EC2151 ELECTRIC CIRCUITS AND ELECTRON DEVICES notes

EC2151      ELECTRIC CIRCUITS AND ELECTRON DEVICES

Anna University Syllabus

UNIT-I           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"

SI_in = SI_out

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"

SE = SIZ

Similarly, t any instant the algebraic sum of all the voltages around any closed circuit is zero:
SE - SIZ = 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                   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.

PARALLEL

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 R_A, R_B and R_C connected together at common node N can be transformed into a delta network of three resistances R_AB, R_BC and R_CA by the above equations:

In general terms:
R_delta = (sum of R_star pair products) / (opposite R_star)


Delta-Star Transformation

A delta network of three impedances R_AB, R_BC and R_CA can be transformed into a star network of three impedances R_A, R_B and R_C connected together at common node N by the following equations:

In general terms:
R_star = (adjacent R_delta pair product) / (sum of R_delta)
 

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 n^th order differential equation, than the system is called n^th order system.

            Q(s) = a₀ sⁿ + a₁ s ^n-1+ a₂ s ^n-2 + ……..+a_n-1 s +a_n

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

Ø  Single tuned circuit

Ø  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 µ_n (free electron) and the other is positive of mobility µ_p (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 = (nµ_n + pµ_p )E

For a pure semiconductor n = p = n_i ;               where  n_i  – intrinsic semiconductor

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 = N_c e^{- (E}_C ^–E_F^)/KT   

3/2

2πmn KT

Where,    

h2

                           N_c =  2   

Concentration of holes (p)

The number of holes in the conduction band,   p = N_v e^{- (E}_F ^–E_V^)/KT   

3/2

2πmp KT

Where,   

h2

                           N_v =  2  
Energy Gap (E_G)                                                                                                                                     

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,             E_G = E_GO – 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 D_p dp/dx  A/cm²

Diffusion current density due to electrons,   Jn = - q D_n 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 = - qpµ_pE- q D_p dp/dx 

The total current density for N-type semiconductor Jn = - qnµ_nE- q D_n 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)                 Forward biasing

(ii)               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 (V_B).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.

Ø  Avalanche breakdown

Ø  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

Ø  Voltage regulator

Ø  Fixing reference voltages in electronic circuits such as power supplies and transistor biasing.

Ø  Clippers in wave-shaping circuits.

Ø  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

Ø  Unipolar Junction Transistor

Ø  Bipolar Junction Transistor

Construction of the transistor

Ø  n-p-n transistor

Ø  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

Ø  Emitter

Ø  Collector

Ø  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.

Ø  Active region.

Ø  Cut-off region.

Ø  Saturation region.

Sl.No.	Region	Emitter Base	Collector Base	Operation of a transistor
1	Active	Forward biased	Reverse biased	acts as an amplifier
2	Cut-off	Forward biased	Reverse biased	acts as an open switch
3	Saturation	Forward biased	Reverse biased	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 I_E.
• 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 I_B & 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 I_C.
• Current conduction I PNP transistors is by movement of holes.
• Current conduction in the external circuit is by electrons.

Types of configuration

Ø  Common Base configuration

Ø  Common Emitter configuration

Ø  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 I_E flows in the input circuit. Collector current I_C flows in output circuit.

·         The ratio of collector current I_C, to emitter current I_E, is called the Current amplification factor (α).

·         If there is no input ac signal, then the ratio of I_C to I_E is called dc alpha (α_dc).

·         ac alpha refers to the ratio of change in I_C to change in I_E.

·         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 V_CB,the curve is just like a forward-biased PN junction.

·         With an increase in the value of V_CB,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  V_CB  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 I_B flows in the input circuit. Collector current I_C 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 I_C, to change in emitter current I_E, 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 V_EC,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 I_B increases less rapidly with increase in V_BE .

·         An increment in the value of V_CE, causes the input current I_B to be lower for a given level of V_BE . This is because of the effect called early effect.

Output Characteristics

·         The curve drawn between Collector current I_C and Collector – emitter voltage V_CE for a given value of base current I_B 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 I_B equal to V_CO, the emitter current I_E is zero, so no current flows in the load resistor R_L.

·         With increases in input current I_B, the transistor passes through the active region and finally reaches saturation.

Input Characteristics

·         To determine the input Characteristics, V_EC is kept at a suitable fixed value.

·         The base-collector voltage V_Bc is increased in equal steps and the corresponding increase in I_B is noted.

·         This is repeated for different values of V_EC.

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 BV_CBO (for CB configuration).

·         This breakdown voltage is a characteristic of the transistor alone.

·         Breakdown occurs because of the avalanche multiplication of current I_CO that crosses the collector junction.

·         As a result of this multiplication, the current becomes MI_CO in which M is the factor by which the original current I_CO is multiplied by the avalanche effect.

·         At a high voltage BV_CBO, 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 V_CB 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 Reach-through.

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 V_GG applied and V_DD=0
• When V_DS applied and V_GG=0
• When V_DD applied and V_{GG is} applied.

Where,

Ø  V_GG – Gate supply voltage.

Ø  V_DS– Drain Source voltage.

Ø  V_DD– 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.

Ø  Transfer characteristics

Ø  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 n-doped 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

Ø  Drain characteristics

Ø  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

Ø  Tunnel diode is used as Ultra-high speed switch.

Ø  Used in relaxation oscillator.

Ø  Used as an amplifier.

Ø  Used as logic memory storage device.

Ø  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

Ø  Used as pulse and phase shifter.

Ø  Used as SPST and MPST switches.

Ø  Used in amplitude modulation.

Ø  Used as photo detectors in fiber optic systems.

Ø  Used as T-R switch.

Ø  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

Ø  Used as a tuning device in receivers.

Ø  It is used in High frequency.

Ø  It is used in adjustable band-pass filter

Ø  It is used in FM modulation.

Ø  It is used in automatic frequency control devices.

Ø  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 (V_BO).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

Ø  Forward Characteristics

Ø  Reverse Characteristics

Forward Characteristics

• It is the current drawn between anode-cathode voltage (V_AK) and anode current (I_A) 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 V_AA and V_GG .
• 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.

Ø  Gate triggering

Ø  Forward break over voltage

Ø  Light triggering

Ø  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

Ø  Power control device

Ø  Relay control

Ø  Regulated power supplies

Ø  Static switches

Ø  Motor control

Ø  Battery charges

Ø  Heater controls

Ø  Phase controls

Ø  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

Ø  Non sinusoidal oscillator

Ø  Timing circuits

Ø  Saw tooth generators

Ø  Triggering device for SCR and TRIAC

Ø  Switching circuits

Ø  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

Ø       Temperature control

Ø       Triggering of TRIAC

Ø       Light dimming circuits

Ø       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

Ø  Heater control

Ø  Phase control

Ø  Light dimming control

Ø  Static switch to turn a.c. power ON and OFF.

Ø  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

Ø  Optical light controls

Ø  Phase control

Ø  In relays

Ø  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

Ø  PN homojunction laser

Ø  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.

Ø  Surface - emitting LED

Ø  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
• Dynamic Scattering Displays.
• 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

Ø  Used as numerical counters for counting production items.

Ø  Analog quantities can also be displayed as a number on a suitable device. (e.g.) Digital multimeter.

Ø  Used for solid state video displays.

Ø  Used for image sensing circuits.

Ø  Used for numerical display in pocket calculators.

ASSIGNMENT QUESTIONS

UNIT – I

1. Define the following terms: i) Mesh ii) Loop iii) Node iv) Branch
2. State and explain Kirchhoff’s laws
3. Explain voltage division & current division method using suitable example.
4. Derive the relationship to express three delta connected resistances into star.
5. 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.

UNIT – II

1. Determine the expression of resonant frequency and bandwidth of a series resonant circuit.
2. Derive the expression for the half power bandwidth of a parallel resonant circuit.
3. What is Q-factor? Find values of Q-factor for an inductor and capacitor.
4. In a single tuned resonant circuit, the applied voltage in a primary coil E_g = 20 volts (R_g=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.
5. Derive the expression of maximum value of E₀ and I₀.

UNIT – III

1. Explain the PN junction diode.
2. Explain the diode current equation.
3. Derive the expression for transition capacitance and diffusing capacitance.
4. Explain different methods of breakdown in PN junction diodes.
5. Describe the operation of Zener diode and explain its characteristics.

UNIT – IV

1. Describe Common Emitter configuration and its characteristics.
2. Explain the breakdown in transistor.
3. Describe the transistor switching times.
4. Explain the Characteristics of JFET with the help of neat sketches.
5. With the help of suitable diagrams explain the working of different types of MOSFET.

UNIT –V

1. Explain the operation of PIN diode.
2. Explain the following terms: i) Photoconductive sensor ii) Photo emissive sensor
3. Describe the operation of LED and LCD.
4. Explain the operation of TRIAC and DIAC.
5. Draw the equivalent circuit of UJT and explain its operation.
6. Write short notes on light activated SCR.