Historical Timelines Properties Related to Band...
Transcript of Historical Timelines Properties Related to Band...
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Properties Related to Band Theory
History of Semiconductors Conductivity Intrinsic and Extrinsic Semiconductors Direct & Indirect Bandgap Semiconductors p-n Junction Photoelectricity Nanoscaled Semiconductor
Semiconductors Historical Timelines
Semiconductor Products are Proliferating
LANs WANs
Routers Hubs
Switches
Workstations Interne
t Servers
Video Games
Voice Over IP
Digital Cameras
Wireless Handset
s PDAs
PCs Storage Systems
Set-Top Boxes
Internet Browsers
Scanners
Digital Copiers
Internet
The Next Big Thing…A Lot of Little Things
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$27.0
$12.8
0
5
10
15
20
25
30
2000 2005
Billio
ns
US
$
China Semiconductor Consumption
China to grow faster than the world at CAGR (Compound Annual Growth Rate) of 17% from 20012005, reaching $27B in 2005
China currently produces only 1 of every 4 chips it consumes
The Electronics Ecosystem
Materials
Semiconductor Equipment
Semiconductors
Electronic End Equipment
SEMI MEMBERSHIP
$990B
2001
Estimate 2004
$21B
$28B
$139B
$879B
$28B
$46B
$218B
Three Types of Solid Materials Based on Electrical Conductivity
1084-4-8-12-16-20 101010 10 10 10 10
glass
diamond
fused silica
silicon
germanium
iron
copper
insulators semiconductors metals
0-2410
Conductivity ( -1cm-1 )
isolator
= alloy increasing resistivity
resistivity below Tc = 0 !!
decreasing resistivity
Temperature Dependence of the Electrical Conductivity of Metals and Semiconductors
(Isolators)
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Electrical Conductivity
Conductivity of metals decreases with temperature as atomic vibrations scatter free electrons.
Conductivity of semiconductors increases with temperature as the number of carriers increases.
Creation of Carriers in Intrinsic Semiconductors by Thermal Excitation
Thermally induced electrical conductivity
T=0 K Conduction band empty Valence band completely filled No electrical conductivity T>>0 K The thermal energy is responsible for
the promotion of electrons to the conduction band.
Creation of electronhole pairs: carriers electrical conductivity
Experimental Observation: Conductivity of Semiconductors
Semiconductor block connected to the terminals of a battery
No conductivity observed at low or room temperature or in the dark.
When we increase the temperature or expose the semiconductor to light, we observe that it starts conduction.
Semiconductors
Intrinsic Semiconductors: If a semiconductor crystal contains no impurities, the only charge carriers present are thus produced by thermal breakdown of the covalent bonds. The conducting properties are thus characteristic of the pure semiconductor. Such a crystal is termed an intrinsic semiconductor. Extrinsic Semiconductors: If a semiconductor crystal contains n-type or p-type impurities, the conducting properties are chiefly due to the impurities. Such a crystal is termed an extrinsic semiconductor.
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Conductivity of Intrinsic Semiconductors
•The valence band of semiconductors is completely filled. However, the band gap between the valence and conduction bands is small, and electrons can be promoted to the conduction band. •In semiconductors, only the electrons promoted to the conduction band and the holes created in the valence band will be carriers. •The smaller the gap, the easier to promote electrons to the conduction band. •At the same temperature, smaller gap semiconductors will show a larger conductivity. •The higher the temperature, the larger the number of carriers. •Conductivity increases with temperature
TK2
E
B
g
Ce
the Response of Equilibrium to Temperature
The van’t Hoff equation
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0
12
2
o
P
0
T/1T/1
R/HKlnKln
RT
H
dT
Klnd
R
H
)T/1(d
)K(lnd
Temperature Effects
Intrinsic semiconductors
Concentration of holes and free electrons increase with temperature. Because increasing thermal energy will excite more e- across the band gap.
Ge has a greater charge concentration than Si. Because Ge has a smaller band gap than Si (0.67 vs 1.11)
Carrier Mobility
The intrinsic carrier mobility is defined as the drift velocity per unit electric field.
Similar to metals, charge carriers in semiconductors lose mobility with increasing dopant concentration.
E
VD
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Carrier Mobility
Temperature also affects carrier mobility. Note regardless of dopant concentration, high temperatures reduce mobility.
Delocalized Bonding Model
energy
Conduction band
Valence band
electrons
holes
Bonding Picture of Silicon
Delocalized bonding picture
mobile holes:acid species electrons:basic species
Semiconductors and Acid-Base Analogy
Chemical Equilibrium in Solution H2O H++OH-
Kw=[H+][OH-] [H+]1014 ions/cm3
Chemical Equilibrium in Solid
Si(crystal)h++e-
K=[h+][e-]=p•n
[h+]1.5x1010cm-3
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Donor States: ntype Semiconductors
If an atom in the lattice is substituted by an atom of a different element with more valence electrons, once the impurity is accommodated to the lattice and the new bonds are formed, there will be a remaining negative charge.
Example: Pentavalent Sb impurity in a silicon crystal (tetrahedrally coordinated)
Extrinsic Semiconductors:
Valence Band
Donor States: ntype Semiconductors
Acceptor States: ptype Semiconductors
If an atom in the lattice is substituted by an atom of a different element with less valence electrons, once the impurity is accommodated to the lattice and the new bonds are formed, there will be a remaining positive charge.
Example: Trivalent boron (B) impurity in a silicon crystal (tetrahedrally coordinated)
Extrinsic Semiconductors:
Valence Band
Acceptor States: ptype Semiconductors
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Intrinsic vs. Extrinsic
Valence Band
Conduction Band
Intrinsic Extrinsic (doped)
Conduction Band
Valence Band
h+
e-
Conduction Band
Valence Band
ntype ptype
Silicon Si:P Si:Al
ptype semiconductors Addition of acceptor states
ntype semiconductors Addition of donor states
Examples: P, As, or Sb impurities in Si or Ge.
Examples: B, Al, Ga, or In impurities in Si or Ge.
Extrinsic (Doped) Semiconductors
We can enhance the electrical properties of a semiconductor by adding impurities to it. The addition of impurities is called doping and the doped semiconductor is called extrinsic.
Example: the addition of 1 Boron atom every 105 Silicon atoms enhance the conductivity of Silicon by a factor of 103 at room temperature.
Extrinsic semiconductors are the basic materials in the electronics technology. Great importance in current technology: lasers, solar cells, rectifiers, transistors, ...
Band Diagram (n and p type)
Electrons can jump to Al atom
Electrons can jump from P atom to Conduction Band
n type
p type
Acceptor level in Band Gap
Ea
Donor Level in Band Gap
Ed
Temperature Effects
undoped
Extrinsic ntype Semiconductors Low Temperatures Thermal energy is insufficient to excite electrons from the donor state Intermediate Temperatures e’s from donor state are excited into the conduction band. e concentration equal to dopant concentration. High Temperatures Enough thermal energy to excite an effective amount of valence e’s into the conduction band
ptypes behave similarly with temperature
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Fermi Level
Ef
metal
Ef
undoped semiconductor
Ef
Ef
ptype semiconductor ntype semiconductor
Ed Ea
The pH of aqueous solutions and the Fermi level in semiconductors play analogous roles in determining the extent of ionization in the two media.
Analogy Between pH and Fermi Level (Ef)
Extent of Ionization: Weak Acid Acceptor Analogy
When pH = pKa
[HA] = [A]
Acidbase system Semiconductor
When Ef ~ Ea, [A] ~ [A]
Ea = acceptor energy level
aa
a
KlogpK
]HA[
]A][H[K
AHHA
AhA
Energy Levels for Impurities in Silicon
Donors
Acceptors
e
h+
shallow
shallow
deep
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Interaction of Light and Electrons
absorption
spontaneous
emission
stimulated
emission
Optical Properties of Semiconductors
longest wavelength absorption to promote e corresponds to Eg
Eg is energy between “HOMO” of valence band and “LUMO” of conduction band
Eg
Absorption Emission
Eg
Band Gap Energy and Color
.
Bandgap e
nerg
y (e
V)
Color thatcorresponds toband gap energy
Apparent colorof material(unabsorbed light)
4
3
2
1
red
yellow
greenblue
violet
colorless
black
yellow
orange
ultraviolet
infrared
red
Semiconductor Glossary
Direct Bandgap Semiconductor: semiconductor in which the bottom of the conduction band and the top of the valence band occur at the momentum k=0; in this case, energy released during bandtoband electron recombination with a hole is converted primarily into radiation (radiant recombination); wavelength of emitted radiation is determined by the energy gap of semiconductor. e.g. GaAs, InP, etc. Indirect Bandgap Semiconductor: semiconductor in which bottom of the conduction band does not occur at effective momentum k=0, i.e. is shifted with respect to the top of the valence band which occurs at k=0; energy released during electron recombination with a hole is converted primarily into phonon; e.g. Si, Ge, GaP.
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An important property of direct semiconductors is that electrons may easily drop from the conduction band to the valence band by emitting a photon
This process is known as electron-hole recombination since the electron drops to occupy a hole state in the valence band the energy of the photon emitted by the semiconductor is determined by the size of its energy gap recombination is therefore analogous to the level transitions that occur in atomic systems
Bandstructure in Three Dimensions
E
k
PHOTON
Electron-hole Recombination in a direct semiconductor such as GaAs
An electron drops from the conduction band to the valence band and its excess energy is emitted in the form of a photon
Note that in the figure shown here the initial and final wavevector states are the same … this is an important property of direct semiconductors
In indirect semiconductors, the bottom of the conduction band and the top of the valence band occur at different points in kspace
An electron cannot therefore drop from the conduction band to the valence band just by emitting a photon since this would violate momentum conservation instead the electron must simultaneously emit a photon and exchange momentum with the crystal lattice the probability of this double process occurring is very small, so indirect semiconductors turn out to be much poorer emitters of light than direct ones
E
k
PHOTON
Electron-hole recombination in an indirect Semiconductor
In order to conserve energy and momentum, an electron must drop to the valence band by emitting a photon and exchanging momentum with the crystal
Because this process has a low probability, indirect semiconductors such as Si or Ge cannot be used in optoelectronic applications as light emitters
Bandstructure in Three Dimensions
E
k
PHOTON
The opposite process to recombination is electronhole generation in which an electron is excited from the valence band into the conduction band by absorbing a photon
Since this process also must conserve momentum the electron is excited into a state with the same kvalue as the initial valenceband state Both direct and indirect semiconductors may therefore be used as photodetectors to detect electromagnetic radiation The absorption of these materials strongly increases once the photon energy exceeds the direct band gap
Bandstructure in Three Dimensions
absorption of light by direct (left) and indirect (right) semiconductors
E
k
PHOTON
Luminescence
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Solid line: direct bandgap materials Dotted line: indirect bandgap materials
Matched system to reduce the strain effect and epitaxial growth defects!
What's Luminescence?
The spontaneous emission of light upon electronic excitation is called luminescence.
Absorption and Luminance pn Junction
What happens if we bring a ptype semiconductor in contact with a ntype semiconductor?
Electrons close to the junction diffuse across the junction into the ptype region. Holes are filled by recombination.
Equilibrium is established resulting in a potential difference.
If the two regions are connected in a circuit a variety of applications are possible.
p n
- - - -
e
+ + + +
h+
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Biasing the pn Junction
Biasing introduction of a voltage into the circuit containing the pn junction.
Forward bias negative voltage is applied to ntype side. Decreases energy barrier for electrons and holes to flow through the junction.
Reverse bias positive voltage applied to ntype side. Raises energy barrier for current flow.
p n
V + —
e
“Majority Carrier” and Current Flow in ptype Silicon
p Type Silicon + -
Hole Flow
Current Flow
“Majority Carrier” and Current Flow in ntype Silicon
Electron Flow
n Type Silicon + -
Current Flow
the pn Junction
p n 0 Volts
Hole Diffusion
Electron Diffusion
Holes and Electrons “Recombine” at the Junction
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A Depletion Zone (D) and a Barrier Field Forms at the pn Junction
The Barrier Field Opposes Further Diffusion
(Equilibrium Condition)
p ++ n 0 Volts
Hole (+) Diffusion
Electron () Diffusion
D
Barrier Field
Donor Ions
Acceptor Ions
“Forward Bias” of a pn Junction
•Applied voltage reduces the barrier field •Holes and electrons are “pushed” toward the junction and the depletion zone shrinks in size •Carriers are swept across the junction and the depletion zone •There is a net carrier flow in both the p and n sides = current flow!
p + n + Volts
Volts
Current
“Reverse Bias” of a pn Junction
p +++ n Volts
D + Volts Current
•Applied voltage adds to the barrier field •Holes and electrons are “pulled” toward the terminals, increasing the size of the depletion zone. •The depletion zone becomes, in effect, an insulator for majority carriers. •Only a very small current can flow, due to a small number of minority carriers randomly crossing D (= reverse saturation current)
Forward Bias Holes and free electrons flow together and recombine at the junction. Current flows. Reverse Bias Holes and free electrons flow away from each other. The center of the diode quickly becomes a dead zone with no charge carriers. Current is reduced.
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Energydiagram of pn Junction
When ptype and ntype semiconductors touch, the Fermi levels do not align until equilibrium is reached.
Applications of pn Junction Diode
Rectifier
Photodetectors
solar cells
LEDs
diode lasers
Optoelectronics
Why call it pn Junction as a Diode?
PN junction
Simple Application: Rectifier
One of the most important uses of a diode is rectification. The normal p-n junction diode is wellsuited for this purpose as it conducts very heavily when forward biased (lowresistance direction) and only slightly when reverse biased (high resistance direction). If we place this diode in series with a source of ac power, the diode will be forward and reverse biased every cycle. Since in this situation current flows more easily in one direction than the other, rectification is accomplished.
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pn Rectifying Junction
A diode’s properties can be seen when the voltage is examined
Optoelectronics
In optoelectronic applications of semiconductor devices, the basic idea is that the device is used to either detect or to emit electromagnetic radiation
In detection, the incident light is converted into a
measurable electrical signal by exploiting internal carrier processes within the device. Examples of such devices include photodetectors and solar cells
In emission, on the other hand, the internal
processes allow the conversion of an electrical signal into detectable light and examples of such devices include LEDs and lasers
Photodetector
Photodetector converts optical energy into electrical energy, thus making possible data reading in the optical storage systems, such as CD or DVD drives. Modern photodetectors are typically semiconductor photodiodes.
socalled "reverse bias pn photodiode" with the carriers flowing away from the pn junction thus creates a depletion region. There is very little current flowing through this junction until the light illuminates the surface of the photodiode. Then, the absorbed photons create pairs of electrons and holes mostly in the depletion area. Those new carriers move quickly in opposite directions, and moving electrons create current in the external circuit.
LED (LightEmitting Diodes)
LEDs are pn junction devices constructed of gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), or gallium phosphide (GaP). Silicon and germanium are not suitable because those junctions produce heat and no appreciable IR or visible light.
The junction in an LED is forward biased and when electrons cross the junction from the n- to the p-type material, the electronhole recombination process produces some photons in the IR or visible in a process called electroluminescence. An exposed semiconductor surface can then emit light.
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Lightemitting diodes (LEDs) operate in the opposite manner to photodetectors by exploiting the enhanced diffusion of carriers that occurs across the depletion region in forwardbiased junctions in direct semiconductors, the additional carriers recombine through bandtoband processes giving rise to the emission of light from the junction
PHOTON
PHOTON
PHOTON
when the p-n junction is forward biased, large numbers of electrons and holes diffuse across the depletion region
in a direct semiconductor, these carriers may recombine by emitting photons
the photon flux increases as the forwardbias voltage and so the corresponding diffusion current is increased
LED
With forward bias, electrons reaching the ptype side can recombine with the abundant holes and emit light according to the energy difference. Holes reaching the ntype side can recombine with the abundant electrons emitting light.
Color of luminescence is controlled by the composition of the solid solutions in the semiconductors.
A great advantage of semiconductor optoelectronic devices is that they can be fabricated in a highly compact manner and can even be incorporated into integrated circuits
semiconductor
lens
In contrast to photodetectors, an important requirement for light emitting diodes is that they be fabricated from a direct semiconductor
While gallium arsenide is a direct semiconductor, its energy gap (1.42 eV) corresponds to a photon wavelength (870 nm) that lies outside of the visible spectrum
For display applications, it is therefore necessary to use alloys of GaAs which allow access to photon frequencies in the visible range of the spectrum.
ALLOY COLOR
GaAs0.6P0.4Ga RED
As0.35P0.65:N ORANGE-RED
GaAs0.14P0.86:N YELLOW
GaP:N GREEN
GaP:ZnO RED
AlGaAs RED
AlInGaP ORANGE
AlInGaP YELLOW
AlInGaP GREEN
SiC BLUE
GaN BLUE
color characteristics of commercial LEDs
changes in alloy composition are exploited to modify the energy band gap or to introduce impurity levels that mediate photon emission in indirect semiconductors
the development of the blue GaN LED (from 1994) now allows the possibility of manufacturing full color LED-based displays
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Band Gap Engineering in Semiconductors: Solid Solutions
Trends in cubic unit cell lattice parameter and Eg as a function of the composition x for the solid solution ternary semiconductor AlxGa1-xAs:
Band gap engineering enables a range of optical and electronic devices to be fabricated.
The roughly linear dependence of the physical properties on composition is known as Vegard’s law and proves that the distribution of the Al and Ga is random :
P(AlxGa1-xAs) = xP(AlAs) + (1x)P(GaAs) P = physical property Any physical property is the atomic fraction
weighted average of the two end members.
Semiconductor Heterostructures In addition to alloying, fabricating artificial structures with
tailored optical and electronic properties has been possible using crystal growth techniques, such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). These techniques allow monolayer control in the chemical composition of the growing crystal. When two different semiconductors are grown into a single structure, the structure is called heterostructure. One such structure is superlattice in which two (or more) semiconductors A and B are grown alternately with thickness dA and dB,respectively.
GaAsAlGaAs superlattice. On the left is a sequence of nearly thirty different layers, while on the right the individual atomic resolution is indicated.
In1-xGaxN bandgap (room temp)
Define Emission Color via Band Engineering
InGaN growth 780C 760C 720C 690C 630C
Indium (%) ~5% ~10% ~20% ~30% ~35%
ΔΕ(eV) 3.18 2.95 2.64 2.38 2.14
Emission (nm) 390 420 470 520 580
FWHM (nm) 7 27 30 48 61
)x1(x43.1)x1(77.0x42.3)x(EG
Wu, et al, Superlattices Microstruct., 2003,34, 63
Electrically Driven SQW Nanowire Multicolor LEDs
—— Tuning emission colors from ultraviolet to visible —— Can be assembled and individually addressed
Lieber*, Nano Lett., 2005, 5, 2287
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What is a solar cell?
A solar cell is a kind of semiconductor device that takes advantage of the photovoltaic effect, in which electricity is produced when the semiconductor's p-n junction is irradiated.
Solar Cell (Photovoltaic)
If light of sufficient energy strikes the semiconductor, electrons are promoted into the high energy state and move toward the ntype semiconductor. Holes are also generated and move toward the ptype semiconductor.
This creates an electric current.
Magnitude of current depends on intensity of light.
p n
e-
h+
h
V
Electron Flow in a Solar Cell
ntype ptype
h
e-
h+
How A Solar Cell Works
When sunlight strikes a solar cell, only certain bands (or wavelengths) of light will cause electrons to move within the semiconductor, thereby producing electric current. The energy "band gap" of the semiconductor determines the ideal portion of the light spectrum that will create this effect. To allow it all to happen, the semiconductor layers must be constructed so as to produce an electric field (shown as the layer above).
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Amorphous Silicon Solar Cells
Amorphous silicon solar cells are cells containing noncrystalline silicon, which are produced using semiconductor techniques. Amorphous silicon solar cells are mostly used as power sources for devices requiring little electricity or as modulated light sensors. They are common in pocket calculators, watches, light detectors for cameras, and television and car navigator screens.
Solar Vehicle Project
“Spirit of Canberra” II solar vehicle (Australian)
Amorphous Silicon and Solar Cell House
Self-supplying Solar Cell House in Germany
Water + primary energy sources Hydrogen + oxygen water
Clean Energy by Means of Advanced Materials
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Transistor
Collector (n) Emitter (n)
Base (p)
Both pn junctions are reversed biased
First transistor 1947 John Bardeen, Walter Brattain, William Shockley.
1947
Pictorial History of Transistors How a Transistor Works
• The transistor can function as: – An insulator – A conductor
• The transistor's ability to fluctuate between these two states that enables to switch or amplify.
• The transistor has many applications, but only two basic functions: switching and modulation (amplification).
• In the simplest sense, the transistor works like a dimmer. – With a push the knob of the dimmer, the light comes on
and off. You have a switch. Rotate the knob back and forth, and the light grows brighter, dimmer, brighter, dimmer. Then you have a modulator.
• Both the dimmer and the transistor can control current flow. • Both can act as a switch and as a modulator/amplifier. • The important difference is that the “hand” operating the
transistor is millions of times faster.
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MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) MOSFET transistor: consists of two small islands of
p-type semiconductor created within n-type silicon substrate. Islands are connected by narrow p-type channel.
Metal contacts are made to islands (source and drain), one more contact (gate) is separated from channel by a thin (< 10 nm) insulating oxide layer.
Gate serves the function of the base in a junction transistor (the electric field induced by the gate controls the current through the transistor)
MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor)
Voltage applied from source encourages carriers (holes in the case shown below) to flow from the source to the drain through the narrow channel.
Width (and hence resistance) of channel is controlled by intermediate gate voltage. For example, if positive voltage is applied to the gate, most of the holes are repelled from the channel and conductivity is decreasing.
Current flowing from the source to the drain is therefore modulated by the gate voltage (amplification and switching)
p n n
gate
electrode
Potential as seen
by electrons
When Vbias > 0
Gate voltage > Vt
e e e e e e
e e e
e e e
e e e
e e e
e e e
p+ p+ p+ p+
p+ p+ p
+ p+
Vbias
Metal-
Oxide-
Semiconductor
Field-
Effect
Transistor
Electron
potential
energy
(negative of
electric
potential)
npn MOSFET (n-FET)
Moderate bias
e e e e e e
e e e
e e e
p+ p+ p+ p+
p n n
gate
electrode
Vbias
e e e
e e e
Strong bias
e e e
e e e
e
e
Very strong bias
Zero bias
Punch-Through
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p n n
gate
electrode
Vbias
e e e e e e
e e e
e e e
p+ p+ p+ p+
e e e
e e e
e e e
e e e
e
e
p n n
e e e e e e
e e e
e e e
p+ p+ p+ p+
e e e
e e e e e
e
Smaller size & same voltage
higher electric field strengths
easier punch-through
Vbias
Need for Voltage Scaling Nano-Scale MOSFET
• Metal Oxide Semiconductor Field Effect Transistor • Three terminal device
• Source, gate and drain
• Vg controls the conduction from source to drain
• Half thickness of the gate is called “Feature size λ”
• Current feature sizes in production – 90nm (Intel Pentium 5)
• Demonstrated feature sizes up to 20nm (IBM).
Ph
oto
Co
urt
esy:
Fu
jits
u L
ab
s
Challenges
• Difficulties • High electric fields
• Power supply vs. threshold voltage
• Heat dissipation
• Interconnect delays
• Vanishing bulk properties
• Shrinkage of gate oxide layer
• Too many problems to continue miniaturization as physical limits approach
• Proposed solutions are short term
• Open Problems
• Improve lithographic precision (eBeam)
• Explore new materials (GaAs, SiGe, etc.)
• As a long term goal explore new devices
The MOSFET dominates the microelectronic industry (memories, microcomputers, amplifiers, etc.)
Large Si single crystals are grown and purified. Thin circular wafers (“chips”) are then cut from the crystals
Circuit elements are then constructed by selective introduction of specific impurities (diffusion or ion implantation)
A single 8” diameter wafer of silicon can contain as many as 1010 1011 transistors in total
Cost to consumer ~ 0.00001cent each.
Transistors and Microelectronic Devices
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Sand Silicon Computer Chips Silicon Wafer Preparation
Evaluation
Melting
Preparation
Body growth
Cooldown Ingot removal
Slicing
Lapping
Etching
Heat Treatment
Polishing
Silicon Wafer Preparation
Epitaxial Processing
Cleaning
Inspection
Packing
Silicon Wafer Preparation
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Four Stages of Semiconductor Manufacturing
raw semiconductor material mined and purified
crystal growth and wafer preparation
the devices or integrated circuits are actually formed in and on the wafer surface. Chip fabrication!
packaging
Why wide bandgap semiconductors?
Needs: Solidstate amplifiers for:
Broad band wireless communications
Sophisticated controllers for electric grids, Radars
Base stations of future wireless network
Multifunction RF Systems
Military Applications
Requirements:
Ultra high power
High efficiency
High Frequency
Linearity
Manufacturability
Low Cost
High Temperature (300C400C) & Hostile Surrondings Endurance
Silicon Devices can not substain these requirements
Why wide bandgap semiconductors?
Gallium Arsenide GaAs
Indium phosphide InP
Silicon Carbide SiC
Gallium Nitride GaN
GaN will replace GaAs devices and all of its properties comes from electrical and physical characteristics
Important Compound Semiconductors Materials
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SPEED
GHz POWER W/mm NOISE FIGURE
GaAs 1050 1 Good
InP 100200 0.5 Very Good
SiC 10 0.6 (like Si)
GaN 30 10 Good
Commercial devices available Not commercial devices available
Front-end Devices Semiconductor characteristics
Semiconductor
characteristics
Silicon
Gallium
Arsenide
Indium
Phosphide
Silicon
Carbide
Gallium
Nitride
Bandgap(eV) 1.1 1.42 1.35 3.26 3.49
Electron mobility
(cm2/Vs)
1500 8500 5400 700 1000-2000
Saturated (peak)
electron velocity
(x107cm/s)
1.0(1.0) 1.3 (2.1) 1.0 (2.3) 2.0 (2.0) 1.3 (2.1)
Critical breakdown
field MV/cm
0.3 0.4 0.5 3.0 3.0
Thermal
conductivity
1.5 0.5 0.7 4.5 >1.5
Relative dielectric
constant (er)
11.8 12.8 12.5 10 9
Technology development costs can be amortized over several large electronic and opto-electronic applications, like BLUE & WHITE LED and BLUE LASER.
Cost Advantages
Power density increased
Device variability
Reliability
Complexity
Leakage
Power dissipation limits device density
Transistor will operate near ultimate limits of
size and quality – eventually, no transistor can
be fundamentally better
◄
Transistor problems
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The Future of transistors
Molecular electronics
Carbon nanotubes transistors
Nanowire transistors
Quantum computing
CMOS devices will add functionality to CMOS non-volatile memory, opto-electronics, sensing….
CMOS technology will address new markets macroelectronics, bio-medical devices, …
Biology may provide inspiration for new technologies bottom-up assembly, human intelligence
Nanocrystal 3 D
Quantum layer 2 D
Quantum wire 1 D
Quantum dot 0 D
Nanoscaled Semiconductor
Quantum Confinement
Trap particles and restrict their motion Quantum confinement produces new material behavior/phenomena “Engineer confinement” control for specific applications Structures
Quantum dots (0D) only confined states, and no freely moving ones
Nanowires (1D) particles travel only along the wire
Quantum wells (2D) confines particles within a thin layer
Variation of the Optical Properties with the Crystal Size
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Optical Properties Are Directly Dependent on Size
EXAMPLE: Gold and silver nanoparticles Cut to 1000nm the color would be golden color. Cut to 700nm the color would be red.
Cut to 600nm the color would be orange.
The color changes because each color has a specific wavelength.
1.0 1.5 2.0 2.5
1600 1200 800
Wavelength (nm)
Ab
sorb
an
ce (
a.u
.)
Energy (eV)
Ph
oto
lum
ine
sce
nce
5.8 nm
5.0 nm
4.6 nm
4.0 nm
3.3 nm
2.9 nm
2.4 nm
6.4 nm
600
Quantum Confinement in InAs
Nanocrystals
Particle in a box model
E n
e r
g y
r
1Sh 1Ph
1Se
1Pe
1Sh
1Ph
1Se
1Pe
Luminescence from Indirect Gap Semiconductors
It is possible to observe luminescence from indirect gap semiconductors when their crystal size is very small. The origin of this emission is the modification of the electronic structure due to the size, although some theories support some other possible radiative paths in nanocrystals (defects, surface effects,...)
Quantum Wells
The optical properties of a semiconductor are altered by quantum size effects; at least one of the dimensions of material is on the order of De Broglie’s wavelength of an electron: = h/m; if m ~ eV = ~ a few nm;
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Superlattices based on
Bi2Te3, Si/Ge, GaAs/AlAs
Ec
Ev
x
E
Quantum well (QW) Barrier
Top View
Nanowire
Al2O3 template
Nanowires based on
Bi, BiSb, Bi2Te3, SiGe Eg(GaN) 3.4 eV
Eg (InGaN) 2.1-3.3 eV
n-GaN | InGaN | p-GaN
An inner shell of smaller bandgap material sandwiched between the nanowire core and outer shell of larger bandgap materials
5.5V 4V
5 m
6V
Lighting Up with Nanowires
Nanowire Double Heterostructures
Lieber*, Nano Lett., 2004, 4, 1975
Nature, June 10, 2004
Outlook for Nanocrystal LEDs Brightens
Victor Klimov and colleagues at Los Alamos National Laboratory assembled their cadmium selenide dots on top of a so-called quantum well, a thin sheet of semiconductor sandwiched between two barrier layers. A quick flash of laser light aimed at the well generates pairs of electrons and positively charged "holes" in the middle layer. Normally the pairs would recombine and emit a photon, but by making the top layer of the well thinner than 30 Angstroms, the researchers forced the recombined pairs to release their energy as a wiggling electric field. This field generated electron-hole pairs in the adjacent dots; these pairs recombine, producing photons.
Important Semiconductor Materials for Optoelectronics
Materials Type Substrate Devices Wavelength range(mm)
Si SiC Ge
GaAs
AlGaAs
GaInP GaAlInP
GaP GaAsP
InP InGaAs
InGaAsP InAlAs
InAlGaAs GaSb/GaAlSb
CdHgTe ZnSe ZnS
IV IV IV
III-V
III-V
III-V III-V III-V III-V III-V III-V III-V III-V III-V II-VI II-VI] II-VI II-VI
Si SiC Ge
GaAS
GaAS
GaAs GaAS GaP GaP InP InP InP InP InP
GaSb CdTe ZnSe ZnS
Detectors, Solar cells Blue LEDs Detectors
LEDs, Lasers, Detectors, Solar Cells, Imagers, Intensifiers
LEDs, Lasers, Solar Cells, Imagers Visible Lasers, LEDs Visible Lasers, LEDs
Visible LEDs Visible LEDs Solar Cells Detectors
Lasers, LEDS Lasers, Detectors Lasers, Detectors Lasers, Detectors
Long wavelength Detectors Short wavelength LEDs Short wavelength LEDs
0.5-1 0.4
1-1.8 0.85
0.67-0.98
0.5-0.7 0.5-0.7 0.5-0.7 0.5-0.7
0.9 1-1.67 1-1.6 1-2.5 1-2.5 2-3.5
3-5 and 8-12 0.4-0.6 0.4-0.6
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Commercial Applications of Optoelectronic Devices
Materials Devices Applications
Remote control TV, etc., video disk players, range-finding, solar energy conversion, optical fiber communication systems (local networks), image intensifiers Space solar cell Optical fiber communications (long-haul and local loop) Optical fiber communications, Military applications, medicine, sensor Displays, control, compact disk players, laser printers/scanners, optical disk memories, laser medicine equipment Solar energy conversions, e.g. watches, calculators, cooling, heating, detectors Detectors Displays, optical disk memories, etc. Infrared imaging, night vision sights, missileseekers, other military applications Commercial applications (R&D stages only)
Detectors, Infrared LEDs and Lasers Solar cell Infrared LEDs, Lasers (1-1.6mm) 1-1.67mm Detectors 1.67-2.4mm Detectors 0.5-0.7mm LEDs and Lasers Detectors and Solar Cells Detectors Blue LEDs Long wavelength detectors/smitters Visible LEDs
GaAs/AlGaAs
InP/InP InP/InGaP
InP/InGaAs
InGaAlAs/InGaAs GaAs/GaInP/
GaInAlP
Si
Ge SiC
GaSb/GaAlSb/InSb
ZnSe/ZnS