMOS فناوری در پرکاربردتر oترانزیستورMOS . oساختن که جا آن از اما کم‌تری NMOSبود oمساحت و است آسان‌تر

از می‌گیرد، . PMOSهم oخالف بر گرفت دوقطبی پیشی ترانزیستورهای ترانزیستورهای در ،MOSFET ،نتیجه; جریان ،

oیک شارش هم( حفرهیا الکترون ) حاملتنها تک‌قطبی را ترانزیستورها این رو، این از و است پیوندها oمیان درمی‌نامند.

افزایشی • ماسفت کارکرد و ساختاردرین • نام‌های با پایه سه دارای سورس Dفت ،S گیت گیت، Gو پایه که است

. نوع دو دارای فت‌ها می‌کند کنترل را سورس به درین از عبوری کانال Nجریان. Pو نوع فت در هستند مثبت Nکانال سورس به نسبت گیت که زمانی کانال

. می‌کند عبور سورس به درین از جریان بوده FETباشد حساس بسیار ! معموال ها . به نسبت دلیل همین به می‌گردند تحریک نیز بدن ساکن الکتریسیته با حتی و

. میدانی اثر ترانزیستورهای دیگر نوع هستند حساس بسیار ها MOSFETنویز ) اساسی‌ترین ) از یکی فلز اکسید نیمه‌رسانای اثرمیدانی ترانزیستور هستند. است مدار در آن‌ها کمتر نویز ماسفت‌ها مزیت‌های

• . برای دارند فراوانی کاربرد نیز رادیو ام اف باند فرستنده ساخت در فت‌هاکانال فت کردن . Nتست یعنی می‌کنیم پیدا را گیت پایه نخست متر، مالتی با

جهت در و دارد رسانایی مقداری جهت یک در دیگر پایه دو به نسبت که پایه‌ای . از گیت و درین پایه بین مقاومت ! معموال است نهایت بی آن مقاومت دیگررا درین پایه; می‌توان طریق این از که است بیشتر سورس و درین پایه مقاومت. داد تشخیص سورس از

کاهشی • ماسفت• oترانزیستور گونه; این همانند MOSساختار ،

با تنها است، افزایشی ترانزیستورهای ساختاربه را، کانال آن، ساخت oهنگام که تفاوت اینسورس oمیان سیلیسیم، جنس از نوار یک وسیله; . اختالف اگر رو، این از می‌کنند تعبیه درین واز جریانی شود، اعمال دو آن میان پتانسیلولتاژ که هرچند داشت؛ خواهیم درین به سورس. باشد صفر گیت به شده اعمال

CMOSمدارهای ••MOSFET( فلزی اکسید هادی نیمه دیجیتال، های مکمل منطق( CMOSدر

از استفاده با ساختمانی های بلوک عنوان به گیرد،که می قرار استفاده موردp -کانال ماسفت . Nو مدارهای- در را ای عمده نگرانی یک حد از بیش است

بندی بسته کوچکتر های تراشه در را بیشتری ترانزیستورهای از است، مجتمع . است جریان CMOSشده هیچ زیرا دهد، می کاهش را برق مصرف منطق

) به) ورودی که زمانی جز به شود، می مصرف قدرت هیچ نتیجه در و ، آل ایده . است تغییر حال در منطقی های حاضر CMOSگیت حال در کاهش این انجام

هر تکمیل یک nMOSFETبا با pMOSFETبا درن دو هر و گیتس دو هر اتصال و . شد خواهد باعث دروازه روی بر باال ولتاژ یک است انجام nMOSFETهم برای

. pMOSFETو طول در معکوس باعث دروازه روی بر پایین ولتاژ و انجام بهبه ماسفت دو هر رود، می دیگر کشور به کشور یک از ولتاژ سوئیچینگ زمان . تولید و برق مصرف زیادی حد تا ترتیب این شد خواهد انجام خالصه طور

. کاربردی های برنامه دهد می کاهش را زیر CMOSگرما در آنالوگ و دیجیتال. است شده داده توضیح

دیجیتال•انگیزه • است کرده فراهم ریزپردازنده مانند دیجیتال های آوری فن رشد

تکنولوژی پیشبرد سیلیکون MOSFETبرای از دیگری نوع هر از تر سریع . تعویض برای ماسفت از بزرگ مزیت یک است ترانزیستور بر مبتنیجریان از جریان مانع کانال و دروازه بین اکسید ی الیه که است این دیجیتال

دروازه طریق ورودی DCاز امپدانس دادن و برق مصرف کاهش به بیشتر ، . یک در جدا موثر طور به کانال و دروازه بین عایق اکسید است بزرگ بسیارخروجی یک تا دهد می اجازه که آن، از بعد و اولیه مراحل از مرحله منطق

MOSFET ورودی از توجهی قابل تعداد درایو به دو. MOSFET MOSFETتنهامانند ) منطق بر مبتنی ترانزیستور باال( TTLقطبی ظرفیت fanoutچنین

از. برخی به طراحان برای تر آسان را آن شود می باعث نیز انزوا این ندارد . این گرفت نادیده را مستقل طور به منطق مراحل بین بارگذاری حد اثرات : امپدانس فرکانس، افزایش عنوان به شود می تعریف عامل فرکانس با حد. یابد می کاهش ها ماسفت ورودی

آنالوگ•. MOSFETمزایای • نوع دو نیست شدنی تبدیل آنالوگ مدارهای تمام در برتری به را دیجیتال مدارهای در

. بیشتر صرف سوئیچ، دیجیتال مدارات است کنند می رفتار ترانزیستور مختلف های ویژگی اساس بر مداررفتار بر آنالوگ مدارهای که حالی در تعویض، منطقه از خارج را خود منطقه MOSFETوقت در دقیقا

( . قطبی دو ترانزیستور اتصال دارد بستگی شود می برگزار عملیات از سنتی( BJTسوئیچینگ طور بهآن دلیل به انتخاب، از آنالوگ طراح تخلیه ) transconductanceترانزیستور خروجی امپدانس پایین و باال

. ) تعویض منطقه در استقالل ولتاژ• خاصی مزایای دلیل به آنالوگ مدارات انواع از بسیاری در ای گسترده طور به ها ماسفت وجود، این با

( . و طول اندازه تغییر با توان می را آنالوگ مدارهای از بسیاری ویژگیهای و عملکرد شود می استفاده . دوقطبی( ترانزیستورهای اکثر در مقایسه، در است شده طراحی شود می استفاده ماسفت از عرض

. مورد در ماسفت آل ایده های ویژگی عملکرد بر توجهی قابل دستگاه ( gate اندازه تخلیه ) و صفر فعلی ) مدارهای ) خازن تغییر ایجاد همچنین و سوئیچ، آل ایده به نزدیک عناصر را آنها نیز صفر افست ولتاژ منبع . تواند می که دقیق، مقاومتهای عنوان به توان می را ها ماسفت خود، خطی ناحیه در عملی آنالوگ

از بیشتر بسیار کنترل برابر در . BJT مقاومت گاهی ماسفت باال، قدرت مدارهای در شود می استفاده هاعنوان به نه برند می رنج حرارتی فراری از استفاده . BJT اوقات به توان می را آنها همچنین باشد داشته ها

ظاهر سلف عنوان به آنها از شده ساخته آمپر عملیات، دهد می اجازه که است چرخنده مدارهای و خازندیودها برای جز به طبیعی، های دستگاه آنالوگ تمام دهد می اجازه نتیجه در است، گرفته شکل شود می

( یک از کوچکتر صورت به تواند می حال MOSFET که هر (به شده ، ساخته ماسفت از کامل طور به بهبسیار. فضای یک در سیلیکونی تراشه یک روی بر را آنالوگ مدارات کامل برای تا دهد می اجازه این است

است شده ساخته .کوچکتر

• از مدارات IC بعضی ترکیب مدار MOSFET ها سیگنال مخلوط یک درو مدیره هیئت نیاز مورد فضای ساخت دیجیتال، و آنالوگ مجتمع . مدارهای ساختن منزوی برای نیاز یک این است کوچکتر حتیحلقه از استفاده به منجر تراشه، سطح در دیجیتال مدارهای آنالوگ

عایق روی بر سیلیکون و اصلی .(SOI) انزوا مقابل BJT مزیت در هاتوانایی آنالوگ، طراحی فرایند در مسئولیت BJT ماسفت که ها

. است کوچکتر فضای یک در بزرگتر جریان یک به رسیدگیترکیب که دارد وجود ساخت یک MOSFET و BJT فرآیندهای به را

. بی نام به مختلط ترانزیستور ها دستگاه واحد بیین FET دستگاه هابیین FET-دوقطبی) حاوی (ها آنها از یکی تنها میکنند BJT-FET اگر وBicmos (دوقطبی-CMOS) مکمل حاوی آنها . BJT-FET اگر بیین ها

باالتر و عایق گیتس دو هر از استفاده مزایای چنین های دستگاهاست جریان چگالی

ماسفتها • پیشرفتگذشته، • های دهه طول و MOSFETدر کوچک اندازه در مستمر طور به

کانال مجتمع MOSFETطول مدارهای اما میکرومتر، بار یک معمولیترکیب . MOSFETمدرن در کار است نانومتر دهها از کانال طول با های

رابرت شدن پوسته پوسته که Dennardتئوری است شناخت در محوری . شامل فرایند یک تولید به شروع اینتل بود بود ممکن مداوم کاهش این

32 ) اواخر ) در تر کوتاه حتی که کانال با های ویژگی از اندازه نانومتر" 2009سال " . که ، راه نقشه کند می حفظ را هادی نیمه صنعت است

توسعه برای سرعت با. MOSFETمجموعه مشکالت تاریخی، لحاظ ازاندازه ساخت، MOSFETکاهش فرآیند هادی نیمه دستگاه با اند شده

الکتریکی عملکرد فقیرتر با و پایین، بسیار ولتاژهای از استفاده به نیازنوآوری ) و می ایجاب مدار نمایشگاه MOSFETطراحی کوچک های

.) بحث مورد زیر در مقاومت، خروجی پایین و باالتر نشتی جریان