Imaging the Effects of Individual Impurity Atoms in High-T c Superconductors
Prepaid by G-Michael G. 2008 EC · To increase the number of holes in intrinsic silicon, trivalent...
Transcript of Prepaid by G-Michael G. 2008 EC · To increase the number of holes in intrinsic silicon, trivalent...
Prepaid by G-Michael G.
2008 EC
Semiconductor Theory
Electrical/Electronics 2
The atom
An atom is the smallest particle of an element that retains the characteristics of that
element. Each of the known 118 elements has atoms that are different from the atoms
of all other elements. This gives each element a unique atomic structure. According to
the classical Bohr model, atoms have a planetary type of structure that consists of a
central nucleus surrounded by orbiting electrons, as illustrated in Figure 1.
The nucleus consists of positively charged particles called protons and uncharged
particles called neutrons. The basic particles of negative charge are called electrons.
Fig-1 Bohr model
Insulators, Conductors, and Semiconductors
All materials are made up of atoms. These atoms contribute to the electrical properties
of a material, including its ability to conduct electrical current.
Insulators
An insulator is a material that does not conduct electrical current under normal
conditions. Most good insulators are compounds rather than single-element materials
and have very high resistivity. Valence electrons are tightly bound to the atoms;
therefore, there are very few free electrons in an insulator. Examples of insulators are
rubber, plastics, glass, mica, and quartz.
Conductors
A conductor is a material that easily conducts electrical current. Most metals are good
conductors. The best conductors are single-element materials, such as copper (Cu),
silver (Ag), gold (Au), and aluminum (Al), which are characterized by atoms with only one
valence electron very loosely bound to the atom. These loosely bound valence electrons
become free electrons. Therefore, in a conductive material the free electrons are valence
electrons.
Semiconductor Theory
Electrical/Electronics 3
Semiconductors
A semiconductor is a material that is between conductors and insulators in its ability
to conduct electrical current. A semiconductor in its pure (intrinsic) state is neither a
good conductor nor a good insulator. Single-element semiconductors are antimony (Sb),
arsenic (As), astatine (At), boron (B), polonium (Po), tellurium (Te), silicon (Si), and
germanium (Ge). Compound semiconductors such as gallium arsenide, indium
phosphide, gallium nitride, silicon carbide, and silicon germanium are also commonly
used. The single-element semiconductors are characterized by atoms with four valence
electrons. Silicon is the most commonly used semiconductor.
Energy diagrams for insulators, semiconductors, and conductors.
Figure-2 shows energy diagrams for insulators, semi-conductors, and conductors.
Comparison of a Semiconductor Atom to a Conductor Atom
Silicon is a semiconductor and copper is a conductor. Bohr diagrams of the silicon atom
and the copper atom are shown in Figure-3. Notice that the core of the silicon atom has
a net charge of +4 (14 protons - 10 electrons) and the core of the copper atom has a net
charge of + 1 (29 protons - 28 electrons). The core includes everything except the valence
electrons.
Figure-3
Comparison of a
Semiconductor Atom to a
Conductor Atom
Semiconductor Theory
Electrical/Electronics 4
Silicon and Germanium
The atomic structures of silicon and germanium are compared in Figure-4. Silicon is
used in diodes, transistors, integrated circuits, and other semiconductor devices. Notice
that both silicon and germanium have the characteristic four valence electrons.
Figure-4 Silicon and Germanium
N-TYPE AND P-TYPE SEMICONDUCTORS
Semi conductive materials do not conduct current well and are of limited value in their
intrinsic state. This is because of the limited number of free electrons in the conduction
band and holes in the valence band. Intrinsic silicon (or germanium) must be modified
by increasing the number of free electrons or holes to increase its conductivity and make
it useful in electronic devices. This is done by adding impurities to the intrinsic material.
Two types of extrinsic (impure) semi conductive materials, n-type and p-type, are the
key building blocks for most types of electronic devices.
Since semiconductors are generally poor conductors, their conductivity can be
drastically increased by the controlled addition of impurities to the intrinsic (pure) semi
conductive material. This process, called doping, increases the number of current
carriers (electrons or holes). The two categories of impurities are n-type and p-type.
N-Type Semiconductor
To increase the number of conduction-band electrons in intrinsic silicon, pentavalent
impurity atoms are added. These are atoms with five valence electrons such as
Arsenic (As),
Phosphorus (P),
Bismuth (Bi),
Antimony (Sb).
As illustrated in Figure-5, each pentavalent atom (antimony, in this case) forms covalent
bonds with four adjacent silicon atoms. Four of the antimony atom's valence electrons
are used to form the covalent bonds with silicon atoms, leaving one extra electron. This
extra electron becomes a conduction electron because it is not involved in bonding.
Because the pentavalent atom gives up an electron, it is often called a donor atom.
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Electrical/Electronics 5
The number of conduction electrons can be carefully controlled by the number of
impurity atoms added to the silicon. A conduction electron created by this doping
process does not leave a hole in the valence band because it is in excess of the number
required to fill the valence band.
.
Majority and Minority Carriers
Since most of the current carriers are electrons, silicon (or germanium) doped with
pentavalent atoms is an n-type semiconductor (then stands for the negative charge on
an electron). The electrons are called the majority carriers in n-type material. Although
the majority of current carrier’s in n-type material are electrons, there are also a few
holes that are created when electron-hole pairs are thermally generated. These holes are
not produced by the addition of the pentavalent impurity atoms. Holes in an n-type
material are called minority carriers.
P-Type Semiconductor
To increase the number of holes in intrinsic silicon, trivalent impurity atoms are added.
These are atoms with three valence electrons such as
Boron (B),
Indium (In),
Gallium (Ga).
As illustrated in Figure -6, each trivalent atom (boron, in this case) forms covalent bonds
with four adjacent silicon atoms. All three of the boron atom's valence electrons are used
in the covalent bonds; and, since four electrons are required, a hole results when each
trivalent atom is added. Because the trivalent atom can take an electron, it is often
referred to as an acceptor atom. The number of holes can be carefully controlled by the
number of trivalent impurity atoms added to the silicon. A hole created by this doping
process is not accompanied by a conduction (free) electron.
FIGURE -5 pentavalent impurity atom
in a silicon crystal structure. An
antimony (Sb) impurity atom is shown
in the center. The extra electron from
the Sb atom becomes a free electron
Semiconductor Theory
Electrical/Electronics 6
Majority and Minority Carriers
Since most of the current carriers are holes, silicon (or germanium) doped with trivalent
atoms is called a p-type semiconductor. The holes are the majority carriers in p-type
material. Although the majority of current carriers in p-type material are holes, there are
also a few conduction-band electrons that are created when electron-hole pairs are
thermally generated. These conduction-band electrons are not produced by the addition
of the trivalent impurity atoms. Conduction-band electrons in p-type material are the
minority carriers.
THE PN JUNCTION
When you take a block of silicon and dope part of it with a trivalent impurity and the
other part with a pentavalent impurity, a boundary called the PN-junction is formed
between the resulting p-type and n-type portions. The PN-junction is the basis for diodes,
certain transistors, solar cells, and other devices.
A p-type material consists of silicon atoms and trivalent impurity atoms such as boron.
The boron atom adds a hole when it bonds with the silicon atoms. However, since the
number of protons and the number of electrons are equal throughout the material, there
is no net charge in the material and so it is neutral.
An n-type silicon material consists of silicon atoms and pentavalent impurity atoms
such as antimony. As you have seen, an impurity atom releases an electron when it
bonds with four silicon atoms. Since there is still an equal number of protons and
electrons (including the free electrons) throughout the material, there is no net charge
in the material and so it is neutral.
If a piece of intrinsic silicon is doped so that part is n-type and the other part is p-type,
a PN-junction forms at the boundary between the two regions and a diode is created, as
indicated in Figure-7(a). The p region has many holes (majority carriers) from the
impurity atoms and only a few thermally generated free electrons (minority carriers). The
n region has many free electrons (majority carriers) from the impurity atoms and only a
few thermally generated holes (minority carriers).
FIGURE-6 trivalent impurity atom in a
silicon crystal structure. A boron (B)
impurity atom is shown in the center.
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FIGURE -7 Formation of the depletion region. The width of the depletion region is
exaggerated for illustration purposes.
Formation of the Depletion Region
The free electrons in the n region are randomly drifting in all directions. At the instant
of the PN junction formation, the free electrons near the junction in the n region begin
to diffuse across the junction into the p region where they combine with holes near the
junction, as shown in Figure -7(b).
The free electrons in the n region are randomly drifting in all directions. At the instant
of the PN junction formation, the free electrons near the junction in the n region begin
to diffuse across the junction into the p region where they combine with holes near the
junction, as shown in Figure -7(b).
Before the PN junction is formed, recall that there are as many electrons as protons in
then-type material, making the material neutral in terms of net charge. The same is true
for the p-type material.
When the PN junction is formed, the n region loses free electrons as they diffuse across
the junction. This creates a layer of positive charges (pentavalent ions) near the junction.
As the electrons move across the junction, the p region loses holes as the electrons and
holes combine. This creates a layer of negative charges (trivalent ions) near the junction.
These two layers of positive and negative charges form the depletion region, as shown in
Figure -7(b). The term depletion refers to the fact that the region near the PN junction is
depleted of charge carriers (electrons and holes) due to diffusion across the junction.
Keep in mind that the depletion region is formed very quickly and is very thin compared
to the n region and p region. After the initial surge of free electrons across the PN
junction, the depletion region has expanded to a point where equilibrium is established
and there is no further diffusion of electrons across the junction.
Semiconductor Theory
Electrical/Electronics 8
This occurs as follows.
As electrons continue to diffuse across the junction, more and more positive and negative
charges are created near the junction as the depletion region is formed. A point is
reached where the total negative charge in the depletion region repels any further
diffusion of electrons (negatively charged particles) into the p region (like charges repel)
and the diffusion stops. In other words, the depletion region acts as a barrier to the
further movement of electrons across the junction.
The Diode
As mentioned, a diode is made from a small piece of semiconductor material, usually
silicon, in which half is doped as a p region and half is doped as an n region with a PN
junction and depletion region in between. The p region is called the anode and is
connected to a conductive terminal. The N region is called the cathode and is connected
to a second conductive terminal. The basic diode structure and schematic symbol are
shown in Figure -8.
Fig -8 basic diode structure and schematic symbol
Now, the electrons are in the valence band in the p region, simply because they have lost
too much energy overcoming the barrier potential to remain in the conduction band.
Since unlike charges attract, the positive side of the bias-voltage source attracts the
valence electrons toward the left end of the p region. The holes in the p region provide
the medium or "pathway" for these valence electrons to move through the p region. The
valence electrons move from one hole to the next toward the left. The holes, which are
the majority carriers in the p region, effectively (not actually) move to the right toward
the junction, as you can see in Figure -9. This effective flow of holes is the hole current.
You can also view the Hole current as being created by the flow of valence electrons
through the p region, with the holes providing the only means for these electrons to flow.
FIGURE -9 A forward-biased diode showing the flow of majority carriers and the
voltage due to the barrier potential across the depletion region.
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Electrical/Electronics 9
VOLTAGE-CURRENT CHARACTERISTIC OF A DIODE
Forward bias produces current through a diode and reverse bias essentially prevents
current, except for a negligible reverse current. Reverse bias prevents current as long as
the reverse-bias voltage does not equal or exceed the breakdown voltage of the junction.
In this section, we will examine the relationship between the voltage and the current in
a diode on a graphical basis.
V-1 Characteristic for Forward Bias
When a forward-bias voltage is applied across a diode, there is current. This current is
called the forward current and is designated Ip.
V-1 Characteristic for Reverse Bias
When a reverse-bias voltage is applied across a diode, there is only an extremely small
reverse current (IR) through the pn junction. With 0 V across the diode, there is no
reverse current.
FIGURE -10 the complete V-1 characteristic curve for a diode.
Temperature Effects
For a forward-biased diode, as temperature is increased, the forward current increases
for a given value of forward voltage. Also, for a given value of forward current, the forward
voltage decreases. This is shown with the V-I characteristic curves in Figure -11. The
blue curve is at room temperature (25°C) and the red curve is at an elevated temperature
(25°C + ∆T). The barrier potential decreases by 2 mV for each degree increase in
temperature.
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Electrical/Electronics 10
FIGURE-11 Temperature effect on the diode V-1 characteristic. The 1 mA and 1 µA
marks on the vertical axis are given as a basis for a relative comparison of the current
scales.
Bias Connections
Forward-Bias
A diode is forward-biased when a voltage source is connected as shown in Figure -12(a).
The positive terminal of the source is connected to the anode through a current-limiting
resistor. The negative terminal of the source is connected to the cathode. The forward
current (Ip) is from anode to cathode as indicated. The forward voltage drop (Vp) due to
the barrier potential is from positive at the anode to negative at the cathode.
Reverse-Bias
A diode is reverse-biased when a voltage source is connected as shown in Figure -12(b).
The negative terminal of the source is connected to the anode side of the circuit, and the
positive terminal is connected to the cathode side. A resistor is not necessary in reverse
bias but it is shown for circuit consistency. The reverse current is extremely small and
can be considered to be zero. Notice that the entire bias voltage (VBIAs) appears across
the diode.
FIGURE -12 Forward-bias and reverse-bias connections showing the diode symbol.
Semiconductor Theory
Electrical/Electronics 11
RECTIFIER DIODE
Diodes have ability to conduct current in one direction and block current in the other
direction, diodes are used in circuits called rectifiers that convert AC voltage into DC
voltage. Rectifiers are found in all DC power supplies that operate from an AC voltage
source. A power supply is an essential part of each electronic system from the simplest
to the most complex.
Half-Wave Rectifier Operation
A diode is connected to an AC source and to a load resistor, RL, forming a half-wave
rectifier. When the sinusoidal input voltage (Vin) goes positive, the diode is forward-
biased and conducts current through the load resistor, as shown in part (a). The current
produces an output voltage across the load RL, which has the same shape as the positive
half-cycle of the input voltage.
(a) During the positive alternation of the 60Hz input voltage, the output voltage looks
like the positive half of the input voltage. The current path is through ground back to
the source.
(b) During the negative alternation of the input voltage, the current is 0, so the output
voltage is also 0.
(c) 60Hz half-wave output voltage for three input cycles
Semiconductor Theory
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FULL-WAVE RECTIFIERS
Although half-wave rectifiers have some applications, the full-wave rectifier is the most
commonly used type in DC power supplies. In this section, you will use what you learned
about half-wave rectification and expand it to full-wave rectifiers. You will learn about
two types of full-wave rectifiers: center-tapped and bridge.
FIGURE -13 Full-wave rectification.
Types of Full-Wave Rectifiers
1. Center-Tapped Full-Wave Rectifier Operation
2. Bridge Full-Wave Rectifier Operation
Center-Tapped Full-Wave Rectifier Operation
A center-tapped rectifier is a type of full-wave rectifier that uses two diodes connected to
the secondary of a center-tapped transformer, as shown in Figure -14. The input voltage
is coupled through the transformer to the center-tapped secondary. Half of the total
secondary voltage appears between the center tap and each end of the secondary winding
as shown.
FIGURE 1- 14 A center-tapped full-wave rectifier.
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Electrical/Electronics 13
FIGURE -15 Basic operation of a center-tapped full-wave rectifier. Note that the current
through the load resistor is in the same direction during the entire input cycle, so the
output voltage always has the same polarity.
Bridge Full-Wave Rectifier Operation
The bridge rectifier uses four diodes connected as shown in Figure -16. When the input
cycle is positive as in part (a), diodes D1 and D2 are forward-biased and conduct current
in the direction shown. A voltage is developed across RL that looks like the positive half
of the input cycle. During this time, diodes D3 and D4 are reverse-biased.
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Electrical/Electronics 14
FIGURE -16 Operation of a bridge rectifier.
TYPES OF DIODES
It is sometimes useful to summarize the different types of diode that are available. Some
of the categories may overlap, but the various definitions may help to narrow the field
down and provide an overview of the different diode types that are available.
Zener diode: The Zener diode is a very useful type of diode as it provides a stable
reference voltage. As a result it is used in vast quantities. It is run under reverse
bias conditions and it is found that when a certain voltage is reached it breaks
down. If the current is limited through a resistor, it enables a stable voltage to be
produced. This type of diode is therefore widely used to provide a reference voltage
in power supplies. Two types of reverse breakdown are apparent in these diodes:
Zener breakdown and Impact Ionization. However the name Zener diode is used
for the reference diodes regardless of the form of breakdown that is employed.
FIGURE -17 zener diode characteristics and symbol
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Electrical/Electronics 15
Backward diode: This type of diode is sometimes also called the back diode.
Although not widely used, it is a form of PN junction diode that is very similar to
the tunnel diode in its operation. It finds a few specialist applications where its
particular properties can be used.
FIGURE – 18 backward diode symbol
BARITT diode: This form of diode gains its name from the words Barrier Injection
Transit Time diode. It is used in microwave applications and bears many
similarities to the more widely used IMPATT diode.
Gunn Diode: Although not a diode in the form of a PN junction, this type of diode
is a semiconductor device that has two terminals. It is generally used for
generating microwave signals.
FIGURE – 19 Gunn diode symbol
Laser diode: This type of diode is not the same as the ordinary light emitting
diode because it produces coherent light. Laser diodes are widely used in many
applications from DVD and CD drives to laser light pointers for presentations.
Although laser diodes are much cheaper than other forms of laser generator, they
are considerably more expensive than LEDs. They also have a limited life.
Light emitting diodes: The light emitting diode or LED is one of the most
popular types of diode. When forward biased with current flowing through the
junction, light is produced. The diodes use component semiconductors, and can
produce a variety of colors, although the original color was red. There are also very
many new LED developments that are changing the way displays can be used and
manufactured. High output LEDs and OLEDs are two examples.
FIGURE – 21 LED symbol
FIGURE – 20 laser diode symbol
Semiconductor Theory
Electrical/Electronics 16
Photodiode: The photo-diode is used for detecting light. It is found that when
light strikes a PN junction it can create electrons and holes. Typically photo-diodes
are operated under reverse bias conditions where even small amounts of current
flow resulting from the light can be easily detected. Photo-diodes can also be used
to generate electricity. For some applications, PIN diodes work very well as
photodetectors.
FIGURE – 22 photo diode symbol
PIN diode: This type of diode is typified by its construction. It has the standard
P type and N-type areas, but between them there is an area of intrinsic
semiconductor which has no doping. The area of the intrinsic semiconductor has
the effect of increasing the area of the depletion region which can be useful for
switching applications as well as for use in photodiodes, etc.
FIGURE – 23 basic pin diode structure
Schottky diodes: This type of diode has a lower forward voltage drop than
ordinary silicon PN junction diodes. At low currents the drop may be somewhere
between 0.15 and 0.4 volts as opposed to 0.6 volts for a silicon diode. To achieve
this performance they are constructed in a different way to normal diodes having
a metal to semiconductor contact. They are widely used as clamping diodes, in RF
applications, and also for rectifier applications.
FIGURE – 24 schottky diode symbol
Step recovery diode: A form of microwave diode used for generating and shaping
pulses at very high frequencies. These diodes rely on a very fast turn off
characteristic of the diode for their operation.
Tunnel diode: Although not widely used today, the tunnel diode was used for
microwave applications where its performance exceeded that of other devices of
the day.
FIGURE – 25 Tunnel diode symbol
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Electrical/Electronics 17
Varactor diode or varicap diode: This type of diode is used in many radio
frequency (RF) applications. The diode has a reverse bias placed upon it and this
varies the width of the depletion layer according to the voltage placed across the
diode. In this configuration the varactor or varicap diode acts like a capacitor with
the depletion region being the insulating dielectric and the capacitor plates formed
by the extent of the conduction regions. The capacitance can be varied by changing
the bias on the diode as this will vary the width of the depletion region which will
accordingly change the capacitance.
FIGURE – 26 Varactor diode or varicap diode symbol
TRANSISTOR FUNDAMENTALS
A TRANSISTOR is a three or more element solid-state device that amplifies and
switching by controlling the flow of current carriers through its semiconductor
materials.
Semiconductor devices that have-three or more elements are called TRANSISTORS. The
term transistor was derived from the words ‘’TRANS fer’’ and ‘’res ISTOR’’. This term
was adopted because it best describes the operation of the transistor - the transfer of an
input signal current from a low-resistance circuit to a high-resistance circuit. Basically,
the transistor is a solid-state device that amplifies by controlling the flow of current
carriers through its semiconductor materials.
There are many different types of transistors, but their basic theory of operation is all
the same. As a matter of fact, the theory we will be using to explain the operation of a
transistor is the same theory used earlier with the PN-junction diode except that now
two such junctions are required to form the three elements of a transistor.
The three elements of the two-junction transistor are
1. the EMITTER, which gives off, or emits," current carriers (electrons or holes);
2. the BASE, which controls the flow of current carriers; and
3. The COLLECTOR, which collects the current carriers.
CLASSIFICATION
Transistors are classified as either NPN or PNP according to the arrangement of their N
and P materials. Their basic construction and chemical treatment is implied by their
names, "NPN" or "PNP." That is, an NPN transistor is formed by introducing a thin region
of P-type material between two regions of N-type material. On the other hand, a PNP
transistor is formed by introducing a thin region of N-type material between two regions
of P-type material. Transistors constructed in this manner have two PN junctions, as
shown in figure 27.
One PN junction is between the emitter and the base; the other PN junction is between
the collector and the base. The two junctions share one section of semiconductor
material so that the transistor actually consists of three elements.
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Electrical/Electronics 18
Figure - 27. NPN and PNP transistor configuration
NPN Transistor Operation
Just as in the case of the PN junction diode, the N material comprising the two end
sections of the NP N transistor contains a number of free electrons, while the center P
section contains an excess number of holes. The action at each junction between these
sections is the same as that previously described for the diode; that is, depletion regions
develop and the junction barrier appears. To use the transistor as an amplifier, each of
these junctions must be modified by some external bias voltage. For the transistor to
function in this capacity, the first PN junction (emitter-base junction) is biased in the
forward, or low-resistance, direction. At the same time the second PN junction (base-
collector junction) is biased in the reverse, or high-resistance, direction. The letters of
these elements indicate what polarity voltage to use for correct bias. For instance, notice
the NPN transistor below:
Figure – 28 NPN Transistor Operation
PNP Transistor Operation
The PNP transistor works essentially the same as the NPN transistor. However, since
the emitter, base, and collector in the PNP transistor are made of materials that are
different from those used in the NPN transistor, different current carriers flow in the
PNP unit. The majority current carriers in the PNP transistor are holes. This is in
contrast to the NPN transistor where the majority current carriers are electrons. To
support this different type of current (hole flow), the bias batteries are reversed for the
PNP transistor. A typical bias setup for the PNP transistor is shown in figure 29.
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Electrical/Electronics 19
Figure – 29 PNP transistor operation
Types of transistor
There are many different types of transistors and they each vary in their characteristics
and each have their own advantages and disadvantages.
Some types of transistors are used primarily for switching applications. Others can be used for both switching and amplification. Still other transistors are in a specialty group
all of their own, such as phototransistors, which respond to the amount of light shining on it to produce current flow through it.
Bipolar Junction Transistors
Bipolar Junction Transistors are transistors which are made up of 3 regions, the base, the collector, and the emitter. Bipolar Junction transistors, unlike FET transistors, are
current-controlled devices. A small current entering in the base region of the transistor causes a much larger current flow from the emitter to the collector region.
Bipolar junction transistors come in two main types, NPN and PNP. A NPN transistor is
one in which the majority current carrier are electrons. Electron flowing from the emitter to the collector forms the base of the majority of current flow through the transistor. The other type of charge, holes, are a minority. PNP transistors are the opposite. In PNP
transistors, the majority current carrier are holes.
Overall, bipolar junction transistors are the only type of transistor which is turned on by
current input (input into the base). This is because BJTs have the lowest input impedance of all transistors. The low impedance (or resistance) allows current to flow through the base of the transistor. Because of this low impedance also do BJTs have the
highest amplification of all transistors? The downside of BJTs is because they have low input impedance, they can cause loading in a circuit. Loading is when a device can draw
significant current from a circuit, thus disturbing a circuit's power source.
Field Effect Transistors
Field Effect Transistors are transistors which are made up of 3 regions, a gate, a source,
and a drain. Unlike bipolar transistors, FETs are voltage-controlled devices. A voltage placed at the gate controls current flow from the source to the drain of the transistor.
Field Effect transistors have very high input impedance, from several mega ohms (MΩ)
of resistance to much, much larger values. This high input impedance causes them to have very little current run through them. (According to ohm's law, current is inversely
affected by the value of the impedance of the circuit. If the impedance is high, the current is very low.) So FETs both draw very little current from a circuit's power source. Thus,
Semiconductor Theory
Electrical/Electronics 20
this is ideal because they don't disturb the original circuit's power elements to which
they are connected to. They won't cause the power source to be loaded down. The drawback of FETs is that they won't provide the same amplification that could be gotten from bipolar transistors. Bipolar transistors are superior in the fact that they provide
greater amplification, even though FETs are better in that they cause less loading, are cheaper, and easier to manufacture.
Field Effect Transistors come in 2 main types: JFETs and MOSFETs. JFETs and
MOSFETs are very similar but MOSFETs have even higher input impedance values than JFETs. This causes even less loading in a circuit.
Figure – 30 Field Effect Transistors symbol
Types of Transistors by Function
Now we will go over the types of transistors by function, meaning what they do or, rather, are designed to do. Some transistors are used primarily for switching. Others more so
for amplification.
Small Signal Transistors
Small Signal Transistors are transistors that are used primarily to amplify low-level signals but can also function well as switches.
Transistors come with a value, called the hFE values, which denotes how greatly a
transistor can amplify input signals. Typical hFE values for small signal transistors range from 10 to 500, with maximum Ic (collector current) ratings from about 80 to
600mA. They come in NPN and PNP forms. Maximum operating frequencies range from about 1 to 300 MHz.
As a design note, small signal transistors are used primarily when amplifying small
signals, such as a few volts and only when using milliamperes of current. When using larger voltage and current (larger power), using many volts or amperes of current, a power transistor should be used.
Small Switching Transistors Small Switching Transistors are transistors that are used primarily as switches but
which can also be used as amplifiers. Typical hFE values for small switching transistors range from 10 to 200, with maximum Ic ratings from about 10 to 1000mA. They come in NPN and PNP forms.
In terms of for design, small switching transistors are used primarily as switches. Though they may be used as an amplifier, their hFE value only ranges to about 200,
which means they are not capable of the amplification of small signal transistors, which can have amplification of up to 500. This makes small switching transistors more useful for switching, though they may be used as basic amplifiers to provide gain. When you
need more gain, small signal transistors would work better as amplifiers.
Power Transistors
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Electrical/Electronics 21
Power transistors are suited for applications where a lot of power is being used- current
and voltage.
The collector of the transistor is connected to a metal base that acts as a heat sink to dissipate excess power.
Typical power ratings range from around 10 to 300 W, with frequency ratings from about 1 to 100 MHz. Maximum Ic values range between 1 to 100 A. Power transistors come in NPN, PNP, and Darlington (NPN or PNP) forms.
High Frequency Transistors High Frequency (RF) Transistors are transistors that are used for small signals that run
at high frequencies for high-speed switching applications.
These are transistors that are used for high frequency signals and must be able to switch on and off at very high speeds. High frequency transistors are used in HF, VHF, UHF,
CATV, and MATV amplifier and oscillator applications. They have a maximum frequency rating of about 2000 MHz and maximum Ic currents from 10 to 600mA. They are
available in both NPN and PNP forms.
Phototransistors
Phototransistors are light-sensitive transistors.
A common type of phototransistor resembles a bipolar transistor with its base lead removed and replaced with a light-sensitive area. This is why a phototransistor has only
2 terminals instead of the usual 3. When this surface area is kept dark, the device is off. Practically, no current flows from the collector to emitter region. However, when the light-
sensitive region is exposed to light, a small base current is generated that controls a much larger collector-to-emitter current.
Just like regular transistors, phototransistors can be both bipolar and field effect
transistors. Field-effect phototransistors (photo-FETs) are light-sensitive field-effect transistors. Unlike photo-bipolar transistors, photo-FETs use light to generate a gate
voltage that is used to control a drain-source current. Photo-FETs are extremely sensitive to variations in light and are more fragile, electrically speaking, than bipolar phototransistors.
Figure – 31 Phototransistors symbol
Unijunction Transistors Unijunction transistors are three-lead transistors that act exclusively as electrically controlled switches; they are not used as amplifiers.
This differs from other transistors in that general transistors usually provide the ability to act as a switch and also as an amplifier. But a unijunction transistor does not provide
any decent type of amplification because of the way it is constructed. It's simply not designed to provide a sufficient voltage or current boost.
The three leads of a unijunction transistor are B1, B2, and an emitter lead, which is the
lead which receives the input current. The basic operation of a UJT is relatively simple. When no potential difference (voltage) exists between its emitter and either of its base
leads (B1 or B2), only a very small current flows from B2 to B1. However, if a sufficiently
Semiconductor Theory
Electrical/Electronics 22
large positive trigger voltage- relative to its base leads- is applied to the emitter, a larger
current flows from the emitter and combines with the small B2-to-B1 current, thus giving rise to large B1 output current. Unlike other transistors- where the control leads provide little additional current- the UJT is just the opposite. Its emitter current is the
primary source of current for the transistor. The B2 to B1 current is only a very small amount of the total combined current. This means that unijunction transistors are not suitable for amplification purposes, but only for switching.
Figure – 32 Unijunction Transistors symbol
How to Test a Transistor
In this article, we will go over different tests that we can use to determine whether a
transistor is good or not, all by utilizing the ohmmeter of a digital multi-meter.
A transistor, internally, is basically a component made up of different diode junctions.
NPN and PNP Transistors are made up two PN junctions sandwiched together. These PN
junctions are diodes.
Knowing that transistors are essentially diodes sandwiched together (placed back-to-
back), we can exploit this principle and test the leads of transistors as if they are separate
diodes. If you want to learn more about diode testing, check out how to test a diode.
To test a transistor, we measure one diode junction with the multi-meter leads situated
one way and then we flip the leads of the multi-meter to the reverse position, to switch
polarity. One side of the diode junction should read a very high resistance, above 1MΩ
of resistance (the anode-to-cathode side) and the other side should read a much lower
resistance, maybe of a few hundred thousand ohms (the cathode-to-anode side). This we
do for each junction.
So in total we'll have six readings, which are shown below:
1. Emitter to base
2. Base to emitter
3. Emitter to collector
4. Collector to emitter
5. Collector to base
6. Base to collector
Each pair should have one side with very high resistance (>1MΩ), and the other side
with a much lower resistance of a few hundred thousand ohms.
If this is the case for all the transistor leads, the transistor is good. If not, the transistor
is defective.