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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.



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



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.



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



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.



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.



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.



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.



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.



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



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.



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.



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



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



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



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.



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.



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,



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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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

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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.

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