Download RAJALAKSHMI INSTITUTE OF THCHNOLOGY

RAJALAKSHMI INSTITUTE OF THCHNOLOGY
Kuttambakkam (PO), Chennai-602 107.
EC2151
ELECTRIC CIRCUITS AND ELECTRON DEVICES
SEM:II
Branch: IT
UNIT-I
Staff-in-Charge: Mr. Sathish Babu.P
CIRCUIT ANALYSIS TECHNIQUES
Ohm’s Law
Temperature remaining constant, the potential difference (E) across the ends of a
conductor is proportional to the current (I) flowing through it.
Mathematically, V=IR
Kirchhoff's Current Law (KCL)
"The algebraic sum of all currents entering and exiting a node must equal zero"
Iin = Iout
Similarly, at any instant the algebraic sum of all the currents at any circuit node is zero.
SI = 0
Kirchhoff's Voltage Law (KVL)
"The algebraic sum of all voltages in a loop must equal zero"
E = IZ
Similarly, t any instant the algebraic sum of all the voltages around any closed circuit is zero:
E - IZ = 0
Series and Parallel Resistor Combinations
There are two basic ways in which to connect more than two circuit components:
Series and Parallel.
• For analysis, series resistors/impedances can be replaced by an equivalent
resistor/ impedance.
• Parallel resistors/impedances can be replaced by an equivalent resistor/
impedance.
Series Resistance
Two elements are in series if the current that flows through one must also flow
through the other.
SERIES
R1
R2
Req = R1 + R2 + R3
Req is equivalent to the resistor network on the left in the sense that they have the
same i-v characteristics.
Parallel Resistance
Two elements are in parallel if they are connected between (share) the same two
(distinct) end nodes.
R1
PARALLEL
1
1
1
1
 

Req R1 R2 R3
R2
Inductors
Series and parallel inductances
Where, L = Inductance in henrys
Capacitors
Series and Parallel Capacitances
Where, C = Capacitance in farads
Mesh current method
The Mesh Current Method uses simultaneous equations, Kirchhoff's Voltage Law,
and Ohm's Law to determine unknown currents in a network. It differs from the Branch
Current method in that it does not use Kirchhoff's Current Law, and it is usually able to
solve a circuit with less unknown variables and less simultaneous equations.
Steps to follow for the .Mesh Current method of analysis:
1. Draw mesh currents in loops of circuit, enough to account for all components.
2. Label resistor voltage drop polarities based on assumed directions of mesh
currents.
3. Write KVL equations for each loop of the circuit, substituting the product IR for
E in each resistor term of the equation. Where two mesh currents intersect through
a component, express the current as the algebraic sum of those two mesh currents.
4. Solve for unknown mesh currents (simultaneous equations).
5. If any solution is negative, then the assumed current direction is wrong!
6. Algebraically add mesh currents to find current in components sharing multiple
mesh currents.
7. Solve for voltage drops across all resistors (E=IR).
Node voltage method
The node voltage method of analysis solves for unknown voltages at circuit nodes
in terms of a system of KCL equations. This analysis looks strange because it involves
replacing voltage sources with equivalent current sources.
Node voltage rules:
1. Convert voltage sources in series with a resistor to an equivalent current source
with the resistor in parallel.
2. Change resistor values to conductance.
3. Select a reference node(E0)
4. Assign unknown voltages (E1)(E2) ... (EN)to remaining nodes.
5. Write a KCL equation for each node 1,2, ... N. The positive coefficient of the first
voltage in the first equation is the sum of conductances connected to the node.
Repeat for coefficient of second voltage, second equation, and other equations.
These coefficients fall on a diagonal.
6. All other coefficients for all equations are negative, representing conductances
between nodes. The first equation, second coefficient is the conductance from
node 1 to node 2, the third coefficient is the conductance from node 1 to node 3.
Fill in negative coefficients for other equations.
7. The right hand side of the equations is the current source connected to the
respective nodes.
8. Solve system of equations for unknown node voltages.
NETWORK THEOREMS:
Thevenin's Theorem:
• Any circuit with sources (dependent and/or independent) and resistors can be
replaced by an equivalent circuit containing a single voltage source and a single
resistor.
• Thevenin’s theorem implies that we can replace arbitrarily complicated networks
with simple networks for purposes of analysis.
Norton's Theorem:
Any circuit with voltage sources, resistances (impedances) and open output
terminals can be replaced by a single current source in parallel with single resistance
(impedance), where the value of current source is equal to the current passing through the
short circuit output terminals and the value of the resistance (impedance) is equal to the
resistance seen into the output terminals.
Super Position Theorem:
In a linear, lumped element, bilateral electric circuit that is energized by two or
more sources the current in any resistor is equal to the algebraic sum of the separate
currents in the resistor when each sources acts separately. While one source is applied,
the other sources are replaced by their respective internal resistances.
Super Position Theorem is not valid for power responses. It is applicable only
for computing voltage and Current responses.
Maximum Power transfer Theorem:
The maximum Power transferred to a load resistor occurs when it has a value
equal to the resistance of the network looking back at it from the load terminals (all
sources being replaced by their respective internal resistances).
Duality:
Two electrical networks which are governed by the same type of equations are
called duality.
For the networks to be duals it is necessary that the variables & elements of one
network should also be the duals of variables & elements of other networks.
Method of drawing duality (or) dual network:
a) Place a dot in each independent loop of the given network. These dots placed
inside the loops correspond to the independent node in the dual network.
b) A dot is placed outside the given network. This corresponds to the reference node
of the dual network.
c) All the dots are connected by dotted lines crossing all the branches. The dotted
lines should cross only one branch at a time.
d) The dual elements will form the branches connecting the corresponding nodes in
the dual network.
Star-Delta Transformation:
The Star-Delta transformation techniques are useful in solving complex network.
A star network of three resistances RA, RB and RC connected together at common
node N can be transformed into a delta network of three resistances RAB, RBC and RCA by
the above equations:
In general terms:
Rdelta = (sum of Rstar pair products) / (opposite Rstar)
Delta-Star Transformation
A delta network of three impedances RAB, RBC and RCA can be transformed into a star
network of three impedances RA, RB and RC connected together at common node N by
the following equations:
In general terms:
Rstar = (adjacent Rdelta pair product) / (sum of Rdelta)
UNIT-II
TRANSIENT RESONANCE IN RLC CIRCUITS
Transient State:
If a network contains energy storage elements, with change in excitation, the
current and voltages change from one state to another state is called transient state. The
behavior of the voltage or current when it is changed from one state to another state is
called transient state.
Transient Time:
The time taken for the circuit to change from one steady state to another steady
state is called the transient time.
Natural response:
If we consider a circuit containing storage elements which are independent of
sources, the response depends upon the nature of the circuit, it is called natural response.
Transient response:
The storage elements deliver their energy to the resistances, hence the response
changes with time, gets saturated after sometime, and is referred to the transient response.
Laplace Transform:
The Laplace transform of any time dependent function f(t) is given by F(s).
Where S→A
complex frequency given by
S=σ + jω
Inverse Laplace Transform:
Inverse Laplace transforms permits going back in the reverse direction i.e. from s
domain to time domain.
Order of a System:
The order of the system is given by the order of the differential equation
governing the system. If the system is governed by nth order differential equation, than
the system is called nth order system.
Q(s) = a0 sn + a1 s n-1+ a2 s n-2 + ……..+an-1 s +an
the order of the system is equal to ‘n’.
Initial Value Theorem
The initial value theorem states that if x (t) and x’ (t) both are laplace
transformable, then
Final Value Theorem
The final value theorem states that if x (t) and x’ (t) both are laplace
transformable, then
Driving Point impedance
The ratio of the Laplace transform of the voltage at the port to the laplace
transform of the current at the same port is called driving point impedance.
Transfer Point impedance
The ratio of the voltage transform at one port to the current transform at the other
port is called transfer point impedance.
Resonant Circuit
 The circuit that treat a narrow range of frequencies very differently than all other
frequencies are referred to as resonant circuit.
 The gain of a highly resonant circuit attains a sharp maximum or minimum at its
resonant frequency.
Resonance
Resonance is defined as a phenomenon in which applied voltage and resulting
current are in phase.
Bandwidth
The Bandwidth is defined as the frequency difference between upper cut-off
frequency (f2) and lower cut-off frequency (f1).
Half Power frequencies
The upper and lower cut-off frequencies are called the half-power frequencies. At
these frequencies the power from the source is half of the power delivered at the resonant
frequency.
Selectivity
Selectivity is defined as the ratio of bandwidth to the resonant frequency of
resonant circuit.
Q factor
The quality factor, Q, is the ratio of the reactive power in the inductor or capacitor
to the true power in the resistance in series with the coil or capacitor.
Series Resonance in RLC circuit







In series RLC circuit resonance may be produced by either varying frequency for
given constant values of L and C or varying either L and C or both for a given
frequency.
At resonance inductive reactance is equal to the capacitive reactance.
If f < f0 the current I leads the resultant supply voltage V and so the circuit
behaves as a capacitive circuit at the frequencies which are less than f0.
At f = f0, the voltage and current are in phase. The circuit behaves as pure
resistive circuit at the resonant frequency with unit power factor.
If f > f0, the current I lags the resultant supply voltage V and so the circuit
behaves as an inductive circuit at the frequencies which are more than f0.
At resonance series RLC circuit acts as a voltage amplifier.
Series resonance circuit is always driven by a voltage source with very small
internal resistance to maintain high selectivity of the circuit.
Parallel Resonance
 A parallel circuit is said to be in resonance when applied voltage and resulting
current are in phase that gives unity power factor condition.
 Parallel resonance is also known as Anti resonance.
 At anti resonance the parallel resonant circuit acts as current amplifier.
Reactance curves
The graph of individual reactance versus the frequency is called Reactance Curve.
Types of Tuned circuits
 Single tuned circuit
 Double tuned circuit
Single tuned circuit
In RF circuit design, tuned circuits are generally employed for obtaining
maximum power transfer to the load connected to secondary or for obtaining maximum
possible value of secondary voltage.
A single tuned circuit is used for coupling an amplifier and radio receiver circuits.
Double tuned circuit
 In double tuned circuits, a variable capacitor is used at input as well as output
side.
 With the help of adjustable capacitive reactance, impedance matching is possible
if the coupling is critical, sufficient or above.
 It is also possible to adjust phase angle such that impedance at generator side
becomes resistive.
 The magnitude matching can be achieved by adjusting mutual inductance to the
critical value, which effectively fulfills maximum power transfer condition.
UNIT-III
SEMICONDUCTOR DIODES
Energy Bands
The range of energies possessed by an electron in a solid is known as Energy
band.
Classification of Energy Band
1. Conduction band
2. Forbidden band
3. Valence band
Classification of semiconductors
1. Intrinsic Semiconductors.
2. Extrinsic Semiconductors.
Intrinsic Semiconductors
 A Semiconductor which is in its extremely pure form is known as an intrinsic
semiconductor.
 If potential difference is applied across intrinsic semiconductor, the electrons
will move towards the positive terminal while the holes will drift towards the
negative terminal.
 The total current inside the semiconductor is the sum of currents due to free
electrons and holes.
Extrinsic Semiconductors
 The conductivity can be increased by the addition of a small amount of suitable
metallic impurity. It is also known as impurity semiconductor.
 The process of adding impurity atoms to the intrinsic semiconductor is called
doping.
 The purpose of adding impurity is to increase either the number of free electrons
or holes in a semiconductor.
 Two types of impurity atoms are added to the semiconductor.
Pentavalent impurity atoms - containing five valance electrons
Trivalent impurity atoms
- containing three valance electrons
 Extrinsic semiconductors are classified as,
N-type Semiconductor
P-type Semiconductor
N-type Semiconductor
 A small amount of pentavalent impurity is added to a pure semiconductor is
known as N-type Semiconductor.
 When a pentavalent impurity is added to a pure semiconductor, it displaces
some of its atoms. E.g. ARSENIC (As), ANTIMONY (Sb)

In N-type semiconductor, major part of the current flows due to the movement
of Electrons. Therefore electrons in an N-type semiconductor are known as
majority carriers and holes as minority carriers.
P-type Semiconductor



A small amount of trivalent impurity is added to a pure semiconductor is
known as P-type Semiconductor.
E.g. Gallium (Ga), Indium (In), Aluminium (Al), Boron (B) etc.
In P-type semiconductor, major part of the current flows due to the movement
of holes. Therefore holes in a P-type semiconductor are known as majority
carriers and electrons as minority carriers.
Conductivity of a semiconductor
Each hole-electron pair created two charge carrying particles is formed. One is
negative of mobility µn (free electron) and the other is positive of mobility µp (hole).
These particles move in opposite direction in an electric field.
Hence the current density J = σE
Where n , p = magnitude of free electrons & holes respectively.
σ = Conductivity of semiconductor
E= applied electric field
q = Charge of electron or hole.
Hence σ = (nµn + pµp )E
where ni – intrinsic semiconductor
For a pure semiconductor n = p = ni ;
CARRIER CONCENTRATIONS IN AN INTRINSIC SEMICONDUCTOR
In order to calculate the conductivity of a semiconductor, it is necessary to know the
concentration of free electrons n and the concentration of holes p.
Concentration of Electrons (n)
The number of electrons in the conduction band, n = Nc e- (EC –EF)/KT
Where,
2πmn KT
Nc = 2
h2
3/2
Concentration of holes (p)
The number of holes in the conduction band, p = Nv e- (EF –EV)/KT
Where,
2πmp KT
3/2
Nv = 2
h2
Energy Gap (EG)
The energy required to break a covalent bond in a semiconductor is known as
energy gap. The Energy gap at any temperature is given by,
EG = EGO – βT
Drift Current
Drift current is defined as the flow of electric current due to the motion of the
charge carriers under the influence of an external electric field applied across the
semiconductor material.
Diffusion Current
In a semiconductor material, the charge carriers have the tendency to move from
the region of higher concentration to that of lower concentration of the same type of
charge carriers. This movement of charge carriers takes place resulting in a current called
diffusion current.
Diffusion current density due to holes,
Jp = - q Dp dp/dx A/cm2
Diffusion current density due to electrons, Jn = - q Dn dn/dx A/cm2
Total Current Density
Total current is the sum of drift current and diffusion current.
The total current density for P-type semiconductor Jp = - qpµpE- q Dp dp/dx
The total current density for N-type semiconductor Jn = - qnµnE- q Dn dn/dx
Diffusion Length (L)
The average distance that a charge carrier can diffuse during its lifetime is called as
diffusion Length L.
Theory of PN Junction Diode
When a P-type semiconductor s joined to a N-type semiconductor the contact surface
is called PN junction or PN diode.
The voltage across PN junction can be applied in two ways.
(i)
(ii)
Forward biasing
Reverse biasing
The N-type material has high concentration of free electrons and, P-type material
has high concentration of holes. At the junction, there is a tendency for the free N-type of
diffuse over to the P-side and holes from the P-side to the N-side. This process is called
diffusion.
Thus a barrier is set up against further movement of charge carriers. This is called
Potential barrier or Junction barrier (VB).The potential barrier is of the order of 0.1V
to 0.3 V.
The mobile charges have been depleted in this region. It is known as depletion
layer.
ZENER DIODE
A zener diode is a special purpose diode that is operated in reverse-biased
conditions. Its operation depends on the zener breakdown phenomenon.
Symbol
Anode
Cathode
V-I Characteristics of Zener diode
The operation of zener diode is same as that of ordinary p-n diode order forward
biased condition, whereas under reverse biased condition breakdown of the junction
occurs.
Breakdown voltage depends upon the amount of doping. If the diode is heavily
doped, depletion layer will be thin and consequently breakdown occurs at lower reverse
voltage and further, the breakdown voltage is sharp. The breakdown voltage can be
selected with the amount of doping.
The sharp increase in current under breakdown condition is due to the following
two mechanisms.
 Avalanche breakdown
 Zener breakdown
Avalanche breakdown
When doping concentration is less like in ordinary diode then under reverse
biased condition a small amount of reverse saturation current flows and is constant as
long as the temperature is constant.
When the reverse voltage is increased width of the depletion layer increases at the
same time the electrons due to force of attraction by the plates acquire some high velocity
and during their motion inside the diode they collide with the electrons in covalent bonds
and bring them out.
Due to this multiplication process a large current flows and this kind of
breakdown is called Avalanche multiplication or breakdown. Once when this
breakdown occurs the diode gets damaged.
Zener breakdown
When doping is heavy then in reverse bias even-before the minority charge carries
acquire sufficient velocity the breakdown occurs and is known as Zener breakdown.
In reverse bias under heavy doping condition the width of the depletion layer will be
very thin strong electric field exists inside the diode. When reverse voltage increased at
once electric field the electrons which are present in the covalent bond are brought due to
strong force of attraction. Now, suddenly a large amount of current flows. Nothing but
Breakdown occurs. In Zener diode first zener breakdown occurs and later avalanche
breakdown.
Applications of Zener diodes
 Voltage regulator
 Fixing reference voltages in electronic circuits such as power supplies and
transistor biasing.
 Clippers in wave-shaping circuits.
 Square wave generation.
UNIT-IV
TRANSISTORS
Introduction of transistors
Transistor is a semiconductor device that can amplify electronic signals such as radio and
television signals.
Advantage of the transistor
1. Smaller in size
2. No filament and no need of power for heating filament
3. Low operating voltage
4. Higher efficiency
Types of the transistor
 Unipolar Junction Transistor
 Bipolar Junction Transistor
Construction of the transistor
 n-p-n transistor
 p-n-p transistor
n-p-n transistor
It is formed by sand witching p-type semiconductor between two n-type.
p-n-p transistor
It is formed by sand witching n-type semiconductor between two p-type.
Terminals for the transistor
 Emitter
 Collector
 Base
Functions of Emitter, Collector & Base:
Emitter : To supply majority charge carriers.
Collector: To collect majority charge carriers.
Base:
It passes most of the injected charge carriers to the collector.
Transistor Biasing
 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.
 Active region.
 Cut-off region.
 Saturation region.
Sl.No.
1
2
3
Region
Active
Cut-off
Saturation
Emitter Base
Forward biased
Forward biased
Forward biased
Collector Base
Reverse biased
Reverse biased
Reverse biased
Operation of a transistor
acts as an amplifier
acts as an open switch
acts as an closed switch
Operation of NPN transistors






Emitter is forward biased & as a result large forward current flows across the
emitter junction due to flow of majority carriers.
Injected electrons diffuse into the collector region due to the extremely small
thickness of the base.
Collector is reverse bias and creates a strong electrostatic field between base
&collector.
Field immediately collects the diffused electrons which enter the collector
junction.
Flow of electrons into the base region when confronted with the holes, a few
electrons combine & neutralize
Rest of the electrons of the injected electrons diffuse into the collector region and
is collected by the collector electrode.
Operation of PNP transistors







Forward bias causes the holes in the P-type emitter to flow towards the base.
Reduces the potential barrier at the junction
Holes cross the junction & penetrate into the N-region. This constitutes emitter
current IE.
Width of the base region is very thin & lightly doped; hence a small amount of the holes
recombine with free electrons of N-regions. This constitutes base current IB & is very small.
Rest of the holes drift across the base and enter the collector region and are swept
away by the negative collector electrode. This constitutes base current IC.
Current conduction I PNP transistors is by movement of holes.
Current conduction in the external circuit is by electrons.
Types of configuration
 Common Base configuration
 Common Emitter configuration
 Common Collector configuration
Common Base configuration
 Input is connected between emitter & base. Output is connected between collector
&base.
 Emitter-base junction is forward biased. Collector-base junction is reverse biased.
 Emitter current IE flows in the input circuit. Collector current IC flows in output
circuit.
 The ratio of collector current IC, to emitter current IE, is called the Current
amplification factor (α).
 If there is no input ac signal, then the ratio of IC to IE is called dc alpha (αdc).
 ac alpha refers to the ratio of change in IC to change in IE.
 The higher the value of α, better the transistor. α can be increased by making base
thin and lightly doped.
Characteristics of CB configuration
The performance of transistors, when connected in a circuit, may be determined
from their characteristic curves that relate different d.c. currents and voltages of a
transistor.Such curves are known as Static characteristic curves.
Input Characteristics
 The curve drawn between Emitter current and Emitter – Base voltage for a given
value of collector-Base voltage is known as input Characteristics.
 For a given value of VCB,the curve is just like a forward-biased PN junction.
 With an increase in the value of VCB,it conducts better. This is because of the
effect called early effect or Base width modulation.
Output Characteristics
 The curve drawn between Collector current and Collector – Base voltage for a
given value of emitter current is known as output Characteristics.
 The collector current varies with VCB for very low voltage but transistor is never
operated in this region.
Common Emitter configuration
 Input is connected between base & emitter. Output is connected between collector
& emitter.
 Emitter-base junction is forward biased. Collector-base junction is reverse biased.
 Base current IB flows in the input circuit. Collector current IC flows in output
circuit.
 CE is commonly used because its current, voltage and power gains are quite high
and output to input impedance ratio is moderate.
 The rate of change in collector current IC, to change in emitter current IE, is called
amplification factor (β).
Input Characteristics
 The curve drawn between Base current and Base – Emitter voltage for a given
value of collector-emitter voltage is known as input Characteristics.
 For a given value of VEC,the curve is just like a forward-biased PN junction diode.
 Input resistance is larger in CE configuration than in CB configuration. This is
because the input current IB increases less rapidly with increase in VBE .
 An increment in the value of VCE, causes the input current IB to be lower for a
given level of VBE . This is because of the effect called early effect.
Output Characteristics
 The curve drawn between Collector current IC and Collector – emitter voltage VCE
for a given value of base current IB is known as output Characteristics.
 Output characteristics in CE configuration have some slope while CB
configuration has almost horizontal characteristics. This indicates that output
resistance in case of CE configuration is less than that in CB configuration.
Common Collector configuration
 Input is connected between base & collector. Output is connected between
collector & emitter.
 The Collector forms the terminal common to both the input and output.
 Base current flows in the input circuit. Emitter current flows in output circuit.
 With base current IB equal to VCO, the emitter current IE is zero, so no current
flows in the load resistor RL.
 With increases in input current IB, the transistor passes through the active region
and finally reaches saturation.
Input Characteristics
 To determine the input Characteristics, VEC is kept at a suitable fixed value.
 The base-collector voltage VBc is increased in equal steps and the corresponding
increase in IB is noted.
 This is repeated for different values of VEC.
Breakdown in Transistors
 Avalanche Multiplication
 Reach-Through (or) Punch through
Avalanche Multiplication
 The maximum reverse bias voltage which can be applied before breakdown
between collector and base terminals of the transistor under the condition that the
emitter is open-circuited.
 It is represented by the symbol BVCBO (for CB configuration).
 This breakdown voltage is a characteristic of the transistor alone.
 Breakdown occurs because of the avalanche multiplication of current ICO that
crosses the collector junction.
 As a result of this multiplication, the current becomes MICO in which M is the
factor by which the original current ICO is multiplied by the avalanche effect.
 At a high voltage BVCBO, the multiplication factor M becomes infinite and the
region of breakdown is then attained.
 The current increases abruptly and large changes in current accompanies small
changes in voltage.
Reach-Through (or) Punch through
 It results from Early effect (i.e.) as a result of increase in VCB and as the doping of
the base is substantially smaller than that of the collector and the penetration of
the transition region into the base is larger than into the collector
 Since the base is very thin, the transition region spreads completely across the
base to reach the emitter junction.
 At this point, normal transistor action ceases and the emitter and collector are
effectively shorted.
 Hence, a large current flows from the emitter to collector. This is called Reachthrough.
Field Effect Transistor (FET)
 FET is a semiconductor device which depends for its operation on the control of
current by an electric field.
 The output characteristics of FET are controlled by Input voltage and not by the
Input current.
 So, it is also known as voltage-controlled device.
Features of FET
The FET has several advantages over the conventional transistor.
 Its operation depends upon the flow of majority carrier only. So, it is called as
Unipolar device.
 It is relatively immune to radiation.
 It exhibits a high input resistance, typically many mega ohms.
 It is less noisy than a tube of a Bipolar Transistor.
 It exhibits no offset voltage at zero Drain current.
 It has thermal stability.
Types of FET
 Junction Field Effect Transistor (JFET)
 Metal Oxide Field Effect Transistor (MOSFET) (or)
Insulated Gate Field Effect Transistor (IGFET)
Construction of JFET
 JFET is a three terminal semiconductor device in which current conduction is by
one type of carrier either Electrons or holes.
 The JFET consists of a P-type or N-type silicon bar.
 The bar is made up of N-type material which is known as N-channel JFET and if
the bar is made up of P-type material, it is known as P channel JFET.
 The current in FET is carried by the majority carriers.
 One end of the channel is called the source and the other is called the drain.
Operation of JFET
FET works under the three conditions.
 When VGG applied and VDD=0
 When VDS applied and VGG=0
 When VDD applied and VGG is applied.
Where,
 VGG – Gate supply voltage.
 VDS– Drain Source voltage.
 VDD– Drain supply voltage.
Characteristics of JFET
 A family of curves that relate the current and voltage are known as characteristics
curve.
 There are the two important characteristics of a JFET.
 Transfer characteristics
 Drain characteristics
Characteristics Parameters of JFET
The parameters of JFET are
 Transconductance
 Drain resistance
 Drain conductance
 Amplification factor
Metal Oxide Semiconductor FET (MOSFET)
 MOSFET is a three terminal device. Those terminals are source, gate and drain.
 The gate of a MOSFET is insulated from the channel.
 Because of this, the MOSFET is also known as an IGFET (Insulated gate FET).
 The MOSFET is a second category of FET.
 The MOSFET differs from the JFET is that it has no pn junction structure; instead
the gate of the MOSFET is insulated from the channel by a silicon dioxide layer.
Types of MOSFET
 Depletion – type MOSFET
 Enhance – type MOSFET
Construction of MOSFET
 Two highly doped n regions are diffused into a lightly doped p type substrate.
 These two highly doped regions are represents source and drain. In some cases
substrate is internally connected to the source terminal.
 The source and drain terminals are connected through metallic contacts the ndoped regions linked by an n-channel.
 The gate is also connected to a metal contact surface but remains insulted from
the n-Channel by a very thin layer of dielectric material, Silicon Dioxide.
 This layer act as one parallel plate capacitor.
 Thus, there is no direct electrical connection between the gate terminal and the
channel of a MOSFET increasing the input impedance of the device.
Characteristics of MOSFET
The different characteristics of a D-MOSFET are
 Drain characteristics
 Transfer characteristics
UNIT-V
SPECIAL SEMICONDUCTOR DEVICES
Tunnel Diodes




When the impurity concentration is of the order of one part to 103 parts then
tunnel diode is formed.
This diode has negative resistance region.
Due to which it is used as an oscillator.
This diode is uses the tunneling phenomenon.
Tunneling
The process that an electron from n-side of a pn diode directly penetrates through
the junction into the p-side of diode is called tunneling. It is a quantum –mechanical
behaviors.
Operation
 When a tunnel diode is under unbiased condition then there will not transfer of
electrons from n-side to p-side hence the net current will be zero.
 When the diode is reverse biased under this condition the electrons from n-side
are attracted by the positive plate and hence move away from the junction.
 As a result the energy level in the n-side decreases when compared to the
unbiased state.
 Now, there will be some empty state in valence band of p-side quite opposite to
the empty conduction band.
 Hence tunneling takes place from p to n-side.
 As reverse bias is increased this current increase.
Applications
 Tunnel diode is used as Ultra-high speed switch.
 Used in relaxation oscillator.
 Used as an amplifier.
 Used as logic memory storage device.
 Used as microwave oscillator.
Advantages
 High speed operation
 Ease of operation
 Low noise
 Low cost
 Low power
Disadvantages
 It is two terminal device, there is no isolation between the input and output circuit.
 Voltage range over which it can be operated is 1 V or less.
PIN Diode



It has highly improved switching time in comparison with a PN diode.
PIN diodes are used in microwave switches.
In PIN diode high resistivity intrinsic layer is sandwiched between the P and N
regions. This results in improved switching time.
 Quite often instead of I-region we actually use either a high resistivity P-region is
called π region and the high resistivity N-region is called γ region.
 The I-region has typically resistivity of 10 Ωm.
Applications of PIN Diode
 Used as pulse and phase shifter.
 Used as SPST and MPST switches.
 Used in amplitude modulation.
 Used as photo detectors in fiber optic systems.
 Used as T-R switch.
 Used as attenuator and duplexer.
Varactor diode



Varactor diode is a specially manufactured reverse biased PN junction diode with
a suitable impurity concentration profile.
It is also called as varicap or voltacap.
It is used as a variable reactance capacitance.
Characteristics of Varactor diode
 The diode conducts normally in the forward direction.
 At relatively low voltage the reverse current saturates and then remains constant.
 It is rising rapidly at avalanche point.
 At the saturation point the maximum junction capacitance is obtained and a point
just above avalanche the minimum junction capacitance is obtained.
 Therefore there are two conditions which are limiting the reverse voltage swing
and the capacitance variation.
Applications
 Used as a tuning device in receivers.
 It is used in High frequency.
 It is used in adjustable band-pass filter
 It is used in FM modulation.
 It is used in automatic frequency control devices.
 It is used in parametric amplifier.
SCR




SCR consist of four semiconductor layers forming a PNPN structure.
It has three PN junctions namely J1, J2, J3.
There are three terminals called anode (A), cathode (K) and the gate (G).
The anode terminal is taken out from P1 layer, and the gate (G) terminal from the
P2 layer. It conducts the current in forward direction only.
Operation of SCR
 SCR is forward bias with a small voltage, it is in ‘OFF’ and no current flows
through the SCR.
 The applied forward voltage is increased, a certain critical voltage called forward
break over voltage (VBO).It reaches at the junction J2 breakdown. At this case the
SCR switched ‘ON’ position.
 If the SCR is reverse bias, the junction J1 and J3 are reverse bias and junction J2 is
forward bias.
 It has found that most of the voltage will drop across junction J1 only.
 When the applied reverse voltage is small, the SCR is OFF, and there is no
current flow through the device.
SCR characteristics
 It is the relationship between the anode –cathode voltage and anode current at
different gate current.
 Two types of V-I characteristics
 Forward Characteristics
 Reverse Characteristics
Forward Characteristics
 It is the current drawn between anode-cathode voltage (VAK) and anode current
(IA) at different gate current.
 Adjust the gate current to zero value by keeping the switch open.
 Increase the applied voltage across the SCR in small suitable steps at each step.
 Note the anode current & plot the graph.
Reverse Characteristics
 The reverse characteristic is obtained by reversing the connections of the d.c.
supplies VAA and VGG .
 Adjust the gate current to any suitable value.
 Increase the reverse applied voltage in suitable steps.
 Note the anode current for each steps.
 Now we plot a graph with anode current and anode cathode voltage.
Turning ON (Triggering) SCR
The SCR can be turned ON, from OFF position by anyone of the following methods.
 Gate triggering
 Forward break over voltage
 Light triggering
 Rate-effect
Latching
Once the SCR is turned ON, it starts to conduct and remains in conduction state
even when the gate signal is removed. This ability of the SCR to remain conducting, even
when the gate signal is removed, is known as latching.
Turning OFF
One of the following methods is applied to turn OFF the SCR.
 Reversing polarity of anode-to-cathode voltage called as Gate turn OFF switch
(GTO).
 The second method is anode current interruption. Changing anode current by
means of momentarily series or parallel switching arrangement.
 Third method is forced commutation. In this, the current through SCR is reduced
below the holding current.
Applications of SCR
 Power control device
 Relay control
 Regulated power supplies
 Static switches
 Motor control
 Battery charges
 Heater controls
 Phase controls
 For speed control of DC shunt motor
Advantages of SCR
 SCR controls large current in the load by means of a small gate current.
 SCR size is very compact.
 Switching speed is high.
UniJunction Transistor (UJT)
 UniJunction transistor is a three terminal semiconductor device consisting of only
one PN junction.
 It differs from ordinary PN diode in the sense that it has three terminals namely
Emitter, Base1 and Base 2.
 The behavior of UJT differs from other transistors like BJT and FET in the sense
that it has no ability to amplify.
 However, it has ability to control large ac power with a small signal.
 It also exhibits a negative resistance characteristic which allows it to be used as an
oscillator.
Applications of UJT
 Non sinusoidal oscillator
 Timing circuits
 Saw tooth generators
 Triggering device for SCR and TRIAC
 Switching circuits
 Voltage regulated supply
Diac (Diode A.C. switch)




A DIAC is two terminal semiconductor device and three layer bidirectional
device, which can be switched form of it’s OFF to ON state for either negative or
positive polarity of applied voltage.
The two leads are connected to p-region of silicon separated by an n-region. It
consists of two 4-layer diodes connected in parallel in opposite direction.
The diodes are P1N1P2N2 and P2N1P1N3.
It has two main terminals namely Main terminal 1 and Main terminal 2.
Applications of DIAC
 Temperature control
 Triggering of TRIAC
 Light dimming circuits
 Motor speed control
Triac (Triode A.C. switch)
 TRIAC is a three terminal semiconductor switching device which can conduct in
either forward or reverse direction.
 The TRIAC is the combination of two SCR’s connected in parallel but in opposite
direction.
 The anode of one SCR is connected to the cathode of another SCR.
 The gates are connected together.
 It consists of two four layer switches in parallel and the switches are P1N1P2N2
and P2N1P1N4.
 The TRIAC has two main terminals namely main terminal1 and main terminal2
and one Gate terminal.
Applications of TRIAC
 Heater control
 Phase control
 Light dimming control
 Static switch to turn a.c. power ON and OFF.
 Speed control of motor.
Light Activated SCR (LASCR)
 LASCR is similar to that of a SCR except the light triggering.
 It has a window and lens which focuses light on the gate junction area.
 It can be triggered ON by a light input on the gate area, but does not turn OFF,
when light source is removed.
 The LASCR acts like a latch.
 To reduce the holding current, it can be turned OFF.
 Depending on its size a LASCR is capable of handling larger amount of current.
 It can be handled by a photo transistor or a photo diode.
Applications of LASCR




Optical light controls
Phase control
In relays
Motor control
LASER DIODE
 The term Laser comes from the acronym for light amplification for stimulated
emission of radiation.
 The Laser medium can be a gas, liquid, amorphous solid or semiconductor.
 Two commonly used Laser structure
 PN homojunction laser
 Double hetrostructure laser
Laser Action
 The light traveling through a semiconductor, then a single photon is able to
generate an identical second photon.
 This photon multiplication is the key physical mechanism of lasing.
 The carrier inversion is the first requirement of lasing.
 It is achieved at the PN junction by providing the conduction bandwidth electrons
from the N-doped side and the Valence band with the holes from the P-doped
side.
 The photon energy is given by the band gap, which depends on the semiconductor
material. The optical feedback and the confinement of photon in an optical
resonator are the second basic requirement of lasing.
Photodiode
 It is a light sensitivity device used to convert light signal into electrical signal.
 It is also called Photo detector.
 The light energy fall on the junction through lens, when, the PN photodiode
junction is reverse bias.
 The hole-electrons pairs are created.
 The movement of the hole-electron pairs in a properly connected circuit results in
current flows.
 The current is proportional to the intensity of light and the frequency of the light
falling on the junction of the photo diode.
 It is used in demodulator, encodes and light detectors systems.
Phototransistor
 The photo transistor is a light detector.
 It combines a photodiode and phototransistor.
 The phototransistor cannot be directly used in control applications. Because, it
produces a very low current.
 Before applying to control circuit the current should be amplified.
 A lens focuses the energy on the base-collector junction.
 It has three terminal, but only two leads are generally used (emitter and collector).


The base current is supplied by the current created by the light falling on the
Base-collector photodiode junction.
In phototransistors, the current is dependent mainly on the intensity of light
entering into the lens and the voltage applied to the external circuit.
Photoconductive sensors
 Photoconductive sensor is also called as Light Depending Resistor (LDR).
 It is made of thin layer of semiconductor material (cadmium sulfide).
 There is no light falls on the sensor the resistance is very high and the current is
low.
 Hence, the voltage drop across R is high. It is used in control circuits to control
the current.
Photovoltaic sensors
 It is a light-sensitive semiconductor device, and it produces a voltage, when the
voltage increases and the intensity of light falling on the semiconductor junction
of this photovoltaic cell increases.
 It consists of a piece of semiconductor material (silicon or germanium).
 The photovoltaic cells are produced more power, as in solar cells. These are
called photovoltaic devices.
 It is used in light meters.
LIGHT EMITTING DIODE (LED)
 An LED is a semiconductor p-n junction diode which converts electrical energy to
light energy under forward biasing.
 It emits light in both visible and IR region.
 The amount of light output is directly proportional to the forward current.
 LED structure can be divided into two categories.
 Surface - emitting LED
 Edge - emitting LED
 Surface emitting LED’s emit light perpendicular to the PN junction plane.
 Edge-emitting LED emits light parallel to the PN in the plane.
Principle and Working
 Injection luminescence is the principle used in LED’s.
 When LED is forward biased, the majority charge carriers moves from p to n and
similarly from n to p region and becomes excess minority carriers.
 These excess minority carriers diffuse through the junction and recombines with
the majority carriers in n and p region respectively to produce light.
 The light thus produced is emitted from the p-n junction of the diode.
Advantages of LED
 They are smaller in size.
 Its cost is very low.
 It has long life time.





It operates LED’s are available in different colours at low cost.
even at very low voltage.
Response time of LED is very fast in the order of 10 9 seconds.
Its intensity can be controlled easily.
It can be operated at a wide range of temperature (0-70˚) C.
Applications of LED
 Used for numeric display in pocket calculators.
 Used for applying input power to lasers.
 Used for entering information into optical computer memories
 Used for solid video displays.
 Used in image sensing circuits.
Liquid Crystal Display (LCD)
 Liquid crystal display is not a semiconductor device as LED.
 LCD’s display the light, it doesn’t radiate light energy.
 Therefore, LCD’s require an external (or) internal source of light so that it can
either transmit (or) reflect the incident light.
 LCD is a passive type display device used to display alpha numeric character and
is seven segment display, watches calculators etc., in which the digits are
displayed by the transmission (or) deflection of the incident light, with very low
power consumption.
 Molecules in ordinary liquids have random orientation but in a liquid crystal they
are oriented in a definite crystal pattern.
 Types of LCD’s
o Dynamic Scattering Displays.
o Twisted nematic display (or) Field effect display
Advantages of LCD
 Low power is required
 Good contrast
 Low cost
Disadvantages of LCD
 Speed of operation is slow
 LCD occupy a large area
 LCD life span is quite small, when used on d.c. Therefore, they are used with a.c.
suppliers.
Applications of LCD
 Used as numerical counters for counting production items.
 Analog quantities can also be displayed as a number on a suitable device. (e.g.)
Digital multimeter.
 Used for solid state video displays.
 Used for image sensing circuits.
 Used for numerical display in pocket calculators.
RAJALAKSHMI INSTITUTE OF THCHNOLOGY
Kuttambakkam (PO), Chennai-602 107.
EC2151
SEM:II
ELECTRIC CIRCUITS AND ELECTRON DEVICES
Branch: CSE
Staff-in-Charge: K.KIRUBA RANI
ASSIGNMENT QUESTIONS
UNIT – I
Define the following terms: i) Mesh ii) Loop iii) Node iv) Branch
State and explain Kirchhoff’s laws
Explain voltage division & current division method using suitable example.
Derive the relationship to express three delta connected resistances into star.
Two resistances 15Ω and 20Ω are connected in parallel. A resistance of 12Ω is
connected in series with the combination. A voltage of 120 V is applied across the
entire circuit. Find the current in each resistance, voltage across 12Ω resistance
and power consumed in all the resistances.
6. A resistance R is connected in series with a parallel circuit comprising two
resistances of 12 and 8Ω. The total power dissipated in the circuit is 700 watts
when the applied voltage is 200V. Calculate the value of R.
7. Explain the loop analysis of analyzing a given network, with a suitable example.
8. State and explain Superposition theorem.
9. State and explain Thevenin’s theorem.
10. A bridge network formed by four arms is as AB=2Ω, BC=3Ω, CD=4Ω, DA=5Ω.
A 6 Ω resistance is connected between B and D. A battery source of 9V is
connected with internal resistance of 1 Ω between A and C such that A is +ve and
C is –ve. Calculate current through 6Ω resistance by, i) Norton’s theorem ii)
Thevenin’s theorem.
1.
2.
3.
4.
5.
1.
2.
3.
4.
5.
UNIT – II
Determine the expression of resonant frequency and bandwidth of a series
resonant circuit.
Derive the expression for the half power bandwidth of a parallel resonant circuit.
What is Q-factor? Find values of Q-factor for an inductor and capacitor.
In a single tuned resonant circuit, the applied voltage in a primary coil Eg = 20
volts (Rg=0), R1=R2= 5Ω, L1=L2= 32 μH, M =20μ H, secondary side capacitance
C2 = 0.5μ F. Determine the resonant frequency and the output voltage at this
frequency.
Derive the expression of maximum value of E0 and I0.
UNIT – III
1.
2.
3.
4.
5.
Explain the PN junction diode.
Explain the diode current equation.
Derive the expression for transition capacitance and diffusing capacitance.
Explain different methods of breakdown in PN junction diodes.
Describe the operation of Zener diode and explain its characteristics.
1.
2.
3.
4.
5.
1.
2.
3.
4.
5.
6.
UNIT – IV
Describe Common Emitter configuration and its characteristics.
Explain the breakdown in transistor.
Describe the transistor switching times.
Explain the Characteristics of JFET with the help of neat sketches.
With the help of suitable diagrams explain the working of different types of
MOSFET.
UNIT –V
Explain the operation of PIN diode.
Explain the following terms: i) Photoconductive sensor ii) Photo emissive sensor
Describe the operation of LED and LCD.
Explain the operation of TRIAC and DIAC.
Draw the equivalent circuit of UJT and explain its operation.
Write short notes on light activated SCR.