spl thanks to Mr Suresh Durai

DEPARTMENT OF ELECTRONICS AND COMMUNICATION
ENGINEERING

TWO MARKS QUESTIONS & ANSWERS

EC2151  ELECTRIC CIRCUITS AND ELECTRON
DEVICES EDC ECAED ECED
ELECTRIC CIRCUITS AND ELECTRON DEVICES, anna university question banks for second semester, anna university second semester notes and question banks, CSE FIRST, VIVAQUESTIONS, QUESTION BANKS, LAB MANUALS,

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SYLLABUS

EC2151 ELECTRIC CIRCUITS AND ELECTRON
DEVICES
(For ECE, CSE, IT and
Biomedical Engg. Branches)

UNIT ICIRCUIT ANALYSIS TECHNIQUES12
Kirchoff’s current and voltage laws – series
and parallel connection of independent sources – R, L and C –
Network Theorems – Thevenin, Superposition,
Norton, Maximum power transfer and duality – Stardelta
conversion.

UNIT IITRANSIENT RESONANCE IN RLC CIRCUITS12
Basic RL, RC and RLC circuits and their responses
to pulse and sinusoidal inputs – frequency response –
Parallel and series resonances – Q factor –
single tuned and double tuned circuits.

UNIT IIISEMICONDUCTOR DIODES12
Review of intrinsic & extrinsic semiconductors
– Theory of PN junction diode – Energy band structure –
current equation – space charge and diffusion
capacitances – effect of temperature and breakdown mechanism
– Zener diode and its characteristics.

UNIT IVTRANSISTORS12
Principle of operation of PNP and NPN transistors
– study of CE, CB and CC configurations and comparison
of their characteristics – Breakdown in transistors
– operation and comparison of NChannel and PChannel
JFET – drain current equation – MOSFET – Enhancement
and depletion types – structure and operation –
comparison of BJT with MOSFET – thermal effect
on MOSFET.

SPECIAL SEMICONDUCTOR DEVICES
(Qualitative Treatment
only)12
Tunnel diodes – PIN diode, varactor diode –
SCR characteristics and two transistor equivalent model – UJT –
Diac and Triac – Laser, CCD, Photodiode, Phototransistor,
Photoconductive and Photovoltaic cells – LED,
LCD.

TOTAL: 60 PERIODS

TEXT BOOKS:

1. Joseph A. Edminister, Mahmood, Nahri, “Electric
Circuits” – Shaum series,Tata McGraw Hill,
(2001)
2. S. Salivahanan, N. Suresh kumar and A. Vallavanraj, “Electronic Devices
and Circuits”,Tata McGraw
Hill,
2nd Edition, (2008).
3. David A. Bell, “Electronic Devices and Circuits”, Oxford University
Press, 5th Edition, (2008).

REFERENCES:

UNIT V

1. Robert T. Paynter, “Introducing Electronics Devices and Circuits”,
Pearson Education, 7 th Education,

(2006).
2. William H. Hayt, J.V. Jack, E. Kemmebly and steven M. Durbin, “Engineering
Circuit Analysis”,Tata
McGraw
Hill, 6th Edition, 2002.
3. J. Millman & Halkins, Satyebranta Jit, “Electronic Devices &
Circuits”,Tata McGraw Hill, 2 nd
Edition, 2008.

2

UNIT – I
ELECTRIC CIRCUITS AND ELECTRON DEVICES

1.

What is charge?
The charge is an electrical property of the
atomic particles of which matter consists. The
unit of charge is the coulomb.

Define current?
The flow of free electrons in a metal is called
electric current. The unit of current is the
ampere. Current(I) = Q/t, Where Q is total
charge transferred & T is time required
for transfer of charge.

What is voltage?
The potential difference between two points
in an electric circuit called voltage. The unit
of voltage is volt. It is represented by V
OR v.
Voltage
= W/Q = workdone/Charge

Define power.
The rate of doing work of electrical energy
or energy supplied per unit time is called the
power. The power denoted by either P of p.
It is measured in Watts. (W).
Power
= work done in electric circuit/Time
P = dw/dt = dw/dq.dq/dt
P = VI

What is network?
Interconnection of two or more simple circuit
elements is called an electric network.

Distinguish between a branch and a node
of a circuit.
A part of the network which connects the various
points of the network with one another
is called a branch. A point at which two or
more elements are jointed together is called
node.

Distinguish between a mesh and a Loop
of a circuit.
A mesh is a loop that does not contain other
loops. All meshed are loops. But all loops
are not meshes. A loop is any closed path of
branches.

Define active and passive elements.
The sources of energy are called active element.
Example: voltage source, current source.
The element which stores or dissipates energy
is called passive element.
Example: Resistor, Inductor, Capacitor.

Define unilateral and bilateral elements.
In unilateral element, voltage – current relation
is not same for both the direction.
Example: Diode, Transistors.
In bilateral element, voltage – current relation
is same for both the direction.
Example: Resistor

3

2.

3.

4.

5.

6.

7.

8.

9.

10.

Define linear and nonlinear elements.
If the element obeys superposition principle,
then it is said to be linear elements.
Example: Resistor.
If the given network is not obeying superposition
principle then it is said to be non linear
elements.
Example: Transistor, Diode.

Define Lumped and distributed elements.
Physically separable elements are called Lumped
element.
Example : Resistor, Capacitor, Inductor.
A distributed element is one which is not separable
for electrical purpose.
Example : Transmission line has distributor
resistance, capacitance and inductance.

How are the electrical energy sources
classified?
The electrical energy sources are classified
into:
1. Ideal voltage source2. Ideal current source.

Define an ideal voltage source.
The voltage generated by the source does not
vary with any circuit quantity. It is only a
function of time. Such a source is called an
ideal voltage source.

Define an ideal current source.
The current generated by the source does not
vary with any circuit quantity.
It is only a function of time. Such a source
is called as an ideal current source.

What are independent source?
Independent sources are those in which, voltage
and current are independent and are not
affected by other part of the circuit.

What are dependent sources?
Dependent sources are those in which source
voltage or current is not fixed, but is
dependent on the voltage or current existing
at some other location in the circuit.

What are the different types of dependent
or controlled sources?
1. Voltage Controlled Voltage Sources (VCVS)
2. Current Controlled Voltage Sources (CCVS)
3. Voltage Controlled Current Sources (VCCS)
4. Current Controlled Current Sources (CCCS)

What is resistance?
It is the property of a substance which opposes
the flow of current through it. The
resistance of element is denoted by the symbol
“R”. It is measured in Ohms.
R = PL / A Î©

11.

12.

13.

14.

15.

16.

17.

18.

4

19.

Write down the VI relationship of circuit
elements.

Circuit
Element

Resistance

Inductance

Capacitance

Voltage

Current

Power

V = iR

V = L di /dt

V = 1/c ∫ idt

i=V/R

i = 1/L ∫ vdt

i = C dv / dt

P = vi

P = Li di / dt

P = CV dv / dt

20.

What is average value?
It is defined as area under one complete cycle
to period.
The average value of the sine wave is the total
area under the halfcycle curve divided by
the distance of the curve.
Area under one
complete cycle
Average value = _____________________________
Period

Define R.M.S. value.
The r.m.s value may be determined by taking
the mean of the squares of the
instantaneous value of current over one complete
cycle.

RMS

=

(Area
under hatched line) ²
______________________
Period

21.

22.

Define form factor.
The ratio of RMS value to the average value
is called the Form factor.

RMS value
Form factor (Kf) = ______________
Average Value

23.

Define peak factor.
Peak factor is defined as the ratio of the
maximum value to the rms value.

Maximum value
Peak Factor (Kp) = ________________
RMS value

5

24.

Define Ohm’s law.
The current flowing through the electric circuit
is directly proportional to the potential
difference across the circuit and inversely
proportional to the resistance of the circuit,
provided the temperature remains constant.

Define Kirchoff’s current law.
Kirchhoff’s current law states that in a node,
sum of entering current is equal sum of
leaving current.
∑Iat junction point = 0

Define Kirchoff’s voltage law.
Kirchhoff’s Voltage Law (KVL) states that the
algebraic sum of the voltages around any
closed path is zero.
Around a closed path ∑ V= 0.

Two resistances with equal value of “R”
are connected in series and parallel. What
is the equivalent resistance?
Resistance in seriesR
eq =R1+R2

R1R2
Resistance
in parallelR eq = __________
R1+R2
Two inductors with equal value of “L”
are connected in series and parallel what is
the equivalent inductance?

Inductance in series

Inductance in parallel

L eq = L1 + L2

L1L2
L eq = _______
L1+L2

25.

26.

27.

28.

29.

Two capacitors with equal value of “C”
are connected in series and parallel. What
is the equivalent capacitance?

Capacitance in series

Capacitance in parallel

C1C2
C eq = ____________
C1+C2

C eq = C1 + C2

30.

Write down the formula for a star connected
network is converted into a delta
network?
R R +R R +R R
A BB CC A
R= ____________________________
ABR
C
R R +R R +R R

6

A BB CC A
R= ____________________________
BCR
A
R R +R R +R R
A BB CC A
R= ____________________________
CAR
B

31.

Write down the formula for a delta connected
network is converted into a star
network?
R*R
ABCA
R= ____________________________
AB
R+R + R
ABBC CA

R*R
ABBC
= _____________________________

∑R
AB

R*R
BCCA
= _____________________________

∑R
AB

Where ∑ R

32.

=
AB

R

+
AB

R

+

BC

R

CA

R
B

R
B

Write few advantages of sinusoid waveform?
1. The response of the sinusoidal input for
second order system is sinusoidal.
2. The wave form can be written in terms of
sinusoidal function according to fourier
theorem.
3. The derivatives and integral also sinusoidal.
4. Easy for analyses.
5. Easy to generate.
6. More useful in power industry.

7

33.

Distinguish between a cycle, time periods
and frequency.
One complete set of positive
and negative instantaneous values of the voltage
or current is called cycle.

The time taken by an alternating quantity to complete one cycle is called
time
period (T).
2Ï€
Time period (T)=
_________
W
The number of cycle that a alternating quantity completed per second is
known as
frequency. It is measured in HZ.
1
Frequency (f)= ________
T

What is instantaneous value?
The value of an alternating current, at any particular moment is
called its
instantaneous value.

What are peak value and peak to peak
value?
The peak value of the sine wave during positive or negative half
only.
The sum of positive and negative value is called a peak to peak value.
The
peak to peak value of a sinusoidal alternating
voltage is equal to two times the peak value.

What is average value?
The average value of the sine wave is the total area under the halfcycle
curve
divided by the distance of the curve.
1T
V=___∫V (t) dt
avT0

Define effective value or RMS value
of a sinusoidal voltage.
The R.M.S value may be determined by taking
the mean of the squares of the
Instantaneous value of current over one complete cycle. This is often known
as the effective value.
V=____________
rms√ 1/T T2
∫
[V (t) dt
0
38.Define phasor.
Graphical representation of phasors V = Vm <Ï† and I=I <  Î¸
known phasor.

39.

40.

Define phase angle.
The angle between voltage
and current is called phase angle. It is denoted by Ï†.

What is impedance?
The ratio of the phasor voltage to the phasor
current is called impedance.

8

37.

36.

35.

34.

It is denoted as Z and it is measured in Ohms.

V
Impedance (Z) = ____
I
Obtain the equivalent impedance and reactances
for series and parallel connections.

Impedance in series

Z

eq

Impedance in parallel

= Z+Z
12

= ZZ
12
_________
Z1 +
Z2

X)
2

X

41.

Z
eq

Reactances in series j X

=
j (X +
eq1

X

Reactances in parallel j X
eq

41.

12
= j ____________
X + X
12

Define admittance.
The reciprocal of impedance is called admittance.
It is denoted as Y and it is measured
in Siemens (S).

Admittance (Y)

=

11
___ = ___
ZV

42.

Obtain the equivalent admittance for series
and parallel connections.

YY
12
____________
YY
12

Y
1

Y
2

Admittance in series Y

eq

Admittance in parallel Y
Eq

=

=

9

43.

Obtain the equivalent conductance and
susceptance for series and parallel
connections.

Conductance in series

G
eq

G

Eq

=

GG
12
_________
G + G
12
G+
1

BB
12
__________
G + G
12

j(B
1

=

B)
2

Conductance in parallel

=

G

2

Susceptance in series

jB
eq

=

Susceptance in parallel

44.

jB
eq

Define phasor diagram.
The phasor diagram is a name given to a sketch in the complex plane
showing the
relationships of the phasor voltages and phasor
currents throughout a specific circuit.

45.

What is instantaneous power?
The power at any instant of time is known as
instantaneous power.
P (t) =v(t) . I (t)

What is average power?
The average of the instantaneous power over one period is called
average power. Average
power is also defined as the product of voltage
and current.

47.

What is apparent power?
The product of V andI
rmsrms
Apparent power (S) =

is known as the apparent power (s).

V
Eff

I
eff

VA

46.

48.

Define power factor.
The ratio of the average power to the apparent
power is called the power factor.

Power Factor =

Average
power
_________________
Apparent
Power

10

49.

What is power triangle ?
A commonly employed graphical representation
of complex power is known as the
power triangle.

Define complex power

50.

The product of the rms voltage phasor and the complex conjugate of the
rms current
phasor is known as complex power. It is denoted
as S and it is measured in voltamperes (VA)

The complex power is

51.

S

=

1
___
2

vi*

What is reactive power?
It is defined as product of the applied voltage and the reactive
component of the current.
It is also called as imaginary component of
the apparent power. It is represented by “Q” and it is
measured in unit volt ampere reactive (VAR).
Q=VIsin Ï† VAR
Eff
eff

52

What is the equation for determining the number
of independent loops in mesh
current method?
L=bn +1
Where L=number of loops
B=number of branches
N=number of nodes.

On which law is the mesh analysis based?
Mesh analysis is based
on Kirchoff’s voltage law.

On which law is the nodal analysis based?
Nodal analysis is based on Kirchoff’s current
law and Ohm’s law.

52.

53.

54.

What is mesh analysis?
Mesh analysis is one of the basic techniques used for finding current
flowing through the
loop in a network.
Mesh analysis is applicable if the given network contains voltage sources.
If therer exist
current sources in a circuit, then it should
be converted into equivalent voltage sources.

What is nodal analysis?
Nodal analysis is one of the basic techniques used to finding solution
for voltage drop
across the nodes in a given circuit.
Nodal analysis is applicable if the given network contains current sources.
If there exists
voltage sources in the given circuit, then
it can to be converted into equivalent current sources.

56.

analysis.

11

When do we go for supermesh analysis.
If the branches in the network has a current
source, then it is slightly difficult to apply mesh

55.

One way to over come this difficulty is by
applying the supermesh technique.
In this case we have to choose supermesh.
A supermesh is constituted by two adjacent
loops that have common current source.

57.

When do we, go for supernode analysis.

If the branches in the network has a voltage source, then it is slightly
difficult to apply nodal
analysis.

One way to overcome this difficulty is by applying
the supernode technique.
In this case, we have to choose super node.
A supernode is constituted by two adjacent
node that have common voltage source.

State superposition theorem.
Any electric circuit (linear, lumped, bilateral), is energeied by
two or more sources,
the response in any element in the network
is equal to the algebraic sum of the responses caused by
individual sources acting separately.

State Thevenin’s Theorem.
A complex network having linear, bilateral, lumped elements with
open circuited
output terminals can be reduced by a simple
circuit consisting of a single voltage source in series
with a impedance.

State Norton’s theorem.
Any electrical network (linear, lumped, bilateral) with short circuited
terminals can be
reduced by a simple circuit consisting of a
single current source in parallel with a Thevenin’s
equivalent resistance.

State Maximum power transfer theorem.
Power transferred from source to load will
be maximum, when source resistance is
equal to load resistance looking back from
its load terminals.

62.

duality.

Define duality.
Two electrical network
which are governed by the same type of equations are called

61.

60.

59.

58.

12

UNIT – II

TRANSIENT RESONANCE IN RLC CIRCUITS

What is transient state?
If a network contains energy storage elements, with change in excitation,
the current
and voltages change from one state to other
state. The behaviour of the voltage or current when it is
changed from one state to another state is
called transient state.

What is transient time?
The time taken for the circuit to change from
one steady state to another steady state
is called transient time.

What is 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.

What is 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.

5.

Define Laplace transform function.
The laplace transform
of any time dependent functionf (t) is given by F (s).
Where
S A complex frequency given by S = Ïƒ + jw
∞st
F (S) =L [F (T) ] = ∫ F (t) e dt
0

4.

3.

2.

1.

What is inverse Laplace transform?
Inverse Laplace Transform permits going back in the reverse direction
i.e. from s
domain to time domain.

1
L

7.

1
[F(s)] = f (t) = _________
2Ð›j

Ïƒ1 + j∞
∫
Ïƒ1  j∞

st
F (s) eds

6.

Define time constant or RL Circuit.
The time taken to reach 63.2% of final value in a RL Circuit is called
the time
constant of RL circuit.
Time constant (t)=L/R

Define time constant of RC Circuit.
The time taken to reach 36.8% of initial current in an RC circuit
is called the time
constant of RC circuit.
Time constant (t)=RC.

8.

13

9. What is meant by natural frequency?
If the damping is made
zero then the response oscillates with natural frequency without any
opposition, such a frequency is called natural
frequency of oscillations, denoted as ÏŽ
n.
10. Define damping ratio.
It is the ratio of actual
resistance (R) in the circuit to the critical resistance (R cr). It is
denoted by greek letter Zeta (Î¾).
RR
Îž=____ = _____ √ C / L
R
cr2

11. Define initial value theorem.
The initial value theorem states that if f (t) and f’ (t) both are
laplace transformable,
Then
Lim f (t)=lim s F (s)
T0s∞

12. Define final value theorem.
The final value theorem states that, if f (t) and f ‘ (t) both are
laplace transformable,
then
Lim f (t)=lim s F (s)
T∞s0

13. What is driving point impedence ?
The ratio of the Laplace transform of the voltage at the point to
the laplace transform of the
current at the same port is called driving
point impedance.
The driving point impedance of the network is define as
V(s)
Z (S) =_____________
I (s)

14.
What is transfer
point impedance?
It is defined as the ratio of voltage transform at one port to the
current transform at the
other port. It is defined by
V (s)
Z (s) =___2_________
21I (s)
1
and
V (s)
Z (s) =___1_________
12I (s)
2

14

15. Define network function.
A network function N (S) is defined as the ratio of the complex amplitude
of an
excponential output P (S) to the complex amplitude
of an exponential input Q (S).

16. Define pole and zero.
The network function N (S) will become infinite. Hence the roots
of denominator
polynomial P1, P2, P3,……Pm are called poles
of network function.
The network function N(S) will become
zero. Hence the roots of numerator
polynomial Z1, Z2, Z3…………..Zn. are called zeros
of network function.

17. Define resonant circuit.
The circuit that treat a narrow range of frequencies very differently
than all other
frequencies. These are referred to as resonant
circuit. The gain of a highly resonant circuit attains a
sharp maximum or minimum as its resonant frequency.

18. When the circuit is said to be in
resonance?
1. A network is in resonance
when the voltage and current at the network input terminals are
in phase.
2.
If inductive reactance of a network equals capacitive reactance then the network
is said to
be resonance

19. What is resonant frequency ?
The frequency at which resonance occurs is called resonance frequency.
1
f=________
r2Ð›√LC

20. Define bandwidth.
The bandwidth (BW) is defined as the frequency difference between
upper cutoff
frequency (f2) and lower cutoff frequency
(f1)
Bandwidth = f2f1
Where f2 upper cutoff frequency
F1 lower cutoff frequency

21. Define selectivity.
Selectivity is defined as the ratio of bandwidth
to the resonant frequency of resonant
circuit.
Bandwidth
Selectivity=_______________________
Resonant frequency

22. Define quality factor.
The quality factor is defined as the ratio of maximum energy stored
to the energy
dissipated per cycle.
Maximum energy
stored per cycle
Quality factor (Q) = 2Ð› * ______________________________
Energy dissipated
per cycle

15

23. Define half power frequencies ?
The frequencies at which the power is half the maximum power are
called half power
frequencies.
R
Lower half power frequency, f1 = f r  ____
4Ð› L

R
Upper half power frequency, f2 = f r + ____
4Ð› L

24. Write down the formula for inductive
reactance and capacitive reactance?

Inductive reactance is given by X
L

= 2Ð› fl

1
Capacitive reactance is given by X = _________
C2Ð› fc

Where
F supply frequency
L Inductance of the coil
C Capacitance of the capacitor.

25. Give the expression for quality factor
of series RLC Circuit.
Quality factor is Q = 1 / R √ L /C

26. Give the expression for quality factor
of parallel RLC Circuit.
Quality factor is Q = R √ C / L

16

UNIT III

SEMICONDUCTOR DIODES

1. Give the value of Charge, Mass of an
electron.
Charge of an electron – 1.6 x 10 19 coloumbs
& Mass of an electron  9.11 x 10 31 Kgs
2. Define Potential.
A potential of V volts at point B with respect to point A, is defined
as the work
done in taking unit positive charge from A
to B , against the electric field.
3. Define Current density.
It is defined as the current per unit area
of the conducting medium. J = I / A
4. Define Electron volts.
If an electron falls through a potential of
one volt then its energy is 1 electron volt.
1 eV = 1.6 x 10 19 joules
5. What is Electrostatic deflection sensitivity?
Electrostatic deflection sensitivity of a pair
of deflecting plates of a cathode ray
oscilloscope ( CRO) is defined as the amount
of deflection of electron spot
produced when a voltage of 1 Volt DC is applied
between the corresponding plates.
6. What is the relation for the maximum
number of electrons in each shell?
Ans: 2n2
7. What are valence electrons?
Electron in the outermost shell of an atom
is called valence electron.
8. What is forbidden energy gap?
The space between the valence and conduction
band is said to be forbidden energy gap.
9. What are conductors? Give examples?
Conductors are materials in which the valence
and conduction band overlap each other so there is a
swift movement of electrons which leads to
conduction. Ex. Copper, silver.
10. What are insulators? Give examples?
Insulators are materials in which the valence
and conduction band are far away
from each other. So no movement of free electrons
and thus no conduction.
Ex glass, plastic.
11. Give the energy band structure of
Insulator.
In Insulators there is a wide forbidden energy
gap. So movement of valence
electron from valence to conduction band is
not possible.
12. Give the energy band structure of
Semi conductor.
In Semiconductors there is a small forbidden
energy gap. So movement of
valence electron from valence to conduction
band is possible if the valence
electrons are supplied with some energy.
13. Give the energy band structure of
conductor.
In conductors there is no forbidden energy
gap, valence band and conduction
and over lap each other. so there is a heavy
movement of valence electrons.
14. what are Semiconductors? Give examples?
The materials whose electrical property lies
between those of conductors and
insulators are known as Semiconductors. Ex
germanium, silicon.
15. What are the types of Semiconductor?
1. Intrinsic semiconductor 2. Extrinsic semiconductor.
16. What is Intrinsic Semiconductor?
Pure form of semiconductors are said to be
intrinsic semiconductor.

17

Ex: germanium, silicon.
17. Define Mass – action law.
Under thermal equilibrium the product of free
electron concentration (n) and hole
concentration (p) is constant regardless of
the individual magnitude.
n.p = ni2
18. What is Extrinsic Semiconductor?
If certain amount of impurity atom is added
to intrinsic semiconductor the
resulting semiconductor is Extrinsic or impure
Semiconductor.
19. What are the types of Extrinsic Semiconductor?
1. Ptype Semiconductor
2. N Type Semiconductor.
20. What is Ptype Semiconductor?
The Semiconductor which are obtained by introducing
pentavalent impurity atom
(phosphorous, antimony) are known as Ptype
Semiconductor.
21. What is Ntype Semiconductor?
The Semiconductor which is obtained by introducing
trivalent impurity atom (gallium, indium) are
known as Ntype Semiconductor.
22. What is doping?
Process of adding impurity to a intrinsic semiconductor
atom is doping. The impurity is called
dopant.
23. Which charge carriers is majority
and minority carrier in Ntype
Semiconductor?
majority carrier: electron and minority carrier:
holes.
24.which charge carriers is majority and
minority carrier in Ptype
Semiconductor?
Majority carrier: holes and minority carrier:
electron
25. Why n  type or penta valent impurities
are called as Donor impurities?
n type impurities will donate the excess negative
charge carriers ( Electrons) and therefore they are
reffered to as donor impurities.
26. Why P – type or trivalent impurities
are called as acceptor impurity?
p type impurities make available positive
carriers because they create holes which can accept
electron, so these impurities are said to be
as acceptor impurity.
27. Give the relation for concentration
of holes in the n type material?
pn = ni
2 /ND
Where
pn  concentration of holes in the n – type
semiconductor
ND  concentration of donor atoms in the n
– type semiconductor
28. Give the relation for concentration
of electrons in the p  type material?
np = ni
2 /NA
Where
np  concentration of electrons in p type
semiconductor
ND  concentration of acceptor atoms in the
p – type semiconductor
29. Define drift current?
When an electric field is applied across the
semiconductor, the holes move towards the negative
terminal of the battery and electron move towards
the positive terminal of the battery. This drift
movement of charge carriers will result in
a current termed as drift current.

18

30. Give the expression for drift current
density due to electron.
Jn = q n Î¼nE
Where,
Jn  drift current density due to electron
q Charge of electron
Î¼n  Mobility of electron
E  applied electric field
31. Give the expression for drift current
density due to holes.
Jp = q p Î¼p E
Where, Jn  drift current density due to holes
q  Charge of holes
Î¼p  Mobility of holes E  applied electric
field
32. Define the term diffusion current?
A concentration gradient exists, if the number
of either electrons or holes is greater in one region of a
semiconductor as compared to the rest of the
region. The holes and electron tend to move from
region of higher concentration to the region
of lower concentration. This process in called diffusion
and the current produced due this movement
is diffusion current.
33. Define mean life time of a hole or
and electron.
The electron hole pair created due to thermal
agitation woll disappear as a result of recombination.
Thus an average time for which a hole or an
electron exist before recombination can be said as the
mean life time of a hole or electron.
34. What is the other name of continuity
equation? What does it indicate?
The other name of continuity equation is equation
of conservation of charge.
This equation indicates that the rate at which
holes are generated thermally just equals the rate at
which holes are lost because of recombination
under equilibrium conditions.
35. Define Hall effect?
If a metal or semiconductor carrying current
I is placed in a transverse magnetic field B , an electric
field E is induced in the direction perpendicular
to both I and B This phenomenon is known as Hall
effect.
36. Give some application of Hall Effect.
i). Hall Effect can be used to measure the
strength of a magnetic field in terms of electrical voltage.
ii).It is used to determine whether the semiconductor
is p – type or n type material
iii).It is used to determine the carrier concentration
iv).It is used to determine the mobility.
37. Define the term transition capacitance?
When a PN junction is reverse biased, the depletion
layer acts like a dielectric material while P and
N –type regions on either side which has low
resistance act as the plates. In this way a reverse biased
PN junction may be regarded as parallel plate
capacitor and thus the capacitance across this set up is
called as the transition capacitance.
CT = A / W
Where
CT  transition capacitance
A  Cross section area of the junction
W – Width of the depletion region
38. What is a varactor diode?
A diode which is based on the voltage variable
capacitance of the reverse biased pn junction is said
to be varactor diode. It has other names such
as varicaps, voltacaps.

19

39. Define the term diffusion capacitance.
The diffusion capacitance of a forward biased
diode is defined as the rate of change of injected
charge with voltage.
CD = I / VT
Where, Cd – time constant
I – current across the diode
vT – threshold voltage
40. what is recovery time? Give its types.
When a diode has its state changed from one
type of bias to other a transient accompanies the diode
response, i.e., the diode reaches steady state
only after an interval of time “ tr” called as recovery
time. The recovery time can be divided in to
two types such as
(i) forward recovery time
(ii) reverse recovery time

20

41. What is meant by forward recovery
time?
The forward recovery time may be defined as
the time interval from the instant of 10% diode voltage
to the instant this voltage reaches 90% of
the final value. It is represented as t f r.

42. What is meant by reverse recovery
time?
The reverse recovery time can be defined as
the time required for injected or the excess minority
carrier density reduced to zero , when external
voltage is suddenly reversed.

43. Define storage time.
The interval time for the stored minority charge
to become zero is called storage time. It is
represented as t s.

44. Define transition time.
The time when the diode has normally recovered
and the diode reverse current reaches reverse
saturaton current I0 is called as transition
time. It is represented as t t

45. What are break down diodes?
Diodes which are designed with adequate power
dissipation capabilities to operate in the break down
region are called as break down or zener diodes.

46. What is break down? What are its types?
When the reverse voltage across the pn junction
is increased rapidly at a voltage the junction breaks
down leading to a current flow across the device.
This phenomenon is called as break down and the
voltage is break down voltage. The types of
break down are
i) zener break down
ii)Avalanche breakdown

47. What is zener breakdown?
Zener break down takes place when both sides
of the junction are very heavily doped and
Consequently the depletion layer is thin and
consequently the depletion layer is tin. When a small
value of reverse bias voltage is applied ,
a very strong electric field is set up across the thin depletion
layer. This electric field is enough to break
the covalent bonds. Now extremely large number of free
charge carriers are produced which constitute
the zener current. This process is known as zener
break down.

48. What is avalanche break down?
When bias is applied , thermally generated
carriers which are already present in the diode acquire
sufficient energy from the applied potential
to produce new carriers by removing valence electron
from their bonds. These newly generated additional
carriers acquire more energy from the potential
and they strike the lattice and create more
number of free electrons and holes. This process goes
on as long as bias is increased and the number
of free carriers get multiplied. This process is termed
as avalanche multiplication. Thus the break
down which occur in the junction resulting in heavy
flow of current is termed as avalanche break
down.

21

49. How does the avalanche breakdown voltage
vary with temperature?
In lightly doped diode an increase in temperature
increases the probability of collision of electrons
and thus increases the depletion width. Thus
the electrons and holes needs a high voltage to cross the
junction. Thus the avalanche voltage is increased
with increased temperature.
50. How does the zener breakdown voltage
vary with temperature?
In heavily doped diodes, an increase in temperature
increases the energies of valence electrons, and
hence makes it easier for these electrons to
escape from covalent bonds. Thus less voltage is

22

sufficient to knock or pull these electrons
from their position in the crystal and convert them in to
conduction electrons. Thus zener break down
voltage decreases with temperature.

UNIT IV

TRANSISTORS

51. What is a transistor (BJT)?
Transistor is a three terminal device whose
output current, voltage and /or power
is controlled by input current.
52. What are the terminals present in
a transistor?
Three terminals: emitter, base, collector.
53. What is FET?
FET is abbreviated for field effect transistor.
It is a three terminal device with its output
characteristics controlled by input voltage.
54. Why FET is called voltage controlled
device?
The output characteristics of FET is controlled
by its input voltage thus it is voltage controlled.
55. What are the two main types of FET?
1. JFET 2. MOSFET.
56. What are the terminals available in
FET?
1). Drain, 2).Source and 3). Gate
57. What is JFET?
JFET Junction Field Effect Transistor.
58. What are the types of JFET?
N Channel JFET and P Channel JFET
59. What are the two important characteristics
of JFET?
1. Drain characteristics 2. Transfer characteristics.
60. What is transconductance in JFET?
It is the ratio of small change in drain current
to the corresponding change in drain to source voltage.
61. What is amplification factor in JFET?
It is the ratio of small change in drain to
source voltage to the corresponding change in Gate to
source voltage.
62. Why do we choose q point at the center
of the loadline?
The operating point of a transistor is kept
fixed usually at the center of the active region in order that
the input signal is well amplified. If the
point is fixed in the saturation region or the cut off region the
positive and negative half cycle gets clipped
off respectively.
63. List out the different types of biasing.
._
Voltage divider bias, Base bias, Emitter feed
back bias, Collector feedback bias, Emitter bias.

64. What do you meant by thermal runway?
Due to the self heating at the collector junction,
the collector current rises. This causes damage to the
device. This phenomenon is called thermal runway.
65. Why is the transistor called a current
controlled device?
The output characteristics of the transistor
depend on the input current. So thtransistor is called a
current controlled device.
66. Define current amplification factor?

23

It is defined as the ratio of change in output
current to the change in input current at constant.
67. What are the requirements for biasing
circuits?
• The q point must be taken at the Centre of
the active region of the output characteristics.
• Stabilize the collector current against the
temperature variations.
• Make the q point independent of the transistor
parameters.
• When the transistor is replaced, it must
be of same type.
68. When does a transistor act as a switch?
The transistor acts as a switch when it is
operated at either cutoff region or saturation region
69. What is biasing?
To use the transistor in any application it
is necessary to provide sufficient voltage and current to
operate the transistor. This is called biasing.
70. What is stability factor?
Stability factor is defined as the rate of
change of collector current with respect to the rate of change
of reverse saturation current.
71. Explain about the various regions
in a transistor?
The three regions are active region saturation
region cutoff region.
72. Explain about the characteristics
of a transistor?
Input characteristics: it is drawn between
input voltage & input current while keeping output voltage
as constant. Output characteristics: It is
drawn between the output voltage &output current while
keeping input current as constant.
16 marksHints
73. Explain the construction, operation,
volt ampere characteristics, and application
of SCR, also explain its two transistor
model.
Maximum mark for this question: 16 marks
Construction (2marks)
Equivalent circuit and two transistor model
(2marks)
Operation (4marks)
Volt ampere characteristics (4marks)
Application (2marks)
74. Explain the construction, operation,
equivalent circuit, volt ampere
characteristics, and application of UJT.
Maximum mark for this question: 16 marks
Construction (4marks)
Equivalent circuit (2marks)
Operation (4marks)
Volt ampere characteristics (4marks)
Application (2marks)
75. Explain the construction, operation,
equivalent circuit, volt ampere
characteristics, and application of DIAC.
Maximum mark for this question: 16 marks
Construction (4marks)
Equivalent circuit (2marks)
Operation (4marks)
Volt ampere characteristics (4marks)
Application (2marks)
76. Explain the construction, operation,
equivalent circuit, volt ampere
characteristics, and application of TRIAC
Maximum mark for this question: 16 marks

24

Construction (4marks)
Equivalent circuit (2marks)
Operation (4marks)
Volt ampere characteristics (4marks)
Application (2marks)

UNIT V

SPECIAL SEMICONDUCTOR DEVICES

78. What is a TRIAC?
TRIAC is a three terminal bidirectional semiconductor
switching device. It can conduct in both the
directions for any desired period. In operation
it is equivalent to two SCR’s connected in antiparallel.
79. Give the application of TRIAC.
1. Heater control
2. Motor speed control
3. Phase control
4. Static switches
80. What are the different operating modes
of TRIAC?
1. Keeping MT2 and G positive
2. Keeping MT2 and G negative.
3. Keeping MT2 positive and G negative.
4. Keeping MT2 negative and G positive.
81. What are the regions in the VI characteristics
of UJT?
1. Cutoff region
2. Negative resistance region.
3. Saturation region
82. What is meant by negative resistance
region of UJT?
In a UJT when the emitter voltage reaches the
peak point voltage, emitter current starts flowing.
After the peak point any effort to increase
in emitter voltage further leads to sudden increase in the
emitter current with corresponding decrease
in emitter voltage, exhibiting negative resistance. This
takes place until the valley point is reached.
This region between the peak point and valley point is
called negative resistance region.
83. Mention the applications of UJT.
1. It is used in timing circuits
2. It is used in switching circuits
3. It is used in phase control circuits

25

4. It can be used as trigger device for SCR
and triac.
5. It is used in saw tooth generator.
6. It is used for pulse generation.
84. What is a DIAC?
DIAC is a two terminal bidirectional semiconductor
switching device. . It can conduct in either
direction depending upon the polarity of the
voltage applied across its main terminals. In operation
DIAC is equivalent to two 4 layer diodes connected
in antiparallel.
85. Give some applications of DIAC.
1. To trigger TRIAC
2. Motor speed control
3. Heat control
4. Light dimmer circuits
87. What is a SCR?
A silicon controller rectifier (SCR) is a three
terminal, three junction semiconductor device that acts
as a true electronic switch. It is a unidirectional
device. It converts alternating current into direct
current and controls the amount of power fed
to the load.
88. Define break over voltage of SCR.
Break over voltage is defined as the minimum
forward voltage with gate open at which the SCR
starts conducting heavily.
89.Why SCR cannot be used as a bidirectional
switch.
SCR can do conduction only when anode is positive
with respect to cathode with proper gate
current. Therefore, SCR operates only in one
direction and cannot be used as bidirectional switch.
90. How turning on of SCR is done?
1. By increasing the voltage across SCR above
forward break over voltage.
2. By applying a small positive voltage at
gate.
3. By rapidly increasing the anode to cathode
voltage.
4. By irradiating SCR with light.
91. How turning off of SCR is done?
1. By reversing the polarity of anode to cathode
voltage.
2. By reducing the current through the SCR
below holding current.
3.By interrupting anode current by means of
momentarily series or parallel switching
92. Define holding current in a SCR.
Holding current is defined as the minimum value
of anode current to keep the SCR ON.
93. List the advantages of SCR.
1. SCR can handle and control large currents.
2. Its switching speed is very high
3. It has no moving parts, therefore it gives
noiseless operation.
4. Its operating efficiency is high.
94. List the application of SCR.
1. It can be used as a speed controller in
DC and AC motors.
2. It can be used as an inverter.
3. It can be used as a converter
4. It is used in battery chargers.
5. It is used for phase control and heater
control.
6. It is used in light dimming control circuits.
95. What is meant by latching.
The ability of SCR to remain conducting even
when the gate signal is removed is called as latching.
96. Define forward current rating of a
SCR.

26

Forward current rating of a SCR is the maximum
anode current that it can handle without
destruction.
97. List the important ratings of SCR.
1. Forward break over voltage
2. Holding current
3. Gate trigger current
4. Average forward current
5. Reverse break down voltage.
98. Compare SCR with TRIAC.
SCRTRIAC
1. unidirectional current1. bidirectional current
2. triggered by positive pulse at gate 2. triggered
by pulse of positive or negative
at
gate
3. fast turn off time3,. Longer turn off time
4. large current ratings4. lower current ratings
99. Differentiate BJT and UJT.
BJTUJT
1. It has two PN junctions1. It has only one
PN junctions
2. three terminals present2. three terminals
present are emitter,
are
emitter, base,collectorbase1,base2
3. basically a amplifying device3. basically
a switching device

100. What is Shockley diode (PNPN diode)?
Shockley diode is a four layered PNPN silicon
diode. It is a low current SCR without a gate. This
device is switched ON when the anode to cathode
voltage is increased to forward switching
voltageVS which is equivalent to SCR forward
break over voltage.

101. What is a thyristor?
Thyristor is a semiconductor device having
three or more junctions .Such a device acts as a switch
without any bias and can be fabricated to have
voltage ratings of severalhundred volts and current
ratings from a few amperes to almost thousand
amperes.

102. What are the types of thyristors?
1. Unidirectional thyristors
2. Bidirectional thyristors
3. Lowpower thyristors
103. Give the various triggering devices
for thyristors.
1. SCR
2. UJT
3. DIAC
4. TRIAC
104. what is backward diode?
The backward diode is a diode in which the
doping level is moderate. The forward current in this
case is very small, very much similar to that
of the reverse current in the conventional diode.
105. what is a photo diode?

27

The photo diode is a diode in which the current
sensitivity to radiation can be made much larger by
the use of the reverse biased PN junction.
Thus this diode conducts heavily in the reverse bias when
there is some radiaton allowed to fall on the
PN junction.
106. What is a LED?
A PN junction diode which emits light when
forward biased is known as Light emitting diode
(LED).
107. What is a tunnel diode?
The tunnel diode is a pn junction diode in
which the impurity concentration is greatly increased
about 1000 times higher than a conventional
PN junction diode thus yielding a very thin depletion
layer. This diode utilizes a phenomenon called
tunneling and hence the diode is referred as tunnel
diode.
108. What is tunneling phenomenon?
The phenomenon of penetration of the charge
carriers directly though the potential barrier instead of
climbing over it is called as tunneling.

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