Mohamed Sathak AJ College of Engineering

Electrical and Electronics Engineering

EC3301 Electronic Devices and Circuits

UNIT II TRANSISTORS AND THYRISTORS

1.What are the requirements for biasing circuits? N/D’21

● It must provide adequate zero signal collector current.

● VCE voltage must not drop below 0.5V for Ge transistors and 1V for silicon transistors at
any time.
● Must be able to stabilize the operating point

2.What is meant by negative resistance region in UJT? N/D’21

What do you mean by negative resistance in UJT?

UJT has negative resistance characteristics (negative resistance region in UJT indicates that if the
emitter voltage (Vg) decreases with an increase of the Emitter current (IE) which makes it useful
as an oscillator.

3. Mention the advantages of MOSFET N/D’20

The operational speed of MOSFET is higher than that of JFET.

Input impedance is much higher as compared to JFET.

It can be easily used in case of high current applications.

These devices provide an easy manufacturing process

4.. State the difference between JFET and BJT

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5.. State the difference between JFET and UJT

6.What is meant by latching in SCR A/M’19

Latching current (IL) is the minimum principal current required to maintain the Thyristor in the
on state immediately after the switching from off state to on state has occurred and the triggering
signal has been removed

7.. List the applications of SCR

Applications
● SCR as a Rectifier
● SCR as a static contactor
● SCR for power control
● SCR for speed control of d. c. shunt motor
● Over light detector
Phase Control.

8. How FET used as a voltage variable resister

Small signal FET drain resistance rd varies with applied gate voltage VGS and FET act like a
Voltage Variable Resistor. Hence, JFET can be used as a Voltage Variable Resistor.

9.FET has lower thermal noise than BJT-Justify

FET is less noisy than BJT because FET is a unipolar device ( current is either due to holes or
electrons) and BJT is a bipolar device ( current is due to both electrons and holes). Since, FET is
a unipolar or majority carrier device and the temperature effect on it is less than BJT.

10. Advantages of IGPT

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 ● Lower gate-driver requirements
● Lower switching losses
● Smaller snubber circuit requirement
● Less size and cost
● High efficiency

Disadvantages of IGBT:
The disadvantages of IGBT are,

● The cost of IGBT is more than MOSFET and BJT.

● Latching up problem.
● The switching speed of IGBT is between BJT and MOSFET.
● Higher turn OFF time compared to MOSFET.

PART B

1.Explain the input and output characteristics characteristics of BJT in common emitter
configuration with neat sketch.

2. Explain the structure, working and characteristics of JFET

3.Brief about the operation of an N channel depletion type MOSFET with neat diagram.
Enumerate characteristics with suitable graph. List the advantages and Disadvantages of
MOSFET N/D’21

4.Draw the basic construction and equivalent circuit of a UJT. Explain the device operation and
characteristics. List the applications of UJT

5. With neat sketches, discuss about the construction, working and characteristics of IGPT. List
the advantages, disadvantages and applications of IGPT N/D21

6.Outline the structure of a SCR and explain its operation. Also illustrate its V-I Characteristics.
List the applications of SCR.

7.Explain various biasing methods of BJT and FET (Refer class notes)

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BJT-Structure, Operation & Characteristics
1. Explain about the transistor (BJT) operation.

Applying external voltage to a transistor is called biasing. In order to

operate transistor properly as an amplifier, it is necessary to correctly
bias the two PN junctions with external voltages. Depending upon external
bias voltage polarities used, the transistor works in one of the three regions.

1. Active region 2. Cut-off region 3. Saturation region

To bias the transistor in its active region the emitter base junction is forward
biased, while the collector- base junction in reverse-biased as shown in Fig.

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The Fig. shows the circuit connections for active region for both NPN and
PNP transistors.

Operation of NPN transistor:

As shown in fig. the forward bias applied to the emitter base junction of an
NPN transistor causes a lot of electrons from the emitter region to cross
over to the base region. As the base is lightly doped with P-type impurity,
the number of holes in the base region is very small and hence the
number of electrons that combine with holes in the P – type base
region is also very small. Hence a few electrons combine with holes
to constitute a base current IB. The remaining electrons (more than
95%) crossover into the collector region to constitute a collector current
IC. Thus the base and collector current summed up give the emitter current

i.e. IE=-(IC+IB)

n the external circuit of the NPN bipolar junction transistor, the magnitudes
of the emitter current IE, the base current IB and the collector current IC
are related by

IE=IC+IB.

Operation of PNP transistor:

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As shown in fig. the forward bias applied to the emitter – base junction of a
PNP transistor causes a lot of hoses from the emitter regions to cross over
to the base region as the base is lightly doped with N- type impurity. The
number of electrons in the base regions is very small and hence the number
of holes combined with electrons in the N – type base region is also very
small. Hence a few holes combined with electrons to constitute a base
current IB

The remaining holes (more than 95%) cross over into the collector region to
constitute a collector current IC. Thus, the collector and base current when
summed up gives the emitter current.
i.e. IE= - (IC+IB).

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Explain the input and output characteristics characteristics of BJT in common emitter
configuration with neat sketch.

● In common emitter configuration, base is the input terminal, collector is the output
terminal and emitter is the common terminal for both input and output. That means the
base terminal and common emitter terminal are known as input terminals whereas
collector terminal and common emitter terminal are known as output terminals.
● In common emitter configuration, the emitter terminal is grounded so the common
emitter configuration is also known as grounded emitter configuration. Sometimes
common emitter configuration is also referred to as CE configuration, common emitter
amplifier, or CE amplifier. The common emitter (CE) configuration is the most widely
used transistor configuration

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The input signal is applied between the base and emitter terminals while the output signal is
taken between the collector and emitter terminals. Thus, the emitter terminal of a transistor is
common for both input and output and hence it is named as common emitter configuration.

The supply voltage between base and emitter is denoted by VBE while the supply voltage
between collector and emitter is denoted by VCE.

In common emitter (CE) configuration, input current or base current is denoted by IB and output
current or collector current is denoted by IC.

The common emitter amplifier has medium input and output impedance levels. So the current
gain and voltage gain of the common emitter amplifier is medium. However, the power gain is
high.

Determining Input and Output Characteristics

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Input characteristics

The input characteristics describe the relationship between input current or base current
(IB) and input voltage or base-emitter voltage (VBE).

To determine the input characteristics, the output voltage V CE is kept constant at zero volts and
the input voltage VBE is increased from zero volts to different voltage levels. For each voltage
level of input voltage (VBE), the corresponding input current (IB) is recorded.

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A curve is then drawn between input current IB and input voltage VBE at constant output voltage
VCE (0 volts).

Next, the output voltage (VCE) is increased from zero volts to certain voltage level (10 volts) and
the output voltage (VCE) is kept constant at 10 volts. While increasing the output voltage (VCE),
the input voltage (VBE) is kept constant at zero volts. After we kept the output voltage (V CE)
constant at 10 volts, the input voltage VBE is increased from zero volts to different voltage levels.
For each voltage level of input voltage (VBE), the corresponding input current (IB) is recorded.

A curve is then drawn between input current I B and input voltage VBE at constant output voltage
VCE (10 volts).

This process is repeated for higher fixed values of output voltage (V CE).

Output characteristics

The output characteristics describe the relationship between output current (I C) and output
voltage (VCE).

To determine the output characteristics, the input current or base current I B is kept constant at
0 μA and the output voltage VCE is increased from zero volts to different voltage levels. For each
level of output Ivoltage, the corresponding output current (IC) is recorded.

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A curve is then drawn between output current I C and output voltage VCE at constant input current
IB (0 μA).

Cutoff Region

When the base current or input current IB = 0 μA, the transistor operates in the cut-off region. In
this region, both junctions are reverse biased.

Active Region

Next, the input current (IB) is increased from 0 μA to 20 μA by adjusting the input voltage (VBE).
The input current (IB) is kept constant at 20 μA.

While increasing the input current (IB), the output voltage (VCE) is kept constant at 0 volts.

After we kept the input current (IB) constant at 20 μA, the output voltage (VCE) is increased from
zero volts to different voltage levels. For each voltage level of output voltage (V CE), the
corresponding output current (IC) is recorded.

A curve is then drawn between output current I C and output voltage VCE at constant input current
IB (20 μA). This region is known as the active region of a transistor. In this region, emitter-base
junction is forward biased and the collector-base junction is reverse biase

Saturation Region

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When output voltage VCE is reduced to a small value (0.2 V), the collector-base junction
becomes forward biased. This is because the output voltage V CE has less effect on collector-base
junction than input voltage VBE.

As we know that the emitter-base junction is already forward biased. Therefore, when both the
junctions are forward biased, the transistor operates in the saturation region. In this region, a
small increase in output voltage VCE will rapidly increases the output current I C.

An application circuit requires a voltage controlled resister component. Which component would
you prefer? Enumerate the characteristics of component, Which satisfies the requirement N/D18

A voltage-controlled resistor (VCR) is a three-terminal active device with one input port and two output
ports. The input-port voltage controls the value of the resistor between the output ports. VCRs are most
often built with field-effect transistors (FETs). Two types of FETs are often used: the JFET and the
MOSFET

FET stands for "Field Effect Transistor" it is a three terminal uni polar solid state device in
which current is control by an electric field.

FET can be fabricated with either N- Channel or P- Channel, for the fabrication of N-Channel
JFET first a narrow bar of N-type of semiconductor material is taken and then two P-Type
junction are defused on opposite sides of it's middle part, called channel. The two regions are
internally connected to each other with a signal lead, which is called Gate terminal. One lead is
called Source terminal and the other is called Drain terminal.

Construction of FET

P-Channel JFET is similarly is constructed except that it use P- type of bar and two N- types of
junctions.

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Source:- It is the terminal through which majority carriers are entered in the bar, so it is called
Source.

Drain:- It is the terminal through which the majority carriers leads the bar, so it is called the drain
terminal.

Gate:- These are two terminals which are internally connected with each other and heavily
doped regions which form two PN-Junctions.

Figure1

Operation

Figure 2

When NO bias is applied: In this case, no bias is applied to the gate terminal. It means the gate
to source voltage (Vgs) in this case is 0. Besides, the voltage at drain terminal is also 0. In this
case, the width of depletion region will remain constant. The electrons will flow from source
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terminal to gate terminal. The direction of flow of current is opposite to the direction of flow of
electron. Thus, current will flow from drain to source terminal.

When small negative bias is applied: When a small negative voltage is applied to the gate
terminal i.e. when the gate to source voltage is negative, then the width of depletion region starts
increasing.
Simultaneously, the positive voltage is applied to the drain to the source terminal. The N-channel
is moderately doped while P-channel is highly doped. Due to this the width of the depletion
region is more in N-Channel than in P-channel.

The wedge shape depletion layer so formed will reduce the magnitude of the current through the
N-channel. This is because as the width of the depletion layer increases the space provided for
electrons to flow from source to drain will decrease and eventually the drain current decreases.

The width of the depletion region is more near the drain terminal and less near the source
terminal. This is because the gate is more negative at the points which are nearer to drain than to
source. Besides, the current flowing from drain to source flows from the region of high
resistance to low resistance, thus the voltage drop will be created.

When the Large negative bias is applied: When the large negative gate to source voltage is
applied, then this large negative electric field will contribute to increment of the width of the
depletion region.

Figure 3

The negative voltage at gate terminal implies that the P-terminal is connected to negative
terminal of battery while N terminal is connected to the positive terminal. This forms the reverse
biased PN junction.

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The reversed biased PN junction will create the depletion region and more the reverse voltage
more will be the width of the depletion region. Thus, at a certain point of negative voltage, a
point will reach when the drain current completely cuts off, and the depletion layer from both the
sides will almost touch each other at region nearer to drain terminal as shown in figure 3

This point of voltage is called pinch-off voltage. Thus, the Pinch-off voltage can be defined as
the reverse voltage applied at the gate to source terminal such that the drain current will cease
completely. At this point of time, no current will flow from the junction field effect transistor

Drain Characteristics

The drain characteristics are plotted for drain current ID against drain source voltage VDS for
different values of gate source voltage VGS. The overall drain characteristics for such various
input voltages is as given under.

As the negative gate voltage controls the drain current, FET is called as a Voltage controlled
device. The drain characteristics indicate the performance of a FET. The drain characteristics
plotted above are used to obtain the values of Drain resistance, Transconductance and
Amplification Factor.

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Brief about the operation of an N channel depletion type MOSFET with neat diagram.
Enumerate characteristics with suitable graph. List the advantages and Disadvantages of
MOSFET.

Definition: MOSFET is an acronym for Metal Oxide Semi-Conductor Field Effect Transistor. It
is a device in which the variation in the voltage determines the conductivity of the device. It is a
semiconductor device that belongs to FET family.

MOSFET is also known as IGFET i.e., insulated gate field effect transistor

MOSFETs are of two types:

1. Depletion type MOSFET

2. Enhancement only MOSFET

It has a gate terminal which is made insulated by an oxide layer so as to prevent direct
contact with the substrate.

This insulated gate feature of MOSFET is responsible for infinite impedance on the practical
basis because no flow of current is noticed in between the gate and the channel.
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N Channel Depletion Mode MOSFET

The diagram shown below describes the construction of a depletion type MOSFET

As we can see in the diagrams shown above for the construction of an N-channel DE-MOSFET a
P-type substrate is used. This lightly doped P-type substrate contains two heavily doped N-type
material thus forming source and drain.

A thin layer of SiO2 is deposited over the surface and holes are then cut through SiO 2. Metals are
deposited through holes which resultantly forms drain and source terminal. A metal plate is also
deposited in between the source and drain terminal which acts as gate terminal for the device.

SiO2 is a type of insulator referred to as dielectric, which generates an opposing electric field
when subjected to an externally applied field.

Working of Depletion Mode MOSFET

In a DE-MOSFET when the gate potential is made negative with respect to the substrate, it
causes repulsion of negative charge carriers out of the initially formed channel. This increases
the channel resistance which resultantly reduces the drain current.

So, from the above discussion, we can conclude that in a DE-MOS, more negative the gate
voltage, the less the drain current that flows through the channel

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N Channel Enhancement Mode MOSFET

This is a type of MOSFET in which no any channel is doped between the source and drain at the
time of construction. In E-MOS, a positive gate to source voltage is required for the channel to
induce electrically. It requires large positive gate voltage for its operation.

Fig: N Channel Enhancement Mode-Structure

Operation

In the case when the gate terminal is made positive with respect to the substrate, more number of
electrons gets attracted towards the channel. Thus, causing more current to flow through the
channel.

When the gate to source voltage is made 0, E-MOS does not conduct. Due to this reason, it is
called normally-off MOSFET. When the positive gate voltage exceeds the threshold value then
drain current starts to flow through the device.

Consider a case when a positive drain to source voltage is applied and the gate terminal is
at 0 potential. In this case, the P-type substrate and the two N regions behave as two PN
junctions connected back to back and P substrate provides the resistance.

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In this condition, both junctions cannot be forward bias simultaneously leading to very small
drain current which is a reverse leakage current.

When the gate is increased due to opposite polarity electrons are attracted and free electrons are
added to those already in the channel .so total electrons are increased and more drain current
flows.

Characteristic Curve of Depletion MOSFET

The drain characteristics of a typical N-channel MOSFET is shown in the diagram below-

The bottom curve shows the condition when no gate voltage is applied due to which a negligible
value of drain current flows from source to drain.

The curve at the upper portion shows the condition when gate voltage V GS is made positive and
lower curves indicate the condition for negative gate voltage

Characteristics Curve of E – MOSFET

The characteristic curve shows various values of VGS for which variation in ID is shown-

From characteristics drain current flows increased and when VGS is increased.

Advantages of MOSFET:

1. The operational speed of MOSFET is higher than that of JFET.

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 2. Input impedance is much higher as compared to JFET.
3. It can be easily used in case of high current applications.
4. These devices provide an easy manufacturing process.
Disadvantages of MOSFET: It is a delicate device and is easily destroyable.

1. Excessive application of gate to source voltage VGS may destroy the thin SiO2 layer.

Draw the basic construction and equivalent circuit of a UJT. Explain the device operation
and characteristics. List the applications of UJT

UJT stands for Unijunction Transistor. The unijunction transistor or UJT is a three-terminal
semiconductor device that has only one PN-junction. The unijunction transistor is widely used in
several electronic circuits like free-running oscillator circuits, synchronized oscillator circuits,
and low to moderate-frequency pulse generators.

Construction of UJT

The unijunction transistor consists of three terminals namely emitter (E), and two bases
(B1 and B2). Since it has two bases and one PN junction, hence it is sometimes also called
a double-base diode.

The base of the UJT is made up of a lightly doped n-type semiconductor (usually silicon), and
two metallic contacts are attached to its ends and are designated as B1 and B2. The emitter is
made up of a heavily doped p-type semiconductor. These two semiconductors give a single PN-
junction in the device.

In a practical UJT, the emitter is fabricated closer to the base B2 than B1 which makes the
unijunction transistor an unsymmetrical device. There is a resistance between the two
bases B1 and B2, with the emitter open-circuited. This resistance is known as Interbase
resistance.

The schematic diagram and circuit symbol for a unijunction transistor is shown in figure-1

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 Figure 1

Figure 2: UJT Equivalent Circuit

● The resistance of the silicon bar is known as the inter-base resistance. Its value varies
from 4 kΩ to 10 kΩ.
● The resistance RB1 is the resistance of the material between the emitter and B1 region.
The value RB1 is variable and it depends upon the bias voltage across the PN-junction.
● The resistance RB2 is the resistance of the material between the emitter and the
B2 region.
● The emitter pn-junction represents a diode.
● When no voltage is applied to the UJT, the value of inter-base resistance is given by

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The value of the intrinsic standoff ratio of UJT (ƞ ) generally lies between
0.51 and 0.82.

The Peak Point Voltage (VP) of the UJT

Figure 3

When a positive voltage is applied at the emitter as shown in figure 3, the PN-junction
remains to reverse-biased till the input voltage is less than the internal voltage (Vi). When the
applied voltage becomes greater than Vi, the PN-junction becomes forward-biased. Thus, the
holes started to move from the positive terminal (B2) to the negative terminal (B1).

The accumulation of holes in the emitter to B1 region decreases the resistance of the n-type
semiconductor bar. Consequently, the internal voltage drop from the emitter to B1 is decreased
and hence the emitter current IE is increased. With the accumulation of a large number of holes,
a condition of saturation will eventually be reached. In this condition, the emitter current is
limited by the emitter power supply, and the unijunction transistor is now said to be in the ON
state

Characteristics Curve of UJT

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 Figure 4 UJT Characteristics

The characteristics curve of the unijunction transistor is shown in figure-4 We can explain the
characteristics of a unijunction transistor by the following three parameters-

● Cut-off Region
● Negative Resistance Region
● Saturation Region

1. Cut-off Region

The part of the characteristics curve where the unijunction transistor does not get sufficient
voltage to turn on is called the cut-off region. In this region, the unijunction transistor remains
off-state

2. Negative Resistance Region

In the negative resistance region, the unijunction transistor(UJT) receives enough voltage to turn
on. In this region, when we increase the voltage applied to the emitter terminal, it attains its peak
value (VP) after a certain time. After this point, the voltage drop across the device starts
decreasing, and this reduction stops at a point of voltage VV, this point is called the valley point.
Although, the current through the device still increasing. Therefore, the resistance of the
unijunction transistor is found to be negative in this region so it is called the negative resistance
regio

.3. Saturation Region

The saturation region is the area on the characteristics curve of the unijunction transistor in
which the voltage and current increase if the emitter voltage is increased.

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Advantages of UJT

● It is a low-cost device.
● Excellent characteristics
● It draws low power under normal operating conditions
Applications of UJT

The unijunction transistor (UJT) is used in the following applications-

● It is used as a relaxation oscillator.

● It is used as a voltage regulator.
● It is used for the generation of saw tooth waveforms.
● It is most widely used as a triggering device for SCRs.
● It may also be used as a phase control circuit, etc.
With neat sketches, discuss about the construction, working and characteristics of IGPT.
List the advantages, disadvantages and applications of IGPT

IGBT standards for Insulated Gate Bipolar Transistor. It is another development in the power
electronics field also called COMFET (conductivity modulated field-effect transistor). IGBT
combines the properties of BJT (bipolar junction transistor) and MOSFET (metal-oxide
semiconductor field-effect transistor).

Symbol of IGPT

The figure-a above shows the symbol of IGBT which is the same as that of n-channel MOSFET
with an arrow in the drain terminal representing injecting contact. The symbol in figure-b
represents when IGBT is considered as the basic BJT with MOSFET gate input.

Structure of IGPT

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The below shows the construction of IGBT with three terminals emitter, collector, and gate
which sometimes are also called the source, drain, and gate terminals respectively. The doping of
each layer in IGBT is similar to layers of vertical MOSFET structure

The main difference between the construction of IGBT and MOSFET is it has an additional
p+ layer known as injecting layer which forms the collector of IGBT. This layer is heavily doped
with an intensity of 1019 per cm3.

Depending upon the existence of the n+ layer, IGBTs are classified into two types,

● Non-punch Through IGBTs (Symmetric IGBTs), and

● Punch Through IGBTs (Asymmetric IGBTs).

Working of IGBT

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When collector is made positive with respect to the emitter, the IGBT is forward biased. When
voltage is not applied between gate and emitter, the junction J2 between N- region and P region
is reverse biased, hence no current flows from collector to emitter.

When gate is applied with positive voltage with respect to emitter by source VG, with gate-
emitter voltage greater than threshold voltage (VGET) of the IGBT, then an n-channel or inversion
layer is induced in the upper part of p-region just below the gate. This n-channel short circuits
the N--region with the N+-emitter regions.

Electrons start to flow from N+ region to N- region through the n-channel. Since the IGBT is
forward biased, hence the P+ collector region injects holes into the N- drift region, so that the
injection carrier density in the N- drift region increases extensively and in turn, the conductivity
of N- region increases significantly. Hence the IGBT gets turned ON and starts to conduct
forward collector current IC.

The collector current (IC) or emitter current (IE) consists of two components viz. one is Hole
Current (Ih) due to the injected holes and another is Electron Current (I e) due to injected
electrons. Hence,

The collector or load current = emitter currentIC=Ih+Ie

Since the major component of the collector current is due to the electron current (Ie). Hence,

Ic≅IE

The ON state voltage drop in IGBT is,

VCE.on = Voltage Drop[in−channel +across drift in N− region +across

forward biased P+ N− junction J1]

Characteristics of IGBT

The figure below represents the static I-V characteristics of n channel IGBT

Static I-V Characteristics

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From the above characteristics, curve is plotted between collector current I C and collector-emitter
voltage VCE for different values of gate emitter voltage VGE. This signifies that the IGTB is a
voltage-controlled device.

In the absence of gate emitter voltage, then due to the reverse biased condition of junction J 2 no
current flows through the device. When gate emitter voltage is provided to the IGTB, then even
if the supplied voltage is less than the threshold value then also, no flow of current will take
place through the device thus it is in the off-state. But for various values of VGE above the
threshold different current flows through the device.

Transfer Characteristics of IGBT

The transfer characteristics of IGBT is the graph between the collector current (I C) and gate-
emitter voltage (VGE). When the value of VGE is smaller than threshold voltage (VGET), the IGBT
remains in the OFF state.

Advantages of IGPT

● Lower gate-driver requirements

● Lower switching losses
● Smaller snubber circuit requirement
● Less size and cost
● High efficiency

Disadvantages of IGBT:
The disadvantages of IGBT are,

● The cost of IGBT is more than MOSFET and BJT.

● Latching up problem.
● The switching speed of IGBT is between BJT and MOSFET.
● Higher turn OFF time compared to MOSFET.

Applications of IGBT
● IGBT are used in dc and ac motor drives

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 ● Used in UPS systems
● Used in power supplies and drivers for solenoids, relays and contactors
● In inverter circuits for ac motor drives.
● In chopper circuits for dc to dc power conversion

Outline the structure of a SCR and explain its operation. Also illustrate its V-I
Characteristics. List the applications of SCR .A/M’19

Definition: Thyristor is a semiconductor device which comprises of four layers made up of P-

type and N-type material arranges in the alternate fashion. The word Thyristor is formed from
two words thyratron and transistor. Besides, the characteristics possessed by a thyristor is the
combination of the properties of thyratron and transistor

Basic thyristor / SCR structure

The thyristor consists of a four layer PNPN structure with the outer layers are referred to as the
anode (P-type) and cathode (N-type). The control terminal of the thyristor is named the gate and
it is connected to the P-type layer located next to the cathode.

As a result the thyristor has three junctions rather than the one junction of a diode, and two
within transistors.

The three junctions are normally denoted as J1, J2, and J3. They are numbered serially with
J1 being nearest to the anode.

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WORKING
Load is connected in series with anode the anode is always kept at positive potential w.r.t
cathode.

(i) When gate is Open

No voltage applied to the gate, j2 is reverse biased while j1 and j3 are FB . J1 and J3 is
just in npn transistor with base open .no current flows through the load RL and SCR is cut off. if
the applied voltage is gradually increased a stage is reached when RB junction J2 breakdown .the
SCR now conducts heavily and is said to be ON state. the applied voltage at which
SCR conducts heavily without gate voltage is called Break over Voltage

(ii) When Gate Is Positive W.R.T Cathode.

The SCR can be made to conduct heavily at smaller applied voltage by applying small
positive potential to the gate.J3 is FB and J2 is RB the electron from n type material start moving
across J3 towards left holes from p type toward right. electrons from j3 are attracted
across
junction J2 and gate current starts flowing. as soon as gate current flows anode current increases.
the increased anode current in turn makes more electrons available at J2 break down and SCR

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starts conducting heavily. the gate looses all control if the gate voltage is removed anode current
does not decrease at all. The only way to stop conduction is to reduce the applied voltage to zero.

BREAKOVER VOLTAGE

It is the minimum forward voltage gate being open at which SCR starts conducting
heavily i.e turned on

PEAK REVERSE VOLTAGE ( PRV)

It is the maximum reverse voltage applied to an SCR without conducting in the reverse direction

. HOLDING CURRENT
It is the maximum anode current gate being open at which SCR is turned off from on
conditions.

FORWARD CURRENT RATING

It is the maximum anode current that an SCR is capable of passing without destruction

CIRCUIT FUSING RATING It is the product of of square of forward surge current

and the time of duration of the surge

V-I Characteristics of SCR

FORWARD CHARCTERISTICS When anode is +ve w.r.t cathode the curve between V &
I is called Forward
characteristics. OABC is the forward characteristics of the SCR at Ig =0. if the supplied voltage i

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s increased from zero point A is reached .SCR starts conducting voltage across SCR suddenly dr
ops (dotted curve AB) most of supply voltage appears across RL

REVERSE CHARCTERISTICS

When anode is –ve w.r.t.cathode the curve b/w V&I is known as reverse characteristics
reverse voltage come across SCR when it is operated with ac supply reverse voltage is increased
anode current remains small avalanche breakdown occurs and SCR starts conducting heavily is
known as reverse breakdown voltage SCR as a switch SCR Half and Full wave rectifier

Applications
● SCR as a Rectifier
● SCR as a static contactor
● SCR for power control
● SCR for speed control of d. c. shunt motor
● Over light detector
Phase Control.

Note: As per your Syllabus Biasing of BJT and FET, MOSFET is given your syllabus refer
it

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