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14. Semiconductor Devices By Ritesh Hariram 14.1. Introduction to Semiconductor Devices Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide, as well as organic semiconductors. Semiconductors are thus materials whose conductivity lies between a good conductor and a good insulator. 14.1.1. Component Data A datasheet, data sheet, or spec sheet is a document that summarizes the performance and other technical characteristics of a product, machine, component (e.g., an electronic component), material, a subsystem (e.g., a power supply) or software in sufficient detail to be used by a design engineer to integrate the component into a system. 14.1.2. Component Data Sources The following are examples of component data sources The internet has a wide variety of sources. Manufacturers make their data sheets public knowledge so getting a hold of this may be in your best interest. Most libraries have a Technical section which should have sources in this regard. Your local electronics hobby store. These stores have an abundance of data sheets. 14.1.3. How to read a data sheet? Consider the diagram below. The diagram shows a typical datasheet for a diode. As mentioned earlier datasheets can be sourced from a variety websites, textbooks and electronic stores. This particular datasheet clearly displays the company logo as well as a picture depicting what the physical component looks like. The datasheet has been modified to show you exactly where important information can be found. (Fairchild Semiconductor)

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Page 1: 14.1. Introduction to Semiconductor Devices...14. Semiconductor Devices By Ritesh Hariram 14.1. Introduction to Semiconductor Devices Semiconductor devices are electronic components

14. Semiconductor Devices

By Ritesh Hariram

14.1. Introduction to Semiconductor Devices

Semiconductor devices are electronic components that exploit the electronic

properties of semiconductor materials, principally silicon, germanium, and gallium

arsenide, as well as organic semiconductors. Semiconductors are thus materials

whose conductivity lies between a good conductor and a good insulator.

14.1.1. Component Data

A datasheet, data sheet, or spec sheet is a document that summarizes the

performance and other technical characteristics of a product, machine, component

(e.g., an electronic component), material, a subsystem (e.g., a power supply) or

software in sufficient detail to be used by a design engineer to integrate the

component into a system.

14.1.2. Component Data Sources

The following are examples of component data sources

The internet has a wide variety of sources.

Manufacturers make their data sheets public knowledge so getting a hold of

this may be in your best interest.

Most libraries have a Technical section which should have sources in this

regard.

Your local electronics hobby store. These stores have an abundance of data

sheets.

14.1.3. How to read a data sheet?

Consider the diagram below. The diagram shows a typical datasheet for a diode. As

mentioned earlier datasheets can be sourced from a variety websites, textbooks and

electronic stores. This particular datasheet clearly displays the company logo as well

as a picture depicting what the physical component looks like. The datasheet has

been modified to show you exactly where important information can be found.

(Fairchild Semiconductor)

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14.1.4. Pin Configuration

In electronics, a pin out (sometimes written "pin-out") is a cross-reference between

the contacts, or pins, of an electrical connector or electronic component, and their

functions

14.1.5. Typical Operating Values

This is a value on the datasheet that describes the typical operating parameters of a

component. It is wise that the user does not exceed these values as it would destroy

the component.

14.1.6. Working Temperature

Electronic components are designed to work within certain temperature parameters.

Working out of these parameters may either damage the component or not produce

the desired result. It is important to consult the manufacturerโ€™s datasheet in this

regard.

14.1.7. Equivalent Components

These are components that perform the same function as the component you are

researching. Often these can be found on certain manufacturerโ€™s datasheets.

14.1.8. Dual in Line Packages

In microelectronics, a dual in-line package (DIP or DIL) is an electronic device

package with a rectangular housing and two parallel rows of electrical connecting

pins. The package may be through-hole mounted to a printed circuit board or

inserted in a socket. Dual-in-line packages were developed in the 1960s when the

restricted number of leads available on transistor-style packages became a limitation

in the use of integrated circuits.

Figure 1: Dual in Line Package

14.1.9. Through Hole Components vs. Surface Mount (SMT)

There are some points that make SMT different from Through Hole, which has been

briefly summarized below:

SMT has helped significantly in solving the space problems that were

commonly noticed with the Through Hole mounting.

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The pin count has increased greatly in SMT when compared to its older

counterparts.

In SMT, the components are leadless and are directly mounted to the board

surface. In Through Hole the components have lead wires that are taken to

the wiring boards via holes.

The Pads on the surface in SMT are not used for connection of layers on the

printed wiring boards.

The components in the Though Hole Technology are larger which leads to

lower component density per unit area. The packing density that can be

achieved with SMT is very high as this allows mounting component on both

sides, when needed.

SMT has made possible the applications that seemed impossible with

through-hole components.

SMT is suitable for high volume production that gives access to lowered cost

of unit assemblies which is not possible with through-hole technology.

With SMT acquiring higher circuit speed is easier due to the reduced size.

SMT fulfils one of the prime marketing requirements while assisting in making

high performance circuits in a very small size.

SMT has a disadvantage as the capital investment involved in its machinery

and production is higher.

Designing, production, skill and technology required in implementing SMT is

very advanced when compared to through-hole technology.

Figure 2 through-hole component

Figure 3: Surface mounted components

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14.2. Semiconductors

14.2.1. Electron current flow vs. Conventional Current Flow

Figure 4: Conventional Current Flow

Figure 5: Electron Current Flow

The direction of current movement is referred to as electron flow. Before the electron

theory, many believed that current actually flowed from positive to negative. This is

called conventional current flow. Figure 4 and Figure 5, illustrate this concept.

(Circuits)

14.2.2. Semiconductors and Solid State

Solid-state electronics are those circuits or devices built entirely from solid materials

and in which the electrons, or other charge carriers, are confined entirely within the

solid material.

Semiconductor devices are electronic components that exploit the electronic

properties of semiconductor materials, principally silicon, germanium, and gallium

arsenide, as well as organic semiconductors. Semiconductors are thus materials

whose conductivity lies between a good conductor and a good insulator.

Solid state is a term that refers to electronic circuitry that is built entirely out of

semiconductors e.g. Solid state drives replacing magnetic drives.

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14.2.3. Silicon vs. Germanium

As explained earlier semiconductors are materials whose conductivity lies between a

good conductor and a good insulator. The diagram below, Figure 6, shows their

crystal lattice. It can be seen that their atoms are orderly and have 4 valence

electrons. They are thus referred to as tetravalent electrons.

NB: Other examples of tetravalent electrons are cadmium sulphide and gallium

arsenide.

(Hyperphysics)

Figure 6: Lattice structure of Silicon and Germanium

14.2.4. Doping

For semiconductors to be used in electronic components such as diodes, transistors,

thyristors and integrated circuits, their conductivity has to be increased considerably.

This can be achieved by adding controlled amounts of impurities to the

semiconductor materials during the manufacturing process. This process of adding

impurity materials to the semiconductors is called doping. The impurity material

atoms must be of similar size as the silicon atoms to fit into the crystal structure

without causing too much distortion. Because the impurity material now provides the

negative charge carriers (free electrons) or the positive charge carriers (holes), the

silicon now becomes an extrinsic semiconductor.

When silicon is doped with phosphorous an n-type extrinsic semiconductor is

formed. Phosphorous has five electrons in its outer valence shell and is therefore

called a pentavalent material. When it is doped into the silicon crystalline structure,

four of its valence electrons can form covalent bonds. The fifth valence electron now

becomes a free electron that is only loosely held by the atom. The impurity atom

(phosphorous) is therefore called the donor atom. The doped impure silicon is now

an n-type semiconductor because the majority charge carriers are negative

electrons. The few positive intrinsic holes due to broken covalent bonds are still

present and are called minority carriers.

When silicon is doped with boron a p-type extrinsic semiconductor is formed. Boron

has three electrons in its outer valence shell and is therefore called a trivalent

material. When it is doped into the silicon crystalline structure, three of its valence

electrons can form covalent bonds. The fourth bond is incomplete because of a

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missing electron. The hole that exists because of the missing electron acts like a

positive charge that can attract an electron from any nearby silicon atom. The

impurity atom (boron) is therefore called the acceptor atom. The doped impure

silicon is now a p-type semiconductor because the majority charge carriers are

positive holes. The few negative intrinsic electrons due to broken covalent bonds are

still present and are called minority carriers. (swart, pp. 184-188)

14.2.5. P and N Type Doping

The figures below provide a graphical display of the crystalline structure of both P

and N semiconductors.

14.2.5.1. Current Flow in a P Type Semiconductor

Figure 7: Current flow in a P-Type Semiconductor

Figure 8: P Type Semiconductor

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14.2.5.2. Current Flow in an N Type Semiconductor

Figure 9: Current flow in an N-Type Semiconductor

Figure 10: N Type Semiconductor

14.2.6. Majority and Minority Carriers

The more abundant charge carriers are called majority carriers. In n-type

semiconductors they are electrons, while in p-type semiconductors they are holes.

The less abundant charge carriers are called minority carriers; in n-type

semiconductors they are holes, while in p-type semiconductors they are electrons.

The PN Junction Diode

14.2.7. Construction of the PN Junction Diode

The formation of a PN Junction diode is simply the bringing together of a p-type

material and an n-type material. The p-type material forms the anode and the n-type

material forms the cathode.

On formation the n-region has an excess of free electrons, with the donor atoms

fixed in position. The p-type material, on the other hand, has an excess of positive

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holes, with the acceptor atoms fixed in position. Because of thermal agitation the n-

region will also contain a few positive holes and the p-region a few free electrons.

When the junction is formed, the free electrons in the n-region tend to diffuse into the

p-region. Similarly, holes tend to diffuse into the n-region. The result of this diffusion

is that the n-region assumes a net positive charge and the p-region a net negative

charge.

Because of this charge distribution a potential difference exists across the junction.

This internal potential barrier tends to oppose the further flow of electrons and holes

and may be represented by a virtual battery across the junction. Electrons diffusing

across the junction recombine with existing free electrons, with the result that the

region in the vicinity of the junction becomes devoid of charge carriers. This is known

as the depletion region.

Figure 11, below shows the circuit symbol and block diagram of the PN Junction

Diode. (swart, pp. 186-193)

Figure 11: Circuit symbol and block diagram of PN Junction diode

14.2.8. The Depletion Region

In semiconductor physics, the depletion region, also called depletion layer, depletion

zone, junction region, space charge region or space charge layer, is an insulating

region within a conductive, doped semiconductor material where the mobile charge

carriers have diffused away, or have been forced away by an electric field. The only

elements left in the depletion region are ionized donor or acceptor impurities.

(Wikipedia)

14.2.9. Forward and Reverse Biasing

14.2.9.1. Forward Biasing

When an external battery is connected with its positive terminal to the p-type material

and its negative terminal to the n-type material, the junction is said to be forward

biased. The threshold of voltage across the diode is about 0.6 V for silicon and about

0.2 V for germanium.

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The external battery opposes and overcomes the effect of the virtual battery and

carriers are easily able to cross the junction. The depletion region disappears and

there is a steady flow of positive holes and free electrons across the junction.

Together, these constitute a relatively large flow of current from the external battery.

This is termed the forward current.

Figure 12, below illustrates the Forward Biased Circuit

Figure 12: Forward Bias Circuit

14.2.9.2. Reverse Biasing

When the polarity of the battery is reversed, with the positive terminal connected to

the n-type material and the negative terminal to the p-type material, the p-n junction

is reversed biased. The battery now helps the action of the junction voltage, thereby

widening the depletion layer. The negative terminal supplies more electrons to the p-

type material and the positive terminal supplies more holes to the n-type material.

The junction voltage increases, repelling electrons and holes further into the material

at the junction. The resistance of the junction increases, blocking off the current.

Only a small leakage current flows, due to minority carriers crossing the junction.

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Figure 13: Reverse Bias Circuit (Swart)

14.2.10. Characteristic Curve and Symbol

Figure 14: Characteristic curve of a diode

When connected in reverse bias (negative to the anode and positive to the cathode),

the depletion region will increase. Only a small leakage current will flow. If the

reverse voltage increases, the depletion region will increase as well. If this voltage

increases continuously, a point will be met when the depletion region takes over the

entire pn-junction and the diode will break down.

When forward biased (positive on the anode and negative on the cathode), then the

depletion region will decrease. If the forward bias voltage is reached (0.6 V for silicon

and 0.3 V for germanium), then conduction will take place. The amount of current

that flows will be directly proportional to the amount of applied voltage (ohmโ€™s law).

14.2.11. The Diode Load Line

A load line is used in graphical analysis of nonlinear electronic circuits; representing

the constraint other parts of the circuit place on a non-linear device, like a diode or

transistor. It is usually drawn on a graph of the current vs the voltage in the nonlinear

device, called the device's characteristic curve. A load line, usually a straight line,

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represents the response of the linear part of the circuit, connected to the nonlinear

device in question. The operating point(s) of the circuit are the points where the

characteristic curve and the load line intersect; at these points the current and

voltage parameters of both parts of the circuit match. (Ref Wikipedia)

From the characteristic curve of the diode we see that this curve is not linear. It is

therefore not easy to calculate at which point the diode is operating at, because we

donโ€™t have a formula for the curve. However, we can determine the operating point

graphically. If the diode is connected to a DC energy source in series with a load

resistor we can find a point on the curve at which the diode is operating. To find this

out we need to plot the characteristic curve of the resistor on that same characteristic

curve of the diode. The intersection of these two graphs thus results in the Q-Point

which is the operating point of the diode.

At the Q-Point ๐ˆ๐ = ๐ˆ๐ซ

๐•๐’=๐•๐ + ๐•๐ซ

= ๐•๐ + ๐ˆ๐ซ ร— ๐‘๐ฅ๐จ๐š๐

Where:

๐•๐’ = ๐’๐ฎ๐ฉ๐ฉ๐ฅ๐ฒ ๐•๐จ๐ฅ๐ญ๐š๐ ๐ž

๐•๐ = ๐ƒ๐ข๐จ๐๐ž ๐•๐จ๐ฅ๐ญ๐š๐ ๐ž

๐•๐ซ = ๐‘๐ž๐ฌ๐ข๐ฌ๐ญ๐จ๐ซ ๐•๐จ๐ฅ๐ญ๐š๐ ๐ž

๐ˆ๐ = ๐ƒ๐ข๐จ๐๐ž ๐‚๐ฎ๐ซ๐ซ๐ž๐ง๐ญ

๐ˆ๐ซ = ๐‘๐ž๐ฌ๐ข๐ฌ๐ญ๐จ๐ซ ๐‚๐ฎ๐ซ๐ซ๐ž๐ง๐ญ

(swart)

Figure 15: Diode DC Load line superimposed onto Diode Characteristic Curve

Example 1

A silicon PN junction diode is connected to a 10V supply in series with a 1.25kฮฉ load

resistor. Use a dc load line to determine the Q-point at which the diode is operating.

Solution

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Resistor current ๐ผ๐ฟ =๐‘‰๐‘ 

๐‘…๐‘™

= 10๐‘‰

1.25๐พฮฉ

=80๐‘š๐ด

From the Q-Point of the graph below we can see that the diode is operating at 0.8V

and 70mA. At this stage the diode is dissipating 56 milli watts, which is the product of

the voltage across it and the current through it. It is important to make sure that the

diode does not have to dissipate more heat than what it is designed for (see diode

datasheet). The voltage across the load resistor at this point will be 9.8V, which is

obtained from the manufacturerโ€™s datasheet as well.

(swart)

Figure 16

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14.3. Practical: The diode load line

A silicon PN junction diode is connected to a 5V supply in series with a 1.2 kฮฉ load

resistor. Use a DC load line to determine the Q-Point at which the diode is operating.

Aim: To construct a simple diode circuit and determine its load line.

Requirements:

1.25Kฮฉ or equivalent

1N4007 diode or equivalent

Power supply 10V โ€“ digital analogue trainer

What you are required to do?

1. Connect the circuit as shown below.

2. Measure the current across the load resistor RL

3. Research the applicable diode and find out its exact forward conduction

voltage.

4. Draw a graph clearly indicating the Q-point and dc load line

5. Show all calculations.

6. Record your observations.

Circuit Diagram

(swart)

Measurements

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Redraw Circuit diagram with measurements

Graph

Calculations

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Observations

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14.4. The Zener Diode

14.4.1. Construction

A Zener diode is a silicon junction diode that is designed for voltage stabilization,

since it has the ability to maintain a constant voltage across its terminals under

reverse bias conditions. When it is forward biased, it behaves like an ordinary

junction diode. It is constructed in the same way as a PN Junction diode, but the

doping levels are much higher, in order to lower the reverse breakdown levels to a

few volts. Zener diodes with reverse breakdown voltages of between 3 V and 30 V

are available.

14.4.2. Principle of operation

A conventional solid-state diode allows significant current if it is reverse-biased

above its reverse breakdown voltage. When the reverse bias breakdown voltage is

exceeded, a conventional diode is subject to high current due to avalanche

breakdown. Unless this current is limited by circuitry, the diode may be permanently

damaged or destroyed due to overheating. A Zener diode exhibits almost the same

properties, except the device is specially designed so as to have a reduced

breakdown voltage, the so-called Zener voltage. By contrast with the conventional

device, a reverse-biased Zener diode exhibits a controlled breakdown and allows the

current to keep the voltage across the Zener diode close to the Zener breakdown

voltage. For example, a diode with a Zener breakdown voltage of 3.2 V exhibits a

voltage drop of very nearly 3.2 V across a wide range of reverse currents. The Zener

diode is therefore ideal for applications such as the generation of a reference voltage

(e.g. for an amplifier stage), or as a voltage stabilizer for low-current applications.

14.4.3. Forward Biasing

When forward-biased, zener diodes behave much the same as standard rectifying

diodes: they have a forward voltage drop which follows the โ€œdiode equationโ€ and is

about 0.7 volts for silicon and 0.3 V for germanium.

14.4.4. Reverse Biasing

In reverse-bias mode, they do not conduct until the applied voltage reaches or

exceeds the so-called zener voltage, at which point the diode is able to conduct

substantial current, and in doing so will try to limit the voltage dropped across it to

that zener voltage point. So long as the power dissipated by this reverse current

does not exceed the diode's thermal limits, the diode will not be harmed.

14.4.5. Avalanche Breakthrough vs Controlled Breakthrough

The breakdown voltage of an insulator is the minimum voltage that causes a portion

of an insulator to become electrically conductive. The breakdown voltage of a diode

is the minimum reverse voltage that will make the diode conduct in reverse.

Avalanche breakdown is a phenomenon that can occur in both insulating and

semiconducting materials. It is a form of electric current multiplication that allows

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very large currents within materials which are otherwise good insulators. It is a type

of electron avalanche. The avalanche process occurs when the carriers in the

transition region are accelerated by the electric field and energies sufficiently to free

electron-hole pairs via collisions with bound electrons.

The Zener effect is a type of electrical breakdown in a reverse biased p-n diode in

which the electric field enables tunnelling of electrons from the valence to the

conduction band of a semiconductor, leading to a large number of free minority

carriers, which suddenly increase the reverse current. Zener breakdown is employed

in a Zener diode.

Either the Zener or the avalanche effect may occur independently, or both may occur

simultaneously. In general, diode junctions which break down below 5 V are caused

by the Zener effect, while junctions which experience breakdown above 5 V are

caused by the avalanche effect. Figure 17 below demonstrates graphically the

relationship between Avalanche Breakthrough and Controlled breakthrough.

(Hyperphysics)

Figure 17: Relationship between Avalanche Breakthrough and Controlled Breakthrough

14.4.6. The Zener as a Voltage Regulator

When the Zener diode is forward biased, it behaves as a normal diode. When a

small reverse voltage is applied the current is negligible. As the reverse voltage is

increased, a value is reached at which the current suddenly increases very rapidly.

The voltage across the Zener Diode after breakdown is termed the โ€œreference

voltageโ€ and its value for a given Zener diode remains practically constant over a

wide range of current values, provided the maximum permissible junction

temperature is not exceeded (see manufactures data sheet). The diagram above

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shows how a Zener Diode can be used as a voltage stabiliser. The purpose of the

series resistor is to limit the reverse current through the diode to a safe value.

Figure 18: Zener Voltage Stabiliser

14.4.7. The Zener Characteristic Curve and Symbol

Figure 19: The Zener Symbol

(Talkingelectronics.com)

Figure 20: Zener Diode Characteristic Curve

R1 1

kZ

1 1

N2804

+

-

+

-

Variable Voltage

Constant Voltage

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14.4.8. Zener Diode Series Resistor Calculations

When a Zener diode is used in a circuit, it must be protected by a series resistor

which limits the current flow to below the diodeโ€™s maximum rated reverse current

(Izmax). This current is derived from the power rating (Pz) of the Zener which is

provided in component catalogues. The series resistor Rs also absorbs the

difference in voltage between the supply voltage and Zener voltage Vz

Example 2

Use a Zener diode to provide a stable 9.1 V level from a D.C. supply that fluctuates

between 10 V and 11 V. The Zener diode has a maximum power rating (Pz) of 182

milli watts. Show through calculations how the series resistor Rs is chosen and draw

the circuit to demonstrate the configuration in which the Zener diode is used.

Solution

E = Vrs + Vz ...................Kirchoffs Laws

Therefore

Vrs = E โˆ’ Vz

= 11V โˆ’ 9.1V

= 1.9V

Zener Current Iz =PZ

Vz

=182mW

9.1V

= 20mA

Since this is the same current that flows through the resistor we can calculate the

current through the resistor.

Series Resistor Rs =Vrs

Iz

=1.9V

20mA

= 95ฮฉ

(Swart, pp. 105-106)

Figure 21

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14.5. Practical: Determine the value of the series

resistor for a Zener Diode

Aim: To construct a zener voltage regulator.

Requirements:

Multi meter

variable DC voltage source

4ฮฉ resistor

Zener Diode IN5347(or equivalent) and

ammeter

What is required from you?

Table 1 below shows data for a list of Zener Diodes. Perform the necessary

calculations and choose the right Zener from the list to provide a stable regulated

10V supply to a 50ฮฉ load from an unregulated 12V supply. Calculate the value of the

series resistor.

Table 1

Type No Voltage Power Rating

IN 5240 10 0.4 IN 5242 12 0.4 IN 4740A 10 1 IN 4742A 12 1 IN 5347 10 5 IN 5349 12 5

Build the appropriate circuit once you have conducted all calculations.

Calculations

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Draw the circuit diagram

Build your circuit and let your teacher check it before you turn on the power.

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14.6. The NPN Transistor

The transistor is a 3 element device made up of semiconductor materials. One of the

popular methods of manufacturing a transistor is by growing a crystal. Transistors

are made, and the junctions between elements are formed, by methods similar to

those used to make a PN Junction Diode.

Transistors are used to control current (switch) or to amplify an input voltage or

current. They are used to amplify an extremely small radio signal to a much larger

signal, strong enough to operate the radioโ€™s loudspeaker. Transistors are also used

as electronic switches in logic and computer circuits.

14.6.1. Construction

NPN is one of the two types of bipolar transistors, consisting of a layer of P-doped

semiconductor (the "base") between two N-doped layers. A small current entering

the base is amplified to produce a large collector and emitter current. That is, when

there is a positive potential difference measured from the emitter of an NPN

transistor to its base (i.e., when the base is high relative to the emitter) as well as

positive potential difference measured from the base to the collector, the transistor

becomes active. In this "on" state, current flows between the collector and emitter of

the transistor. Most of the current is carried by electrons moving from emitter to

collector as minority carriers in the P-type base region. To allow for greater current

and faster operation, most bipolar transistors used today are NPN because electron

mobility is higher than hole mobility.

(citycollegiate)

Figure 22: Construction and Symbol NPN Transistor

14.6.2. Principle of Operation

The emitter is connected to the negative. The collector is connected to the positive.

When a positive voltage is connected to the base and it is sufficient in size to forward

bias the base-emitter pn-junction (Si = 0.6 V and Ge = 0.3 V), the transistor will

become conductive. Electrons moves from the emitter to the base. The influx of

minority carriers into the base will forward bias the base-collector junction. When this

happens, current will flow from the base to the collector and to the positive supply.

General Transistor equation: ๐ผ๐ธ=๐ผ๐ต + ๐ผ๐ถ (Thomson, 2013)

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14.6.3. The Purpose of Biasing and Thermal Runaway

14.6.3.1. Biasing

Transistor Biasing is the process of setting a transistors DC operating voltage or

current conditions to the correct level so that any AC input signal can be amplified

correctly by the transistor. A transistors steady state of operation depends a great

deal on its base current, collector voltage, and collector current and therefore, if a

transistor is to operate as a linear amplifier, it must be properly biased to have a

suitable operating point.

(Talkingelectronics.com)

Figure 23: Transistor Biasing Circuits

14.6.3.2. Thermal Runaway

Thermal runaway refers to a situation where an increase in temperature changes the

conditions in a way that causes a further increase in temperature, often leading to a

destructive result.

If this situation is not rectified in a transistor it could lead to destruction of the device.

To prevent thermal runaway a series resistor is often connected to the emitter of the

transistor. (circuits)

14.6.4. Forward Biasing

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Figure 24: NPN Forward Biased Circuit

(Talkingelectronics.com)

To forward bias an NPN transistor it is connected as shown in the circuit above.

1. The collector is connected to high positive voltage with respect to base i.e. Vbc

is very high. So c-b junction is reverse biased. Vbc >> Vbe.

2. The base is connected to low positive voltage with respect to the emitter i.e.

Vbe is low.

3. When we increase Vbe โ‰ฅ 0.7 V (the value 0.7 V is a typical value of potential

barrier voltage for silicon transistors) the transistor is forward biased.

4. Now large numbers of electrons in the emitter layer is repelled by the negative

terminal of Vbe and they flow towards the b-e junction.

5. They cross the junction and enter into the small base layer. Here some

electrons combine with holes. Also, some of them are attracted by positive

terminal of Vbe and the remaining maximum number of electrons flow into

collector layer, crossing the second junction i.e. c-b junction.

6. The resident electrons of the collector are repelled by these (guest) electrons

and thus, all the electrons present in the collector layer are attracted by the

positive terminal of Vcb.

7. Thus, all these electrons complete their journey back into emitter layer and

produce conventional currents in the transistor as shown in the above circuit.

8. Thus, as per Kirchhoff Current Law, we can write, Ic + Ib = Ie.

9. Now when Vbe is still increased, more electrons are repelled by the negative

terminal of Vbe. So the base-emitter junction is more and more forward biased.

Thus the base current (Ib) increases, which in turn increases Ic.

10. Hence, we can say that collector current (Ic) is the function of base current (Ib).

11. But there is a typical value of Vbe for each transistor, at which the collector

current Ic no longer remains the function of base current Ib.

12. Also, the collector current is directly proportional to the base current.

13. In all this process, the maximum number of electrons from the emitter layer

flow into collector layer. So the collector current is ALMOST EQUAL to the

emitter current. Hence we say that, collector current is proportional to the

emitter current. (yashplushtt5) (Unknown, n.d.)

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14.6.5. Reverse Biasing

Figure 25: NPN Reverse Biased Circuit

(Talkingelectronics.com)

In this method both the junctions are reverse biased as the batteries are connected

in opposite direction as shown in the diagram above. Due to the Vcb battery, the

collector-base junction is reverse biased. Similarly, due to the Vcb battery, the base-

emitter junction is also reverse biased. So charges cannot flow and current in the

transistor is practically zero. This method is not useful as the transistor is in โ€œcut-offโ€

state since the base current is zero.

14.6.6. NPN Input Curve

(Talkingelectronics.com)

Figure 26: NPN Input Curve

Figure 26, shows the base emitter voltage against the base emitter current. IBE/VBE

shows the input CONDUCTANCE of the transistor. As conductance I / V is the

reciprocal of RESISTANCE (V / I) this curve can be used to determine the input

resistance of the transistor. The steepness of this particular curve when the VBE is

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above 1 volt shows that the input conductance is very high, and there is a large

increase in current (in practice, usually enough to destroy the transistor!) for a very

small increase in VBE. Therefore the input RESISTANCE must be low. Around 0.6 to

0.7 volts the graph curves shows that the input resistance of a transistor varies,

according to the amount of base current flowing, and below about 0.5 volts base

current ceases.

14.6.7. NPN Output Curve

(Talkingelectronics.com)

Figure 27: NPN Output Curve

Figure 27, shows the output characteristic curve of the NPN Transistor. The near

horizontal parts of the graph lines show that a change in collector emitter voltage VCE

has almost no effect on collector current in this region. Therefore the graph shows

that the output resistance of the transistor is high.

14.6.7.1. The Transistor Characteristic Curve

Figure 28, below provides us with a graphical representation of the comparison

between a simple transistor circuit and its characteristic curve.

Figure 28: Characteristic Curve

14.6.8. Regions of Operation

The transistor is divided into 3 distinct regions. The active region is where the

transistor operates as an amplifier.

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In the saturation region the transistor is fully switched on and Ib has no effect on the

magnitude of Ic. In the cut-off region the transistor is switched off. When the

transistor is used as a switch it operates in these two regions.

(E-Learning)

Figure 29: Regions of Operation

14.6.9. The transistor DC Load Line

(E-Learning)

Figure 30: DC Load Line

From the characteristic curve we can see that transistor can safely handle 12 V

across its collector emitter terminals, and a collector current of 70 mA. When these

two points are joined by a straight line it is termed the DC Load Line of the transistor.

The load line is an imaginary straight line along which the operating point moves up

and down. The Q-point (quiescent point) is the steady state operating point of the

transistor and it is chosen at the middle of the load line.

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From this diagram it is evident that the DC Load line provides us with a tool to

determine the characteristics of the output signal of the amplifier. Below are just a

few characteristics of a transistor that can be determined from the load line:

1. The load line can be used to determine the parameters within which the

transistor can be operated.

2. The Q-Point is determined by the DC Biasing circuit of the transistor.

3. By changing the supply voltage the load line can be shifted in parallel.

4. The load line can be used to determine the AC current, voltage and power

gain of the signal that is applied to the base of the transistor.

14.6.10. Transistor Power related to the load line

(E-Learning)

Figure 31: Transistor DC Load line with Q-Point

From the diagram above it can be seen that the transistor operates optimally at

40๐‘š๐ด and 6V. These values are taken at the center of the characteristic curve at the

point the Q-Point intersects the load line. Apart from helping us obtain the optimal

operating point of the transistor, these two value can also help us calculate the

Power dissipated by the transistor. Since Power is the product of current and

voltage, similarly if these two values are multiplied they help us obtain the Power of

the Transistor. For the transistor represented in Figure 31 above, its Power can be

calculated as follows:

Transisitor Power (P) = ICQ ร— VCQ

= 40mA ร— 6V

ICQ and VCQ are the transistors respective current and voltage values at the Q โˆ’ Point

14.6.11. Influence of the DC Load Line on the Characteristics

of a Transistor

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Figure 32: DC Load line with Q-Point of Transistor.

The slope of the load line invariably has an effect on the transistor. Similarly the

position of the Q-Point has an effect on the characteristics of the transistor. The

position of the Q Point on the DC load line determines the maximum signal that we

can get from the circuit before clipping can occur. Figure 32 (a) shows the Q-point

near the cut off region. Here the signal starts to clip at point A. This is called cut off

clipping. In Figure 32 (b) the Q Point is located near the saturation region. Clipping

now starts at point B. This is caused by the close proximity to the saturation region.

Figure 32 (c) has the Q point at the centre of the load line. In this condition we get

the maximum possible output signal. The signal at this point is undistorted.

The position of the Q point and slope of the load line play an important part in the

use of a transistor for amplification purposes as it determines what type of amplifier

the transistor can be used for.

14.6.12. Transistor Symbols

Figure 33: NPN Transistor Symbol

Figure 34: PNP Transistor Symbol

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14.7. Transistor Applications

14.7.1. The Transistor as a Switch

The transistor can be used as an electronic switch. Such switches are used in

computer circuits and logic circuits to supply low and high voltage levels.

In Figure 35, when the switch is open there is no current to the base, so the

transistor is in the cut off condition with no collector current. The entire voltage drop

is across the transistor.

The base resistor is chosen small enough so that the base current drives the

transistor into saturation.

In this example the mechanical switch is used to produce the base current to close

the transistor switch to show the principles. In practice, any voltage on the base

sufficient to drive the transistor to saturation will close the switch and light the bulb.

(Hyperphysics)

Figure 35: The Transistor as a switch

14.7.2. Transistor as an Amplifier

The common-emitter amplifier is designed so that a small change in voltage (Vin)

changes the small current through the base of the transistor; the transistor's current

amplification combined with the properties of the circuit mean that small alterations in

Vin produce large changes in Vout.

Various configurations of single transistor amplifier are possible, with some providing

current gain, some voltage gain, and some both.

From mobile phones to televisions, vast numbers of products include amplifiers for

sound reproduction, radio transmission, and signal processing. The first discreet-

transistor audio amplifiers barely supplied a few hundred mill watts, but power and

audio fidelity gradually increased as better transistors became available and

amplifier architecture evolved.

Modern transistor audio amplifiers of up to a few hundred watts are common and

relatively inexpensive.

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(Wikipedia)

Figure 36: Common emitter amplifier

14.7.3. Transistor Gain

The number of times an input signal is amplified is called Gain. If only voltage has

been amplified it is called voltage gain and if only current has been amplified it is

called current gain. It is often expressed using the logarithmic decibel (dB) units ("dB

gain"). The symbol โ€œAโ€ is usually associated with gain.

AP= Power gain

AV= Voltage gain

AI= Current gain

Usually we are interested in power gain (AP). Since gain is only a ratio between

similar parameters, it has no units.

๐๐จ๐ฐ๐ž๐ซ ๐†๐š๐ข๐ง =๐Ž๐ฎ๐ญ๐ฉ๐ฎ๐ญ ๐๐จ๐ฐ๐ž๐ซ

๐ˆ๐ง๐ฉ๐ฎ๐ญ ๐๐จ๐ฐ๐ž๐ซ

๐•๐จ๐ฅ๐ญ๐š๐ ๐ž ๐†๐š๐ข๐ง =๐Ž๐ฎ๐ญ๐ฉ๐ฎ๐ญ ๐•๐จ๐ฅ๐ญ๐š๐ ๐ž

๐ˆ๐ง๐ฉ๐ฎ๐ญ ๐•๐จ๐ฅ๐ญ๐š๐ ๐ž

๐‚๐ฎ๐ซ๐ซ๐ž๐ง๐ญ ๐†๐š๐ข๐ง =๐Ž๐ฎ๐ญ๐ฉ๐ฎ๐ญ ๐‚๐ฎ๐ซ๐ซ๐ž๐ง๐ญ

๐ˆ๐ง๐ฉ๐ฎ๐ญ ๐‚๐ฎ๐ซ๐ซ๐ž๐ง๐ญ

14.7.4. Current Gain

The current flowing between emitter and collector of a transistor is much greater than

that flowing between base and emitter. So a small base current is controlling the

much larger collector current. The ratio of the two currents, IC / IB is constant,

provided that the collector emitter voltage VCE is constant. Therefore, if the base

current rises, so does collector current.

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This ratio is the current gain of the transistor and is given the symbol hfe. A fairly low

gain transistor might have a current gain of 20 to 50, while a high gain type may

have a gain of 300 to 800 or more. The spread of values of hfe for any given

transistor is quite large, even in transistors of the same type and batch.

14.7.5. Voltage Gain

As mentioned earlier, gain is the amplification factor of the transistor. The number of

times an input signal is amplified is called the gain of the amplifier. Voltage gain is

thus the number of times the voltage is amplified in an amplifier. It is given by the

formula:

๐ด๐‘‰ =๐‘‚๐‘ข๐‘ก๐‘๐‘ข๐‘ก ๐‘‰๐‘œ๐‘™๐‘ก๐‘Ž๐‘”๐‘’

๐ผ๐‘›๐‘๐‘ข๐‘ก ๐‘‰๐‘œ๐‘™๐‘ก๐‘Ž๐‘”๐‘’

๐ด๐‘‰ =๐‘‰๐‘œ๐‘ข๐‘ก๐‘‰๐‘–๐‘›

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14.8. Practical: Determine the DC Load Line of a

Transistor

Aim: To analyse the load line of an NPN transistor

Requirements: BC 141 transistor, 10Kฮฉ, 100ฮฉ and 10V power supply

What is required from you?

Construct the circuit below and determine the load line of the transistor from its

characteristic curve.

(technologia)

Draw the Characteristic Curve of the Transistor here

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14.9. Practical: Build a circuit using the transistor as a

switch

Aim: To use a transistor as a switch

Requirements:

5V power supply

Breadboard

Digital Analogue Trainer

Pushbutton switch

Resistors - 220ฮฉ, 2700ฮฉ

Transistor BC 548

Leads

Red L.E.D

What is required from you?

1. On your breadboard construct the circuit below.

2. Measure the voltage across the LED when the switch is โ€œonโ€ and when โ€œoffโ€.

Record your answers.

3. Record the voltages across Vbe and Vce when the LED is on. Record your

answers.

4. Record your observations

Circuit Diagram

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Measurements

Voltage across LED โ€“ ON

Voltage across LED โ€“ OFF

Vbe when LED ON

Vbe when LED OFF

Vce when LED ON

Vce when LED OFF

Observations

____________________________________

____________________________________

____________________________________

____________________________________

____________________________________

____________________________________

____________________________________

____________________________________

____________________________________

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14.10. The PNP Transistor

14.10.1. Construction

The only way that the PNP-transistor is different from the NPN-transistor is that the

base is connected to the n-type material and the collector and emitter are the p-type

materials.

The emitter is connected to the positive. The collector is connected to the negative.

When a negative voltage is connected to the base and it is sufficient in size to the

forward bias the base-emitter pn-junction (Si = 0.6 V and Ge = 0.3 V), the transistor

will become conductive. Electrons moves from the base to the emitter. The influx of

minority carriers into the base will forward bias the base-collector junction. When this

happens, current will flow from the collector to the base and through the emitter to

the positive supply.

Symbol

Figure 37: Composition

14.10.2. Principle of 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 38.

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Notice that the procedure used to properly bias the NPN transistor also applies here

to the PNP transistor. The first letter (p) in the PNP sequence indicates the polarity of

the voltage required for the emitter (positive), and the second letter (n) indicates the

polarity of the base voltage (negative). Since the base-collector junction is always

reverse biased, then the opposite polarity voltage (negative) must be used for the

collector. Thus, the base of the PNP transistor must be negative with respect to the

emitter, and the collector must be more negative than the base. Remember, just as

in the case of the NPN transistor, this difference in supply voltage is necessary to

have current flow (hole flow in the case of the PNP transistor) from the emitter to the

collector. Although hole flow is the predominant type of current flow in the PNP

transistor, hole flow only takes place within the transistor itself, while electrons flow in

the external circuit. However, it is the internal hole flow that leads to electron flow in

the external wires connected to the transistor.

Figure 38: PNP Biasing

Now let us consider what happens when the emitter-base junction is forward

biased.

1. With the bias setup as shown in Figure 38 above, the positive terminal of the

battery repels the emitter holes toward the base, while the negative terminal drives

the base electrons toward the emitter.

2. When an emitter hole and a base electron meet, they combine. For each electron

that combines with a hole, another electron leaves the negative terminal of the

battery, and enters the base.

3. At the same time, an electron leaves the emitter, creating a new hole and enters

the positive terminal of the battery. This movement of electrons into the base and out

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of the emitter constitutes base current flow (IB), and the path these electrons take is

referred to as the emitter-base circuit.

4. In the reverse-biased junction, the negative voltage on the collector and the

positive voltage on the base block the majority current carriers from crossing the

junction.

5. This same negative collector voltage acts as forward bias for the minority current

holes in the base, which cross the junction and enter the collector. The minority

current electrons in the collector also sense forward bias-the positive base voltage-

and move into the base.

6. The holes in the collector are filled by electrons that flow from the negative

terminal of the battery. At the same time the electrons leave the negative terminal of

the battery, other electrons in the base break their covalent bonds and enter the

positive terminal of the battery. Although there is only minority current flow in the

reverse-biased junction, it is still very small because of the limited number of minority

current carriers.

7. The interaction between the forward- and reverse-biased junctions in a PNP transistor is very similar to that in an NPN transistor, except that in the PNP transistor, the majority current carriers are holes.

8. In the PNP transistor, the positive voltage on the emitter repels the holes toward the base. Once in the base, the holes combine with base electrons. But again, remember that the base region is made very thin to prevent the recombination of holes with electrons. Therefore, well over 90 percent of the holes that enter the base become attracted to the large negative collector voltage and pass right through the base. However, for each electron and hole that combine in the base region, another electron leaves the negative terminal of the base battery (V BB) and enters the base as base current (IB). At the same time an electron leaves the negative terminal of the battery, another electron leaves the emitter as IE (creating a new hole) and enters the positive terminal of VBB. Meanwhile, in the collector circuit, electrons from the collector battery (VCC) enter the collector as Ic and combine with the excess holes from the base. For each hole that is neutralized in the collector by an electron, another electron leaves the emitter and starts its way back to the positive terminal of VCC.

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Figure 39: Total current flow in the PNP transistor

Although current flow in the external circuit of the PNP transistor is opposite in direction to that of the NPN transistor, the majority carriers always flow from the emitter to the collector. This flow of majority carriers also results in the formation of two individual current loops within each transistor. One loop is the base-current path, and the other loop is the collector-current path. The combination of the current in both of these loops (IB + IC) results in total transistor current (IE). (radartut)

14.10.3. Relation to NPN Transistor

The relation between the forward and reverse bias circuits are the same for both NPN and PNP transistors. The only difference between these transistors is the polarities of the voltages.

The majority carriers in the PNP transistor are holes whilst the majority carriers in an NPN transistor are electrons.

The most important thing to remember about the two different types of transistors is that the emitter-base voltage of the PNP transistor has the same controlling effect on collector current as that of the NPN transistor.

14.10.4. Symbol

Figure 40: PNP Transistor Symbol

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14.10.5. Applications

The PNP transistor has the same applications as the NPN transistor. Some of its

applications include:

1. Switching circuits

2. Amplifiers.

14.10.5.1. Sample Circuits

Figure 41: The PNP Transistor being used as a switch

Figure 42: The PNP Transistor being used as an amplifier

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14.11. Practical: Build a circuit using a PNP Transistor

as a switch.

Aim: To use a PNP transistor as a switch

Requirements:

12V power supply

Breadboard

Digital Analogue Trainer

Pushbutton switch

Resistors -2k and 1K

Transistor BC 557

Leads

Red L.E.D

What you are required to do?

1. Connect the circuit below

1. Draw your circuit with the switch.

2. Record all observations.

Hint: Take note of the polarity of an NPN Transistor

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Circuit Diagram

Observations

________________________________________

________________________________________

________________________________________

________________________________________

________________________________________

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14.12. Thyristors โ€“ The Silicon Controlled Rectifier

(SCR)

14.13. Thyristors have at least four layers of alternating n- and p-type

materials. They act as bi-stable switches and are activated by applying

a pulse to the gate. Typical examples of Thyristors are The Silicon

Controlled Rectifier (SCR) and The Triac. Thyristors have no

amplification ability; therefore they are used in switching circuits only.

Their characteristics make them particularly useful in power control

circuits such as motor speed control and light dimmers.

14.13.1. Construction

The silicon controlled rectifier (SCR) is made of four alternating layers on n- and p-

type material. The top p-type material is the anode and the bottom p-type material is

the cathode. The bottom p-type material is the gate and is used to trigger the SCR

(turn it on). A depletion region will form around each pn-junction. Figure 43 below

shows the symbol and composition of the SCR.

Figure 43: The SCR

14.13.2. Principle of Operation

In reverse bias, the SCR acts as a normal diode. When the anode is connected to a

positive and the cathode is connected to a negative, then the SCR starts to become

forward biased. When the gate receives a positive pulse, then the bottom pn-junction

becomes forward biased and conduction takes place. The large amount of electrons

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flowing into the bottom p-type material is seen on the top n-type material as a

negative supply. This allows the top pn-junction to become forward biased and

conduction takes place through the cathode and anode of the SCR. The gate pulse

can now be removed.

The SCR will remain active until the supply current drops below the holding current,

the voltage across the anode and cathode drops below the holding voltage or the

supply is reversed. (Thomson, 2013)

Figure 44 below illustrates the SCRโ€™S operation.

p

n

p

n

anode

cathode

gate

1

+

+

-

---3

2

Figure 44: Operation of the SCR

14.13.3. Biasing

Figure 45: SCR Biasing

(Talkingelectronics.com)

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Figure 45 above it is evident that in order to switch an SCR on a gate pulse is

needed.

14.13.4. Symbol

(Wikipedia)

Figure 46: SCR Symbol

14.13.5. Characteristic Curves

Figure 47 shows the typical characteristic curve of a SCR (Panel, 2005)

Figure 47: SCR Characteristic Curve

Break over voltage โ€“minimum voltage required to turn SCR on.

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Holding current โ€“ minimum anode current required to keep the SCR in the on

state.

Latching current โ€“ Minimum anode current required to maintain the SCR in

the on state immediately after the SCR has been turned on and the gate

current has been removed

Peak reverse voltage โ€“ This is the maximum reverse voltage that can be

applied to the SCR without conducting in the reverse direction.

Forward current rating โ€“ maximum anode current that the SCR is capable of

passing without destruction

14.13.6. Applications

Silicon Controlled Rectifiers have no amplification ability; therefore they are used in

switching circuits only. Their characteristics make them particularly useful in power

control circuits such as motor speed control and light dimmers.

SCR Phase Control

Figure 48: Phase Control

14.13.6.1. Operation

At the start of each positive half-cycle the SCR is โ€œOFFโ€. On the application of the

gate pulse, it triggers the SCR into conduction and remains fully latched โ€œONโ€ for the

duration of the positive cycle. If the thyristor is triggered at the beginning of the half-

cycle (ฮธ = 0ยบ), the load (a lamp) will be โ€œONโ€ for the full positive cycle of the AC

waveform (half-wave rectified AC) at a high average voltage of 0.318 ร— Vp

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As the application of the trigger pulse increases along the half-cycle (ฮธ = 0ยบ to 90ยบ),

the lamp is illuminated for less time and the average voltage delivered to the lamp

will also be proportionally less reducing its brightness.

We can therefore use a silicon controlled rectifier as an AC light dimmer as well as in

a variety of other AC power applications such as: AC motor-speed control,

temperature control systems and power regulator circuits, etc.

Thus far we have seen that a thyristor is essentially a half-wave device that conducts

in only the positive half of the cycle when the Anode is positive and blocks current

flow like a diode when the Anode is negative, irrespective of the Gate signal.

14.13.6.2. The SCR as a Relaxation Oscillator

In electronics a relaxation oscillator is a nonlinear electronic oscillator circuit that

produces a non-sinusoidal repetitive output signal, such as a triangle wave or square

wave.

Circuit Diagram

Figure 49: Relaxation oscillator Circuit

From the circuit, when power is applied, capacitor Ct charges and the voltage across

resistor RT decreases. When the voltage across RT drops to 0.6 V less the gate

voltage, the SCR turns on. This is the cathode voltage of the SCR. This turn-on

produces current flow through CT and a voltage spike across R3.

Since the large value of RT prevents there being sufficient current to maintain

conduction, the SCR immediately turns off, and RT begins its charge cycle again.

14.13.6.3. Phase Control

Phase control is the most common form of Thyristor power control. The Thyristor is

held in the off condition, that is, all current flow in the circuit is blocked by the

Thyristor except a minute leakage current. Then the Thyristor is triggered into an

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โ€œonโ€ condition by the control circuitry. For full-wave AC control, a single Triac or two

SCRs connected in inverse parallel may be used. One of two methods may be used

for full-wave DC control, a bridge rectifier formed by two SCRs or an SCR placed in

series with a diode bridge.

(littlefuse)

Figure 50: SCR Phase Control

14.13.6.4. Switch mode applications

The SCR is a bi-stable device. This means that it has a conducting state and a non-

conducting state. This characteristic allows for the SCR to be used for switching of

power signals owing to their long life, high operation speed and freedom from other

defects associated with mechanical and electro-mechanical switches. (circuitstoday)

Figure 51: Static AC Circuit Breaker

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Figure 51 shows a circuit in which two SCRs are used for making and breaking an

AC circuit. The input voltage is alternating and the trigger pulses are applied to the

gates of SCRs through the control switch S. Resistance R is provided in the gate

circuit to limit the gate current while resistors R1 and R2 are to protect the diodes D1

and D2 respectively.

When starting the circuit, switch S is closed, SCR1 will fire at the beginning of the

positive half-cycle because it is forward biased. It will turn-off when the current goes

through the zero value. As soon as SCR1 is turned-off, SCR2 will fire since the

voltage polarity is already reversed and it gets the proper gate current. The circuit

can be broken by opening the switch S. Opening of the gate circuit poses no

problem, as current through this switch is small. As no further gate signal will be

applied to the SCRs when switch S is open, the SCRs will not be triggered and the

load current will be zero. The maximum time delay for breaking the circuit is one half-

cycle.

14.13.6.5. DC-DC Converter [buck/boost]

A buck converter is a voltage step down and current step up converter. Figure 52

below bests demonstrates its basic operation

(Wikipedia)

Figure 52: Circuit Configurations of a Buck Converter

The buck converter is best understood in terms of the relation between current and

voltage of the inductor. Beginning with the switch open (in the "off" position), the

current in the circuit is 0. When the switch is first closed, the current will begin to

increase, and the inductor will produce an opposing voltage across its terminals in

response to the changing current. This voltage drop counteracts the voltage of the

source and therefore reduces the net voltage across the load.

Over time, the rate of change of current decreases, and the voltage across the

inductor also then decreases, increasing the voltage at the load. During this time, the

inductor is storing energy in the form of a magnetic field. If the switch is opened while

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the current is still changing, then there will always be a voltage drop across the

inductor, so the net voltage at the load will always be less than the input voltage

source.

When the switch is opened again, the voltage source will be removed from the

circuit, and the current will decrease. The changing current will produce a change in

voltage across the inductor, now aiding the source voltage. The stored energy in the

inductor's magnetic field supports current flow through the load. During this time, the

inductor is discharging its stored energy into the rest of the circuit. If the switch is

closed again before the inductor fully discharges, the voltage at the load will always

be greater than zero.

The switch in a Buck Converter is replaced by a SCR due to its ability to have

switching applications.

Figure 53: Buck Converter using a SCR

(NPTEL)

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14.14. Practical: Construct a Relaxation Oscillator and

analyse its waveform using an oscilloscope.

Aim: To construct and analyse a Relaxation Oscillator

Requirements:

Capacitor โ€“ 1.5uF

Resistors โ€“ 86K, and 1Mฮฉ Pot

SCR โ€“ ECG 540

Oscilloscope

Digital Analogue Trainer What is required from you?

1. Construct a relaxation Oscillator from the circuit below. 2. Using the oscilloscope analyse its output 3. Draw the output waveform on the oscillogram provided. 4. Record you observations

Circuit Diagram

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Oscillogram

Observations

________________________________________

________________________________________

________________________________________

________________________________________

________________________________________

________________________________________

________________________________________

________________________________________

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14.15. Practical: Construct a Light Dimmer Circuit using

an SCR

Aim: To connect an SCR in light dimming circuit

Requirements:

Power supply 6V AC

Bread Board

47K Pre-set resistor / POT

1๐‘ข๐น Capacitor

Diode 1N4007

6 V Lamp

SCR โ€“ CMR 106-4

Hook up Leads

Circuit Diagram

Connect the circuit as shown above.

Observation

__________________________________________________

__________________________________________________

__________________________________________________

__________________________________________________

__________________________________________________

________________________________________________

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14.16. The Triac

The triac is an AC version of the Silicon Controlled Rectifier. Its name originates from

โ€œtriode for alternating currentโ€ or โ€œtriode for ACโ€. The major drawback of an SCR is

that it can conduct current in one direction only. Therefore, an SCR can only control

DC power or forward biased half cycles of AC in a load. However, in an AC system,

it is often desirable and necessary to exercise control over both positive and

negative half cycles. For this purpose a Triac is used. (Talkingelectronics.com)

Figure 54: Triac Composition

14.16.1. Construction

The triac has four possible conduction paths, depending on the biasing and the

polarity of the connections. The main terminals will always have opposite polarities.

The gateโ€™s polarity (with reference to main terminal 2) will be such that current will

flow from the more negative to the more positive. When this happens, the breakdown

occurs and then conduction takes place.

Figure 55 represents the four conduction paths of the Triac.

(Thomson, 2013)

Figure 55: Conduction Paths of the Triac

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14.16.2. Principle of Operation

Figure 56: Four Quadrants of a Triac

(Hyperphysics)

To explain how TRIACs work, one has to individually analyse the triggering in each

one of the four quadrants. The four quadrants are illustrated in Figure 56 according

to the voltage on the gate and the MT2 terminals with respect to the MT1 terminal.

The MT1 and MT2 terminals are also commonly referred to as A1 and A2,

respectively.

The relative sensitivity depends on the physical structure of a particular triac, but as

a rule, quadrant I is the most sensitive (least gate current required) and quadrant IV

is the least sensitive (most gate current required).

In quadrants 1 and 2, MT2 is positive and current flows from MT2 to MT1 through P,

N, P and N layers. The N region attached to MT2 does not participate significantly. In

quadrants 3 and 4, MT2 is negative and current flows from MT1 to MT2, also through

P, N, P and N layers. The N region attached to MT2 is active, but the N region

attached to MT1 only participates in the initial triggering, not the bulk current flow.

In most applications, the gate current comes from MT2, so quadrants 1 and 3 are the

only operating modes.

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(Hyperphysics)

Figure 57: Quadrant 1 Operation

Quadrant I operation occurs when the gate and MT2 are positive with respect to

MT1. The precise mechanism is illustrated in Figure 57. The gate current makes an

equivalent NPN transistor switch on, which in turn draws current from the base of an

equivalent PNP transistor, turning it on also. Part of the gate current (dotted line) is

lost through the ohmic path across the p-silicon, flowing directly into MT1 without

passing through the NPN transistor base. In this case, the injection of holes in the p-

silicon makes the stacked n, p and n layers beneath MT1 behave like a NPN

transistor, which turns on due to the presence of a current in its base. This, in turn,

causes the p, n and p layers over MT2 to behave like a PNP transistor, which turns

on because its n-type base becomes forward-biased with respect to its emitter,

(MT2). Thus, the triggering scheme is the same as an SCR.

However, the structure is different from SCRs. In particular, TRIAC always has a

small current flowing directly from the gate to MT1 through the p-silicon without

passing through the p-n junction between the base and the emitter of the equivalent

NPN transistor. This current is indicated in Figure 57 by a dotted red line and it is the

reason why a TRIAC needs more gate current to turn on than a comparably rated

SCR.

Generally, this quadrant is the most sensitive of the four. This is because it is the

only quadrant where gate current is injected directly into the base of one of the main

device transistors.

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(Hyperphysics)

Figure 58: Quadrant 2 Operation

Quadrant II operation occurs when the gate is negative and MT2 is positive with

respect to MT1.

Figure 58 gives a graphical explanation of the triggering process. The turn-on of the

device is three-fold and starts when the current from MT1 flows into the gate through

the p-n junction under the gate. This switches on a structure composed by an NPN

transistor and a PNP transistor, which has the gate as cathode. As current into the

gate increases, the potential of the left side of the p-silicon under the gate rises

towards MT1, since the difference in potential between the gate and MT2 tends to

lower: this establishes a current between the left side and the right side of the p-

silicon, which in turn switches on the NPN transistor under the MT1 terminal and as

a consequence also the PNP transistor between MT2 and the right side of the upper

p-silicon. So, in the end, the structure which is crossed by the major portion of the

current is the same as Quadrant I operation.

(Hyperphysics)

Figure 59: Quadrant 3 Operation

Quadrant III operation occurs when the gate and MT2 are negative with respect to

MT1. The whole process is outlined in Figure 59. The process happens in different

steps here too. In the first phase, the pn junction between the MT1 terminal and the

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gate becomes forward-biased (step 1). As forward-biasing implies the injection of

minority carriers in the two layers joining the junction, electrons are injected in the p-

layer under the gate. Some of these electrons do not recombine and escape to the

underlying n-region (step 2). This in turn lowers the potential of the n-region, acting

as the base of a PNP transistor which switches on (turning the transistor on without

directly lowering the base potential is called remote gate control). The lower p-layer

works as the collector of this PNP transistor and has its voltage heightened: actually,

this p-layer also acts as the base of an NPN transistor made up by the last three

layers just over the MT2 terminal, which, in turn, gets activated. Therefore, the red

arrow labelled with a "3" in Figure 59 shows the final conduction path of the current.

(Hyperphysics)

Figure 60: Quadrant 4 Operation

Quadrant IV operation occurs when the gate is positive and MT2 is negative with

respect to MT1. Triggering in this quadrant is similar to triggering in Quadrant III. The

process uses a remote gate control and is illustrated in Figure 60. As current flows

from the p-layer under the gate into the n-layer under MT1, minority carriers in the

form of free electrons are injected into the p-region and some of them are collected

by the underlying np-junction and pass into the adjoining n-region without

recombining. As in the case of a triggering in Quadrant III, this lowers the potential of

the n-layer and turns on the PNP transistor formed by the n-layer and the two p-

layers next to it. The lower p-layer works as the collector of this PNP transistor and

has its voltage heightened: actually, this p-layer also acts as the base of an NPN

transistor made up by the last three layers just over the MT2 terminal, which, in turn,

gets activated. Therefore, the red arrow labelled with a "3" in Figure 60 shows the

final conduction path of the current.

Generally, this quadrant is the least sensitive of the four. In addition, some models of

TRIACs cannot be triggered in this quadrant but only in the other three.

14.16.3. Purpose of Biasing

TRIACs are part of the thyristor family and are closely related to silicon-controlled

rectifiers (SCR). However, unlike SCRs, which are unidirectional devices (that is,

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they can conduct current only in one direction), TRIACs are bidirectional and so

current can flow in either direction. Another difference from SCRs is that TRIAC

current flow can be enabled by either a positive or negative current applied to its

gate electrode, whereas SCRs can be triggered only by a positive gate pulse. To

create a triggering current, a positive or negative voltage has to be applied to the

gate with respect to the MT1 terminal (otherwise known as A1).

Once triggered, the device continues to conduct until the current drops below a

certain threshold called the holding current. It is for the reasons outlined above that

biasing plays an integral part in the operation of the Triac.

14.16.4. Symbol

Figure 61; Triac Circuit Symbol

14.16.5. The Triac Characteristic Curve

Figure 62 shows the typical characteristic curve of a triac

(Swart)

Figure 62: Triac Characteristic Curve

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Since a triac consists of essentially two SCRโ€™S of opposite orientation fabricated in

the same crystal, its operating characteristics in the first and third quadrants of the

characteristic curve are the same except for the direction of applied voltage and

current flow. The following points can be noted from the characteristic curve:

1. The V-I characteristics for the triac in the 1st and 3rd quadrants are essentially

identical to those of an SCR in the 1st quadrant.

2. The triac can be operated with either a positive or negative gate control

voltage but under normal operation the gate voltage is positive in quadrant 1

and negative in quadrant 3.

3. The supply voltage at which the triac is turned on depends upon the gate

current. The greater the gate current, the smaller the supply voltage at which

the triac is turned on. This allows us to use a triac to control AC power in a

load from 0 to full load power in a smooth and continuous manner with no loss

in the controlling device. (Talkingelectronics.com)

14.16.6. Application

Triacs have no amplification ability; therefore they are used in switching circuits only.

Their characteristics make them particularly useful in power control circuits such as

motor speed control and light dimmers. Ref: (Thomson) Ref: (Swart)

Figure 63: Typical Light Dimmer Circuit using Triac

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(Talkingelectronics.com)

Figure 64: A typical Triac electronic changeover circuit

Two triacs are used in this circuit viz. TR 1 and TR2. When TR1 is turned on TR2 is

turned off, the line input is connected across the full transformer primary AC.

However, if it is desired to change the tapping so that the input appears across part

AB of the primary, then TR2 is turned on and TR1 turned off. The gate control

signals are so controlled that both triacs are never switched on together. This avoids

a dangerous short circuit on the section BC of the transformer.

Apart from circuits like the ones mentioned above triacs are used in Relaxation

Oscillators, DC-DC Converters and for other switch mode applications.

(Hyperphysics)

Figure 65: Triac used as an AC Switch

When switch S is thrown to position 1, the triac is cut off and the output power of

lamp is zero. As the switch is thrown to position 2, a small gate current (a few mA)

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flowing through the gate turns the triac on. Consequently, the lamp is switched on to

give full output of 1000 watts.

14.16.7. Circuit Diagrams

(schematicdiagram) Figure 66: A Triac used for Motor Speed Control

(Talkingelectronics.com)

Figure 67: The Triac used in a Simple AC Power Control Circuit

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14.17. Practical: Construct a light dimmer circuit

Aim: To construct and examine a light dimmer circuit using a Triac

Requirements:

โ€ข Resistors -1K, 220K Pot,

โ€ข TIC226- Triac

โ€ข D1 โ€“ Diac 32 V

โ€ข Capacitors -100๐‘›๐น X 2

โ€ข 220V power supply

What are you required to do?

Construct the circuit below.

Observation

____________________________________

____________________________________

____________________________________

____________________________________

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14.18. The Diac

The diac is a two terminal, three layer bidirectional device which can be switched

from its off state to on state for either polarity of the applied voltage.

14.18.1. Construction

The diac can be constructed in either NPN or PNP form. The two leads are

connected to p-regions of silicon separated by an n-region. The structure of the diac

is very similar to that of a transistor. However, there are several important

differences:

There is no terminal attached to the base layer.

The three regions are identical in size.

The doping concentrations are identical.

Figure 68 shows the construction and symbol of the diac.

Figure 68: Diac Construction and Circuit Symbol

14.18.2. Operation

The DIAC, or "diode for alternating current", is a diode that conducts current only

after its break over voltage, VBO, has been reached momentarily.

When this occurs, the diode enters the region of negative dynamic resistance,

leading to a decrease in the voltage drop across the diode and, usually, a sharp

increase in current through the diode. The diode remains "in conduction" until the

current through it drops below a value characteristic for the device, called the holding

current, IH. Below this value, the diode switches back to its high-resistance (non-

conducting) state. This behaviour is bidirectional, meaning typically the same for

both directions of current.

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Most DIACs have a three-layer structure with break over voltage around 30 V. Their

behaviour is somewhat similar to that of a neon lamp, but it is much more precisely

controlled and takes place at a lower voltage.

DIACs have no gate electrode, unlike some other thyristors that they are commonly

used to trigger, such as TRIACs. Some TRIACs, like Quadrac, contain a built-in

DIAC in series with the TRIAC's "gate" terminal for this purpose.

DIACs are also called symmetrical trigger diodes due to the symmetry of their

characteristic curve. Because DIACs are bidirectional devices, their terminals are not

labelled as anode and cathode but as A1 and A2 or MT1 ("Main Terminal") and MT2.

14.18.3. Purpose of Biasing

Conduction occurs in a diac when the break over voltage is reached with either

polarity across the two terminals.

Once break over occurs, current is in a direction depending on the polarity of the

voltage across the terminals. The device turns off when the current drops below the

holding value IH.

The equivalent circuit of a diac consists of four transistors arranged as shown below:

(Talkingelectronics.com)

Figure 69: The Equivalent Circuit of a Diac

In the equivalent circuit Q1 and Q2 are forward-biased, and Q3 and Q4 are

reversed-biased. The device operates on the upper right portion of the characteristic

curve under this bias condition.

When the diac is biased as in part (c), the PNPN structure from A2 to A1 is used. In

the equivalent circuit, Q3 and Q4 are forward-biased, and Q1 and Q2 are reverse-

biased. Under this bias condition the device operates on the lower left portion of the

characteristic curve.

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14.18.4. Symbol

Figure 70: Basic construction and Symbol of the Diac

14.18.5. Characteristic Curve

(Talkingelectronics.com)

Figure 71: Diac Characteristic Curve

For applied voltage less than +VBO and negative voltages less than โ€“VBO, a small

leakage current (IBO) flows through the device. Under such conditions, the diac

blocks the flow of current and effectively behaves as an open circuit.

When the positive or negative applied voltage is equal to or greater than the

breakdown voltage the diac begins to conduct and the voltage drop across it

becomes a few volts. Conduction then continues until the device current drops below

its holding current.

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14.18.6. Application

Diacs are primarily used for triggering of triacs. Examples of diac applications are:

light dimmer circuits, relaxation oscillators and switch mode applications.

14.18.7. Circuit Diagram Application

A diac used in a light dimmer circuit to trigger a triac

(Talkingelectronics.com)

Figure 72: Diac used in Light Dimmer Circuit

14.19. Homework Exercises

1. What is a datasheet and what is its purpose?

2. Explain the term โ€œDual in Line Packageโ€

3. Draw diagrams that best represent the differences between Electron Current

Flow and Conventional Current Flow.

4. Define the term โ€œSolid State Deviceโ€

5. What is doping and why is it important?

6. Draw a diagram that best represents the PN Junction. Show all majority and

minority carriers.

7. Draw the characteristic curve of a diode.

8. Why are Zener Diodes commonly used in Power Supplies?

9. Define the following terminology

a) Avalanche Effect

b) Zener Effect

10. Draw the circuit symbol for a NPN and PNP transistor.

11. Using the characteristic curve and DC load line of a transistor, explain the

term โ€œRegions of a transistorโ€

12. Show diagrammatically how you would use a transistor as a switch.

13. What does the acronym SCR stand for?

14. Explain the operation of the SCR.

15. List 3 disadvantages of an SCR.

16. Show how you would use an SCR as a Relaxation Oscillator.

17. How have Triacs overcome the disadvantages of the SCR?

18. Differentiate between a Diac and a Transistor.

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19. Use a Triac to control the brightness of a bulb that is connected to household

power. Draw the circuit to show how the Triac must be connected. Draw one

cycle of the input and load voltages if the firing angle is set at 135โฐ.

20. How is a Diac turned on?

21. How is a Diac turned off?

22. Explain the function of a Diac.

23. Draw the equivalent circuit of a Diac using Transistors.

24. Draw a circuit showing how a Triac can be used to control AC.

25. Draw a circuit representing a Relaxation Oscillator.

Class Activity 1

1. Conduct research on the following transistor:

BC557B

From your research answer the following questions on the transistor:

1.1. Within what temperature parameters is it advisable to store this transistor?

1.2. List at least 3 uses of this type of transistor.

1.3. Draw a diagram clearly showing the PIN CONFIGURATION of this transistor. 1.4. What is the typical value of DC Current gain of this transistor? 2. List 3 advantages of Surface Mounted Components 3. Define the term โ€œDual in Line Packageโ€

Class Activity 2

1. Show graphically the difference between electron current flow and

conventional current flow.

2. Define the following terms :

a) Semiconductor

b) Solid State Device

c) Doping

3. Fill in the missing words from the statement below:

N-Type Doping is a process where a tetravalent atom combines with a

__________________. P-Type Doping is a process where a tetravalent atom

combines with a __________________________.

4. Draw a clearly labelled diagram showing a PN Junction connected in Forward

Bias.

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5. With reference to PN Junction theory, explain the terms โ€œMajority and Minority

Carriersโ€

Class Activity 3

A silicon PN junction diode is connected to a 10V supply in series with a 1.2kฮฉ load

resistor. Use a DC load line to determine the Q-Point at which the diode is operating.

Class Activity 4

1. Explain the term โ€œBreakdown Voltageโ€ and โ€œAvalanche Breakdownโ€ with respect to the Zener Diode.

2. Draw the characteristic curve of a Zener Diode. 3. With the aid of a suitable diagram explain how you would use a Zener Diode

for voltage stabilization.

Class Activity 5

1. Draw a block diagram and circuit symbol showing all the relevant details pertaining to a NPN transistor.

2. Define the term โ€œtransistor biasingโ€ 3. Draw 4 types of biasing circuits you are aware of. 4. Explain the term โ€œThermal Runawayโ€

Class Activity 6

1. Draw block diagrams to represent the difference between a NPN and PNP

Transistor.

2. Explain the basic operation of a PNP Transistor.

3. List 2 applications of a PNP Transistor.

4. What is the Mathematical relationship between the currents that flow in any

transistor

Class Activity 7

1. What is the collective name for Triacโ€™s and Diacโ€™s?

2. Draw the circuit symbol for the Triac and the Diac.

3. Explain the principal of operation of a Triac

4. Draw a diagram showing how you would use a Diac and a Triac to control

the brightness of your room light.

Page 70: 14.1. Introduction to Semiconductor Devices...14. Semiconductor Devices By Ritesh Hariram 14.1. Introduction to Semiconductor Devices Semiconductor devices are electronic components

14.20. Bibliography

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Circuits, A. a. (n.d.). Retrieved from www.allaboutcircuits.com

E-Learning. (n.d.). Retrieved from http://elearning.vtu.ac.in/P9/notes/06ES32/Unit2-

MSS.pdf

Hyperphysics. (n.d.). Retrieved from http://hyperphysics.phy-

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swart, R. a. (n.d.). Clever Electrical Technology grade 11.

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electronics/CH-21.pdf

Thomson, N. (2013). Discovering Electrical Technology. Pretoria , Gauteng, South

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Wikipedia. (n.d.). Retrieved from http://en.wikipedia.org/wiki/Depletion_region

Wikipedia. (n.d.). Retrieved from

http://en.wikipedia.org/wiki/Bipolar_junction_transistor

Wikipedia. (n.d.). Retrieved from http://en.wikipedia.org/wiki/Transistor

(n.d.). Retrieved from http://www.yashplus.com/2014/01/how-npn-transistor-works-

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electronics/CH-21.pdf

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Wikipedia. (n.d.). Retrieved from

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Wikipedia. (n.d.). Retrieved from http://en.wikipedia.org/wiki/Transistor