1. How "electron volt" is related to "Joule"?

Ans: 1 eV = 1.6x10-19 J

2. What is the forbidden energy gap of Silicon at 0 K?

Ans: Eg(Si) = 1.2 eV

3. Can an intrinsic semiconductor behaves as an insulator at any

temperature?
Ans: Yes, at 0 K.

4. Does an exciton behaves as an electrically charged particle?

Ans: No, it is neutral.

5. What is the unit of mobility of a charged carrier?

Ans: Unit is m2/(V.s)

6. The Fermi level of an n-type semiconductor lies close to which

energy band edge?
A: EF is close to conduction band edge.

7. Effective mass of an electron in semiconductor is usally

different from the electron mass in vacuum. Do you agree or
disagree with the statement?
A: Yes, agree. Effective mass is different from electron mass in
vacuum.

8. What is the expression of Diffusivity-to-Mobility ratio for

electrons?

A:

D/μ=kT/e

9. Write down the name of one atom that can act as donors in
GaAs.
A: Goup VI element

10. An electron at rest is accelerated through a potential

difference of 100 V. What is its final velocity?
Energy EK=e. 100 J=1.6x10-19J.

Velocity = sqrt (2EK/m0)=5.9x106 m/s

11. What is meant by a "compensated semiconductor"?

When semiconductor contains from

ANS. ionized donors and ionized
acceptors.

12.

The band gap energy of ternary compound GaAsP is

1.97eV. Upon direct recombination of electrons and
holes in this material, what is the wavelength of
emitted radiation?

λ= hc/Eg=6.6x10-34.3x108/(1.97x1.6x10-19) m =
ANS.
630 nm.

13. At very high temperature, why an extrinsic

semiconductor behaves like an intrinsic one?

Thermally generated free electrons in

ANS. conduction band will be much higher than
number of electrons contributed by donors.

14. On which factors does the depletion region of a pn

junction depend?

Depletion width depends on donor/ acceptor

ANS.
concentrations, biasing.

15. Why does most of the reverse battery voltage

applied across the diode terminals, appears across
the depletion region?

ANS. Because of high resistance in depletion region.

16. Does the reverse saturation current in a pn

junction diode change with applied reverse bias and
diode temperature?

ICO is independent of reverse voltage but

ANS.
doubles for every 100C rise in temperature.

17. Dynamic resistance of a Zener diode is zero but

its dc resistance is not so. Explain why?

Dynamic resistance = ratio of increment in

reverse voltage to reverse current.
ANS.
dc resistance = ratio of diode voltage to diode
current (in 3rd quadrant)

18. Mention the factors that affect the biasing stability

of a transistor.

ANS. Factors are VBE, ICO, and transistor beta.

19. An npn transistor has alpha = 0.98. What is the

value of transistor beta?

Beta = alpha/(1-alpha)=0.98/(1-
ANS.
0.98)=0.98/0.02=49

20. The term "fill factor" applies to which

semiconductor device?

ANS. Solar cell

21. The junction capacitance of a pn diode is utilized

in __________ diode. Fill in the blank.

ANS. Varactor

22. Both the emitter and collector junctions of a

transistor are found forward-biased. In which region
does the transistor operate?

ANS. Saturation region.

23. If AI, AV, and AP are the current, voltage, and

power gains of a transistor amplifier, how they are
inter-related?

ANS. AV = AP/AI

24. The transistor alpha increases with the increase in

the reverse bias voltage of the collector junction -
explain why?

ANS. Reduction of base width and induced drift field

25. The frequency of the ripple current of a full-wave

rectifier is twice that of a half-wave rectifier. Justify or
correct the statement.

ANS. The statement is correct.

26. Why is the field effect transistor called a unipolar

transistor?

ANS. One type of charge for transport.

27. How can you justify that a FET is a voltage-

controlled device?

Carriers are transported through the channel

ANS.
through drift.

28. Why is the gate current of a MOS FET much smaller (nearlly zero, in practice)?
ANS. There is insulating dielectric (SiO2) layer in between gate terminal and channel.

29. What type of gate voltage is necessary in a p-channel

enhancement MOSFET to cause flow of current?

Negative gate bias is required to

ANS.
cause p-type inversion layer.

30. Write down the relationship among the ac drain resistance

(rd), mutual conductance (gm) and amplification factor (μ) of FET.

ANS.
μ =rdgm

31. From the definition, what is the ac drain resistance if a change

of VDS by 2 V causes a change of drain current by 10 μA at a
constant VGS?

By definition, rd = 2 V/ (10 x10-6 A)

ANS.
= 2x105 Ω= 200 kohm.

32. Transfer characteristic curve of a FET refers to the plot of

________ versus ________ for a constant _______. Fill in the
blanks with VDS, ID, VGS as appropriate.

ANS. ID versus VGS for a constant VDS.

33. n-channel MOSFET has smaller area than the p-channel

MOSFET as far as the device resistance is concerned. Justify the
statemnet.

This is accounted for the fact that

ANS. electron mobility is larger than the
hole mobility.

34.As VGS changes from -1 V to - 1.5 V at constant VDS, the drain

current ID reduces from 7 mA to 5 mA. What is the
transconductance of the FET?
ANS. gm = (ΔID/ΔVGS) =4 mA/V

35. An n-channel MOSFET in the depletion mode has pinch-off

voltage VP = -5 V and operates with VGS = 0 V, VDS = 7 V. Justify
that the operation is in the active region.

For active region, VGS ≥VP; and

ANS.
VDS ≥VGS - VP.

36. What is the method to produce Ingots of single crystal silicon

from the molten polycrystalline silicon?

ANS. Czochralski method

37. The full name of the abbreviation CVD is

ANS. Chemical Vapor Deposition

38. Photon with frequency ν is used to excite a semiconductor of

energy band gap E g. What is the range of wavelength for which the
material will be transparent to light?

ANS. For transparent, hν< E g or λ> hc/ Eg

39. For direct recombination to be fast enough, the mean life time
of electron hole pair (EHP) is usually of the order of ________ (fill
in the value).

ANS. Mean life time ≈10-8 second.

40. A monochrome light with power 10 mW illuminates GaAs (Eg = 1.43 eV). Energy absorbed is 9 mW. What
is the amount of energy converted to heat per second?

ANS. Heat = fraction X absorbed energy = [(2 - 1.43)/2] x 9 mW = 2.57 mJ/s

41. A photodiode is operated in the 3rd quadrant of its current-

voltage characteristics. So the diode must be reverse-biased along
with illumination is on. Do you agree with this?

Yes, diode is in reverse-biased,

ANS.
and light radiation be on.

42. How the population inversion of a pn junction laser is achieved?

ANS. By forward biasing the junction.

43. What is the function of an optical resonant cavity in a laser?

To assist the photon field to grow

ANS.
inside the structure.

44. What is the reason that GaAs can be grown on Ge substrate?

Epitaxial growth needs lattice

ANS. matching and nearly identical
crystal structure.

45. In the indirect optical transition, electron momentum is

conserved. Out of photon, electron, and phonon, which one
maintains this conservation?

Phonon maintains the momentum

ANS.
conservation.

46. Absorption of optical energy in semiconductors due to impurity absorption is usually much smaller than the
intrinsic absorption process. What is the reason of this?

ANS. Impurity concentration is much smaller than the concentration of intrinsic semiconductor atoms.

47. In photoluminescence, if recombination via defect level is slow process (mean

life time ∼1 minute), the process is called ___________ (fill in the blank).
ANS. Phosphorescence.

48. In the p+-i-n+ diode, out of the three regions p+, i (intrinsic), and n+ which one is
highly resistive?

ANS. Intrinsic region

49. The phototransistor can provide amplification whereas photodiode can not. What is the reason behind this?

ANS. Photogenerated current is multiplied by a factor (β+1) due to transistor action.

50. For the laser action to persist, stimulated emission must be

enhanced over spontaneous emission. Do you support the
statement?

Yes, the statement is okay. This is

ANS.
achieved by optical resonant cavity.

51. In the Metal-Oxide-Semiconductor structure, flat band

condition is achieved by applying ________ voltage to the gate.
(fill in the blank)

ANS. Negative voltage

52.MOS capacitor is due to the combination of (i) Gate metal,

SiO2, Semiconductor (ii) Source, Channel, Drain. Which
combination is correct?

ANS. (i) Gate metal, SiO2, Semiconductor

53. The threshold voltage VT at the pinch-off point is decided by

VGS and VDS. Find the relation among these three.

ANS. VT = VGS - VDS.

Short Answer type:

54. (a) Explain the differences in the band structure of a metal, an insulator, and a semiconductor. (3)
(b) Write down the Fermi Dirac distribution function and explain its importance. (2)

ANS.Sketch energy band structures for metal, insulator, semiconductor, label all bands, identify solids in terms
of energy band gap.

Define all symbols in the FD distribution, plot this, state importance of Fermi level EF.

55. (a) Define the terms (i) doping, (ii) n-type semiconductors, (iii) p-type semiconductors. (3)

(b)What is mass-action law for the carrier concentrations in semiconductors. (2)

ANS. Explain doping concept, types of dopants, semiconductor types based on dopant atoms.

Electron and hole concentrations product is a constant, depends on temperature, etc

56. (a) Discuss the process of current flow in a biased n-type

semiconductor. (2)(b) Explain the process of recombination of
electrons and holes in a semiconductor. Comment on how charge
concentrations remain constant even electron hole pair recombination
in progress. (2+1)

ANS.

Explain drift flow of majority and

minority charge carriers due to
applied bias.

Mention equilibrium that

maintains at any temperature
between generation and
recombination rates.

57. (a) Derive the expression for the electrical conductivity in a semiconductor. (3)

(b) At 300 K, the intrinsic concentration of Ge is 2.5x1019m-3, electron mobility is 0.38 m2V-1s-1, hole
mobility is 0.18 m2V-1s-1. Calculate the intrinsic conductivity of Ge at 300K. (2)

ANS.

Derive σ= e(n. μn + p.μp) defining current density, mobility. Mention unit of sigma.Use above
expression and put the given values to find sigma = 2.24 S/m.

58. (a) Discuss the diffusion of current carriers in semiconductors.

By defining diffusion constant, write the expressions for the
diffusion current densities for electrons and holes. (1+2)
ANS.

Write Jp (diffusion), Jn (diffusion).

Define Dp, Dn. Find D/μ ratio for
electrons and holes.

59. (a) What is an abrupt and a linearly graded p-

n junctions? (2)(b) Define the static and dynamic
resistance of a p-n diode. Draw the volt-ampere
characteristics of a p-n diode. (2+1)

ANS.

Schematic of diode with doping profile

(concentration against distance down the diode).

Define the resistances in reference to the V-I

characteristics curve. Static resistance = V/I;

Dynamic resistance = ∂V/∂I.

60. (a) What is a breakdown diode? Discuss the

origin of breakdown mechanism in a p-n junction.
(2)
(b) Explain with a circuit diagram, the use of
Zener diode as a reference voltage source. (3)

ANS.

Explain Zener and Avalanche breakdown

mechanisms.

Draw the circuit with Zener diode in reverese biased

state in parallel with a load resistance, biased with a
supply voltage with a series current limiting resistor.
Explain the operation along with load and line
regulation.

61.
(a) What do you mean by the static
characteristics of a transistor? Draw a circuit
diagram of an n-p-n transistor operating in the
common-emitter configuration and sketch its
output characteristics. (3)
(b) Label the active, cutoff, and saturation region.
(2)

ANS.

Define the characteristic curves. CE mode with

proper labels.

62. (a) Discuss the origin of breakdown

phenomena of a p-n junction. (2)
(b) Discuss with neat sketch, the operation of a
Zener diode as a reference voltage source. (3)

ANS.

(a) Explain Zener and Avalanche breakdown

mechanisms.

(b) Explain operation, comment on regulation.

63. (a) Sketch the input characteristics of an n-p-

n transistor operating in the common emitter
mode. Discuss the nature of the curves
qualitatively.
(b) How can you find the h-parameters hie and
hre from the input characteristics?

ANS.

(a) Draw npn transistor circuit in CE mode, its input

curves with proper labels.

(b) Define above h-parameters and find from input

plots.

64.
(a) Draw the circuit diagram of a half wave rectifier and explain its
principle of operation. (2)
(b) A diode with forward resistance 20
Ω
is used to half-wave rectify a sinusoid of amplitude 40 V, the load
resistance being 1 k
Ω
. Calculate the dc load current and the dc voltage across the diode.
(3)

ANS.

(a) Circuit diagram should be

properly labelled. Plot sinusoidal
input versus rectified output
waveforms.

(b) Find Im = 40/(20+1000) A; I dc =

Im/π=12.48 mA; Vdc(diode) =
IdcRf = 12.5x20 mV = 0.25 V.

65. (a) Define the hybrid parameters of a basic transistor circuit in

any configuration. (2)
(b) Show how the h-parameters can be determined from the input
and output characteristic curves of a transistor.(3)

ANS.

(a) First identify the input and

output ports with current/ voltage
values. Then define parameters.

(b) hi, hr from input curves; hf,

ho from output curves.

66. a) Sketch the structure of an n-channel depletion type MOSFET

and discuss its principle of operation. (3)
b) Draw the typical volt-ampere drain characteristics of an n-
channel MOS transistor. (2)

ANS.

Sketch should include n+ (S), n

(diffused channel), n+ (D).

Plot should show label of axes,

VGS variation, linear/ saturation
active regions.

67. With a neat sketch describe the construction of an

enhancement-type MOSFET using p-type silicon substrate. Mention
the structural differences between enhancement and depletion
types of the MOSFET. What is gate-source threshold voltage of a
MOSFET?

ANS.

See Theory.

Threshold voltage ensures the

channel formation.

68. (a) Sketch the circuit symbols of MOSFETs (both enhancement

and depletion modes). (2)
(b) Draw the typical volt-ampere drain characteristics and the
transfer characteristics of an n-channel MOSFET operated in both
enhancement and depletion modes. (3)

ANS.

(a) enhancement (n channel, p

channel); depletion (n channel, p
channel)

(b) ID versus VDS with VGS as

parameters. Transfer curve -
ID versus VGS for fixed VDS.

69. (a) For an n-channel MOSFET, the drain current is given by ID =

K[(VGS - VT)VDS - 0.5 VDS2]. Derive the expression for the drain
resistance in the linear region of operation. (2)
(b) Find the expression for the saturation drain current. (3)

ANS.

(a)Neglect second term in linear

region and find rDS = ∂VDS/∂IDS.

(b) Prove that at VDS = VGS - VT,

saturation occurs. and find ID.

70. (a) Write down the expression representing the small - signal ac model of a FET. Also sketch the
circuit model. (2)

(b) Define the FET parameters transconductance (gm), ac drain

resistance (rd) and amplification factor (μ). (3)

ANS.

(a) id = gm vgs + vds/rd

(b) Write appropriate

definitions.

71. (a) What is a phototransistor? Explain its principle of operation. (3)

(b) Draw the volt-ampere characteristics of a phototransistor. (2)

ANS.

(a) State function, sketch

phototarnsistor layout as npn
transistor with base open, write
Ic in presence of illumination.

Photo-transistors are operated

in their active regime The
collector of an NPN transistor is
made positive with respect to
the emitter or negative for a
PNP transistor.

The light enters the base region

where it causes hole electron
pairs to be generated. The
hole-electron pairs move under
the influence of the electric
field. As a result the photodiode
current is multiplied by the
current gain β of the transistor.

72. What is a semiconductor laser? Outline the principles of laser action. Explain the terms spontaneous and
radiation induced (stimulated) transitions.

ANS. Explain p-n junction laser in terms of energy band diagram, general lasing action, population inversion,
pumping, etc

73. Explain the principle of operation of a Light Emitting Diode (LED). Why is silicon not the preferred
material for LED?

ANS. Explain excess carrier recombination and electromagnetic energy radiation, relate emitted photon
frequency with energy band gap, probability of recombination in direct band gap and indirect band gap
materials, preferred materials used,

74. Write down the steps involved in the twin tub process of CMOS fabrication. Also sketch the
layout.

ANS. Steps are (i) Choose epitaxial layer as base of the device (ii) N-well and P-well formation (iii)
Gate and Field oxide (iv) Source and Drain implantation (v) contact cuts in both wells (v) metallization.

75. Q.a

As a function of distance across the junction, sketch the curves of (i) charge density, (ii) electric field,
and (iii) potential for the open-circuited p-n junction.

Ans: Theory. (6 Marks)

Q.b Illustrate the V-I characteristic curve and explain the operation of a Zener diode.

Ans: Theory. (6 Marks)

Q.c Compare Avalanche and Zener breakdown mechanisms.

Ans: Theory.

76. Q.a Draw the energy band diagram of metal and semiconductor before and after conduction is
made.

Ans:
Mention CB, VB, Fermi Level, vacuum level, band bending in semiconductor side, etc

(5 Marks)

Q.b Analyze how Zener diode is used in Voltage regulation.

Ans: Mention Zener regulation (supply voltage and load)
(10 Marks)

77. Q.a Sketch the circuit of a half-wave rectifier and explain its operation.

Ans: Sketch proper circuit diagram, mention polarity and direction of voltage, current. Explain
operation with neat sketch of output voltage compared to input one. (7 Marks)

Q.b Derive VDC, Vrms, rectifier efficiency and PIV for it.
Ans: Derive the expressions for the quantites sought. (8 Marks)

78. Q.a Distinguish between the different types of transistor configurations with necessary circuit
diagrams.

Ans: Explain CE, CB, CC configurations (6 Marks)

Q.b Sketch the family of common emitter input and output characteristics for a transistor. Explain the
shapes of the curves qualitatively.
Ans: Theory (9 Marks)

79. Q.a Compare Zener and Avalanche breakdown mechanisms.

Ans: Zener (Avalanche) - ionization due to filed (collision), less (more) breakdown voltage, thin
(relatively thick) depletion region, destruction of junction temporary (permanent), junction electric field
strong (relatively weak), doping level heavily doped (lightly doped), temperature coefficient negative
(positive) (5 Marks)
Q.b In the adjoining diagram, the Zener diode has the breakdown voltage of Vz = 3V. The maximum
power dissipation limit is 20 mW. Find current Iz through the diode and power dissipation Pz in the
diode.
Ans:
Apply KVL in meshes.
IR1+(I-Iz)R2=12; IzR3+Vz-(I-Iz)R2=0; Solving, Iz = 3 mA. Power Pz=IzVz=9 mW < 20 mW (10 Marks)
80. Q.a Explain the term transistor biasing. What do you mean by quiescent point of a transistor
amplifier and what are the factors determining the choice of the Q-point?

Ans: Theory (5 Marks)

Q.b For the circuit shown, transistor β= 100 for the silicon transistor. Find the Q-point (VCE and IC).
Ans:
Find Thevenin voltage VT and Thevenin resistance RT.
Input loop gives IB: VT - IBRT - VBE - (1+beta)RE = 0. This gives IB = 4.886 micro A. IC = beta.IB = 4.886
mA, IE = 4.935 mA. KVL to collector circuit: VCE = VCC - IC RC - IERE = 2.646 V. (10 Marks)
81. Q.a What is Fermi level? Prove that the Fermi level lies exactly in the middle between the bottom
of the conduction band and the top of the valance band of intrinsic semiconductor.

Ans: Fermi level - The highest energy level that an electron can occupy at absolute zero
temperature. (3+4 Marks)

Q.b Derive the Einstein’s relation in semiconductors? Find the diffusion co-efficient of electron in Si at
300 K if μn = 0.19 m2V-1S-1.
Ans: Assume non-uniformly doped semiconductor, no electrical connection, so Jn = Jp = 0. Assume n
∼Nd(x). Electric field F = - (kT/e).[1/Nd(x)].∂Nd(x)/∂x. Put F in the expression of J n = eμn.Nd(x).F +
eDn.∂Nd(x)/∂x. Hence Dn/μn = kT/e.(4+4 Marks)
82. Q.a What is the source of electrons and holes in an intrinsic semiconductor? In general, how
many components of conduction current are there in any semiconductor and what are they?

Ans:
Thermally generated electrons and holes.
Currents carried by majority and minority charge carriers, both field induced (drift) and diffusion
current components. (6 Marks)

Q.b
Derive the equation for the thermal equilibrium concentrations of electrons and holes in terms of
Fermi energy.
Ans: Equilibrium electron concentration n0 = integrate f(E)N(E) wrt E from Ec to infinity.
Equilibrium hole conc p0 = integrate f(E)N(E) wrt E from -∞to Ev. Here N(E) = density of states at E,
f(E) = Fermi function. Assume effective density of states, intrinsic conc = ni, finally get n0 = niexp(-[Ei -
EF]/kT). Similarly, p0 can be obtained.(5 Marks)

Q.c The Fermi energy of a material is 0.5 eV. Find the probability of occupancy of the energy levels
0.4 eV and 0.6 eV. Assume kT = 25 meV.
Ans:
For E=0.6 eV, (E-EF)/kT = 0.1 eV/25 meV = 4; f(E=0.6) = 1/[1+e 4]=1/(55.59)=0.0179; f(E=0.4)=1/[1+e-
4]=1/1.0183]=0.982. Thus below Fermi level, almost all states are occupied, above Fermi level,

occupancy prob. is very small.

(4 Marks)

83. Q.a Explain qualitatively, the formation of energy bands in semiconductors.

Ans: Theory (5 Marks)

Q.b What is the concept of electron effective mass in semiconductors? Explain

Ans: In semiconductor, electrons are influenced by external and internal forces, F ext,
Fint leading to Fext+Fint=mass x acceleration. Since Fint is not known precisely, we take Fext =
m* x acceleration. Same acceleration is assumed but with altered mass.
(4 Marks)
 Q.c Under parabolic approximation, the E-k relation for carriers in semiconductor is given by E
= ℏ2k2/2m. Sketch the e-k diagram for a direct band gap semiconductor showing conduction
band, valence band and forbidden energy gap. Show that the first and second derivatives of
energy will give the carrier velocity and mass, respectively.
Ans: Plot E versus k. Find ∂E/∂k and second derivative to justify.(6 Marks)
84. Q.a What is the significance of continuity equation for charge carriers in semiconductors? In this
light, discuss the time behaviour of excess carriers as a semiconductor relaxes to thermal equilibrium.

Ans: Net time rate of change of electron (hole) equals the influx (or injection), generation rate, and
recombination rate. Discuss with analytical details.(10 Marks)

Q.b A semiconductor is doped with both donor and acceptor dopants. Discuss the charge neutrality
for the condition of complete ionization.
Ans: Total positive charge equals total negative charge. Consider electrons, holes, ionized donors,
ionized acceptors. (5 Marks)
85. Q.a Define the mobility of of charge carriers in a semiconductor. Show that the mobility of the
electrons in a semiconductor is given as μ= eτ/m* where the symbols have usual meanings.

Ans: Define mobility as drift velocity per unit electric field. Under external force F = eE = rate of
change of momentum = m*v/τ. Then find mobility. Define relaxation time also.(6 Marks)

Q.b What do you mean by the lifetime of minority carriers in an impurity semiconductor? Why does
lifetime decrease when trap centres are introduced in the forbidden energy gap?
Ans: Theory (5 Marks)
Q.c Mention some advantages of semiconductor devices.
Ans: No filament power hence low power supply required, mechanically rugged, light weight,
miniature, large life cycle, minimum warm up time, etc. (4 Marks)
86. Q.a Define the three FET parameters gm , rd and μ. Draw the small-signal equivalent circuit.

Ans: Theory. (9 Marks)

Q.b An N-channel MOSFET amplifier operates in saturation mode. A gate bias of 4V is applied and
the source is shorted to ground. A load resistance of RD = 5 kΩ is connected between drain terminal
and the drain supply of VDD = +15V. Find the dc drain current ID and dc drain to source voltage VDS.
Given ID = K (VGS - VT)2 with K = 0.25 mA/V2 and VT = 2V. Verify also that the transistor is indeed in
saturation region.

Ans: ID = 0.25(4-2)2=1 mA; VDS = VDD - IDRD = 15 - 1x5=10V. Verify VDS > VGS - VT (6 Marks)

87. Q.a Draw the basic structure of N-channel enhancement MOSFET and explain the principle of
operation.

Ans: Theory. Check whether p-sub, n+ source, n+ drain, SiO2 insulator, metal contacts to
Gate/Source/Drain/Body are made.(7 Marks)

Q.b Explain how MOS structure can be used as a capacitor. Give the rough plot of capacitor-voltage
characterisitcs of the MOS capacitor.
Ans:
Theory. Include the terms flat-band condition, surface accumulation charge, surface depletion.
In the plot, accumulation, depletion and inversion regions must be shown. (8 Marks)
88. Q.a How depletion MOSFET differs from enhancement MOSFET in construction process? Explain
with suitable diagrams. Explain why depletion mode MOSFET is commonly known as "normally ON"
MOSFET.
Ans: Theory. Diffused channel is additionally built in D- MOSFET structure. Normally ON because
channel is always presents. (7 Marks)

Q.b Discuss the concept of threshold voltage in a MOSFET. What is its importance in device
operation? How can a FET be used as a voltage variable resistor? Explain with a suitable current-
voltage plot.
Ans: It is the Gate to source voltage than ensures channel development. (8 Marks)
89. Q.a Write down the advantages of MOSFET over BJT.

Ans: MOS (BJT) - one(two) type of carrier, transport by drift in channel (diffusion through base), high
frequency operation of FET compared to BJT, FETs more stable than BJT, high impedance than BJT,
FETs less noisy, etc. (6 Marks)

Q.b Draw a voltage-divider biasing circuit of MOSFET and explain the DC load line and AC load line
with suitable equations. Find the drain current and drain-to-source voltage at the Q-point .
Ans: Steps are - draw the biasing circuit, find Thevenin equivalent circuit at the input mesh, find V DS.
Define load line with the coordinates at the extreme end points, Find ac load line by shorting the
source-leg capacitor. (9 Marks)
90. Q.a Draw the circuit for obtaining drain and transfer characteristics of an N-channel MOSFET and
discuss the steps to obtain the same.

Ans: Theory. (6 Marks)

Q.b What is channel length modulation? Discuss the effect of channel length modulation parameter
Λ on the drain current with suitable expression for drain current. If the channel length modulation
parameter λ= 0, what will be ac drain resistance rd and why?
Ans: Explain how channel length L shortens when VDS increases, lambda is measured by
ΔL/L. Current is modeled as ID = K(VGS - VT)2[1 + λVDS]. If no length modulation, derivative of ID with
VDS is zero and its reciprocal defining ac drain resistance becomes infinity.(9 Marks)
91. Q.a What is pumping in the context of a laser and why is a semiconductor very suitable for
efficient pumping?
Ans: Explain population inversion by photon excitation, electron excitation and inelastic collision
methods. (4 Marks)

Q.b Explain emission, absorption and radiation of laser.

Ans: With suitable energy band diagram, explain carrier transition mediated by incident radiation.
Explain absorption, emission (spontaneous/ stimulated). (6 Marks)

Q.c Discuss the working principle of a semiconductor p-n junction laser.

Ans: Discuss the situation when a forward bias is applied in a heavily doped p-n junction, give the
rate of absorption, rate of emission. Draw schematic of a pn junction laser. Discuss how optical cavity
is created in pn laser. (5 Marks)

92. Q.a Discuss in details, the construction and working principle of PIN photodiode. What factors limit
the response time of photodiodes?

Ans: Construction - intrinsic layer is sandwiched between heavily doped p + and n+ regions. This
ensures wide depletion region, EHP generation in this region, carriers swept out due to high electric
field. Explain forward biased, reverse biased status under illumination.
Response time is limited by drift, diffusion, and junction capacitance.
(9 Marks)

Q.b Define quantum efficiency of a photodiode. How are PIN and Avalanche Photo Diode (APD)
different from each other?
Ans: Quantum efficiency - number of carriers collected for every photon = (Jop/e)/(Pop/hν). It is
normally nearly unity. If low level optical signal is to be detected, significant carrier multiplication is
needed. That is, this figure can be 2 or 3. This multiplication is achieved in APD.
(6 Marks)
93. Q.a Explain the construction and working of phototransistors. Mention the advantages and
disadvantages of phototransistors.

Ans: Sketch npn transistor with Base open and Collector is at positive potential wrt Emitter to ensure
reverse bias at collector junction.
Under dark condition, Ic = (β+1)Ico as base is open When base is illuminated by lense-focussed beam,
photo generated carriers contribute to reverse current. Thus, Ic = (β+1)[Ico + Iph].
Due to transistor action, photocurrent is multiplied by (beta +1).
Draw the Ic - Vcc plot with illumination intensity as parameter.
Adv - higher current than photodiode, less expensive, simple and miniature, fast response
Disadv - can not handle high voltages , vulnerable to surges/ spikes of electricity/em energy.

(9 Marks)

Q.b What is LED? Explain its construction and working. Why is a phototransistor an improved version
of a PIN diode? Write down the operation of an optocoupler.
Ans: Due to transistor action photocurrent gets multiplied. (6 Marks)
94. Q.a What are the methods of introducing dopant impurities in a wafer? What is ion implantation?
Mention its advantages and problems involved in ion implantation?

Ans:
Diffusion and ion implantation are the two methods of doping. Explain each process in brief.
(7 Marks)

Q.b
Mention the features in metallization scheme. Why is aluminium the preferred metal for contacts?
Ans:
The Metallization Process takes place in a vacuum evaporation chamber ( pressure is 10-6 to 10-
7 Torr. The material is placed in a basket. Using electron gun, electron beam is focused at the surface

of the material, material starts heating up and vaporizing. The vapours hit substrate and condence to
form a thing film coating. By etching process, aluminium is removed form unwanted places.

Metallization applications can be divided into three categories: gate, contact, and interconnection.
Polysilicon and silicide are frequently used in gates and interconnects in MOS devices.
Aluminum and copper are used ss contact and second-level interconnection to the outside.

In most IC’s, aluminium is the widely used metal for metallization because (i) it is a good conductor
(ii) can form mechanical bonds with silicon (iii) form low resistance, ohmic contacts with
heavily doped n-type and p-type silicon.

(4 Marks)

Q.c
Explain twin-tub process in CMOS fabrication.
Ans:
Steps are -
(i) choice of epi-layer as base of the device
(ii) N-well and P-well formation
(iii) Gate and Field oxide
(iv) Source and Drain implantation
(v) Contact cuts in both wells
(vi) Metallization
(4 Marks)