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Semi Conductor Flashcards

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1
Q

Intrinsic semiconductors are those

A.Which are made of semiconductor material in its purest form
B.Which have zero energy gap
C.Which have more electrons than holes
D.Which are available locally

A

Which are made of semiconductor material in its purest form

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2
Q

Intrinsic semiconductor at room temperature will have, …………….. available for conduction

A.Electrons
B.Holes
C.Both electrons and holes
D.None of the above

A

Both electrons and holes

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3
Q

A pure semiconductor behaves like an insulator at 00 K because

A.There is no recombination of electrons with holes
B.Drift velocity of free electrons is very small
C.Free electrons are not available for current conduction
D.Energy possessed by electrons at that low temperature is almost zero

A

Free electrons are not available for current conduction

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4
Q

The energy gap is much more in silicon than in germanium because

A. It has less number of electrons
B. It has high atomic mass number
C. Its crystal has much stronger bonds called ionic bonds
D. Its valence electrons are more tightly bound to their parent nuclii

A

Its valence electrons are more tightly bound to their parent nuclii

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5
Q
A
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6
Q

A P-type semiconductor results when

A. A pentavalent impurity is added to an intrinsic semiconductor
B. A trivalent impurity is added to an intrinsic semiconductor
C. Either a pentavalent or trivalent impurity is added to an intrinsic semiconductor
D. None of the above

A

A trivalent impurity is added to an intrinsic semiconductor

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7
Q

A semiconductor has…. temperature co-efficient of resistance.

A. Zero
B. Positive
C. Negative
D. None of the above

A

Negative

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8
Q

Which of the following cannot exist outside a semiconductor

A. Hole
B. Electron
C. Both (a) and (b)
D. None of the above

A

Hole

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9
Q

In a N-type semiconductor, the position of the Fermi level

A. Is at the centre of the energy gap
B. Is lower than the centre of energy gap
C. Is higher than the centre of energy gap
D. Can be anywhere depending upon the doping concentration

A

Is higher than the centre of energy gap

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10
Q

As the temperature of a semiconductor increases its

A. Conductivity increases
B. Resistivity increases
C. Atomic number decreases
D. Temperature co-efficient becomes zero

A

Conductivity increases

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11
Q

Which of the following doping elements would not be suitable for converting intrinsic semiconductor to n type

A.Phosphorous
B.Indium
C.Antimony
D.Arsenic

A

Indium

n type : Group 15 -Phosphorus, Arsenic, Antimony
p type - Group 13 - Boron, Al, Gallium, Indium,

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12
Q

In a semiconductor diode, the barrier offers opposition to

A. Majority carriers in both regions
B. Majority as well as minority carriers in both regions

A

Majority carriers in both regions

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13
Q

The temperature co-efficient of an extrinsic semiconductor is…….

A. Zero
B. Positive
C. Negative
D. None of the above

A

Negative

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14
Q

…….. has the highest mobility.

A. Electron
B. Positive ions
C. Negative ions
D. Neutron

A

Electron

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15
Q

Consider the energy level diagram of an intrinsic semiconductor. The Fermi level lies in the

A. Valence band
B. Forbidden band
C. Conduction band
D. It can be at any of the above locations depending upon the doping concentration and temperature

A

Forbidden band

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16
Q

In any specimen the Hall voltage is proportional to magnetic field β as

A. β
B. β2
C. 1/β
D. 1/β2

A

β

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17
Q

Intrinsic concentration of charge carriers in a semiconductor varies as

A. T^3
B. T^2
C. T
D. 1/T

A

T^3

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18
Q

The dynamic resistance of a diode varies as

A. i
B. i^2
C. 1/i
D. 1/i^2

A

1/i

In forward bias, the dynamic resistance decreases with increasing current.
In reverse bias, the dynamic resistance increases with increasing reverse voltage.

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19
Q

The forbidden band in germanium at 00K is

A. 0.03 eV
B. 0.785 eV
C. 1.5 eV
D. 2.0 eV

A

0.785 eV

The energy gap (Eg) of an intrinsic semiconductor remains constant, unaffected by temperature changes

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20
Q

The atomic number of germanium is…….

A.4
B.8
C.16
D.32

A

32

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21
Q

For a germanium P-N junction, the barrier potential is nearly…….

A. 0.15 V
B. 0.3 V
C. 0.45 V
D. 0.6 V/

A

0.3 V

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22
Q

For a silicon P-N junction, the barrier potential is about

A

0.7 V

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23
Q

A zener diode is invariably used with

A. Reverse bias
B. Forward bias
C. Zero bias
D. Any of the above

A

Reverse bias

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24
Q

The crystal diode is used as a
A. Rectifier
B. Amplifier
C. Oscillator
D. Any of the above

A

Rectifier

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25
Q

With increases of reverse bias, the reverse saturation current in P-N diode

A. Increases
B. Remains constant
C. Decreases
D. Increases as inverse of reverse bias

A

Remains constant

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26
Q

The maximum reverse voltage that can be applied to an ordinary semiconductor diode without irreverslible damage is called …..

A. Cut-off voltage
B. Avalanche breakdown voltage
C. Peak inverse voltage
D. Zener voltage

A

Peak inverse voltage

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27
Q

The resistivity of a semiconductor lies

A. Below 10^-6
B. Between 10^-6 to 10^2 ohm-metre
C. Between 10^-6 to 10^4 ohm-metre
D. Above 10^4 ohm-metre

A

Between 10^-6 to 10^2 ohm-metre

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28
Q

Which of the following materials can be used to make a light-emitting diode?

A. Silicon
B. Germanium
C. Gallium arsenide
D. Phosphorescent material

A

Gallium arsenide

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29
Q

For a half wave rectified sine wave the ripple factor is

A.1.65
B.1.45
C.1.21
D.1.00

A

1.21

Ripple Factor : RMS/DC Current
Higher ripple factors indicate greater AC voltage fluctuations

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30
Q

The ripple factor of a full wave rectifier

A

0.48

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31
Q

When forward biased, LED emits light because of

A. Recombination of carriers
B. Light generated in breaking the covalent bonds
C. Light produced by collisions
D. All of the above reasons

A

Recombination of carriers

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32
Q

The LEDs made with GaAs emit light in the

A. Yellow region
B. Infrared region
C. Orange region
D. Red visible region

A

Infrared region

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33
Q

Which column elements are combined to make compound semiconductors?
a) First and fourth
b) Fifth and sixth
c) Second and fourth
d) Third and fifth

A

Third and fifth (13 & 15)

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34
Q

Compound semiconductors are also known as direct band gap semiconductors.
a) True
b) False

A

True

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35
Q

Which method can be used to distinguish between the two types of carriers?
a) Hall effect
b) Rayleigh method
c) Doppler effect
d) Fermi effect

A

Hall effect

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36
Q

A semiconducting crystal 12mm long, 5mm wide and 1mm thick has a magnetic flux density of 0.5Wb/m2 applied from front to back perpendicular to largest faces. When a current of 20mA flows length wise through the specimen, the voltage measured across its width is found to be 37μV. What is the Hall coefficient of this semiconductor?

A

I = 20 mA
B =0.5 Wb/m2
w : 1 mm => Length along magnetic field
Vh = 37 micro V
Vh = BI/new
Rh = 1/ne
= VhW/BI
= 3710^-6110^-3/(2010^-30.5)
= 3.7*10^-6 m^3/C

37
Q

The intrinsic carrier density at room temperature in Ge is 2.37×10^19/m3. If the electron and hole mobilities are 0.38 and 0.18 m2/Vs respectively. Calculate its resistivity.

A

Conductivity = ne(mu_e + mu_h)
= 2.3710^191.610^-19(0.38+.18)
= 2.123 /ohm m

Resistivity = 1/conductivity
=0.4709 ohm m

38
Q

A silicon plate of thickness 1mm, breadth 10mm and length 100mm is placed in a magnetic field of 0.5 Wb/m2 acting perpendicular to its thickness. If 10^-3 A current flows along its length, calculate the Hall voltage developed, if the Hall coefficient is 3.66×10^-3 m3/Coulomb

A

Vh = BI/new = RB/w
= 0.510^-33.66*10^-3/0.001
= 1.83×10^-3 Volts

39
Q

The conductivity of germanium at 20°C is 2/ohm m. What is its conductivity at 40°C? Eg=0.72eV

A

σ = Ce(-E/2KT)
7.51 /ohm m

40
Q

What is the Fermi energy of a n-type semiconductor?
a) E
b) E(F) = (Ec + Ev)/2
c) EF = (Ec + Ed)/2
d) EF = (Ev + Ea)/2

A

EF = (Ec + Ed)/2

41
Q

EF = (Ec+ Ev)/2, this represents the Fermi energy level of which of the following?
a) Extrinsic semiconductor
b) N-type semiconductor
c) P-type semiconductor
d) Intrinsic semiconductor

A

Intrinsic semiconductor

42
Q

For semiconductors, the resistivity is inversely proportional to the temperature for semiconducting materials.
a) True
b) False

A

True

it has a negative temperature coefficient. When the temperature of the semiconductor is increased, large numbers of charge carriers are produced due to the breaking of covalent bonds. These charge carriers move freely, hence conductivity increases and therefore the resistivity decreases.

43
Q

When does a normal conductor become a superconductor?
a) At normal temperature
b) At Curie temperature
c) At critical temperature
d) Never

A

The temperature at which a normal conductor loses its resistivity and becomes a superconductor is known as transition temperature or critical temperature.

44
Q

. In which of the following does the residual resistivity exist?
a) Impure metal at high temperature
b) Pure metal at low temperature
c) Pure metal at high temperature
d) Impure metal at low temperature

A

Impure metal at low temperature

When the temperature is reduced to 0K, the resistivity of the impure metal doesn’t become zero, because there exist some impurities which gives rise to minimum resistivity known as residual resistivity.

45
Q

Meissner effect occurs in superconductors due to which of the following properties?
a) Diamagnetic property
b) Magnetic property
c) Paramagnetic property
d) Ferromagnetic property

A

Diamagnetic property

A diamagnetic material has a tendency to expel magnetic lines of forces. Since the superconductor also expels magnetic lines of force it behaves as a perfect diamagnet. This behaviour is first observed by Meissner and is hence called Meissner effect.

46
Q

What happens when a large value a.c. current is passed through superconductors?
a) Conductivity increases
b) Superconducting property is destroyed
c) It acts as a magnet
d) It becomes resistant

A

Superconducting property is destroyed

When a large value of a.c. current is applied to a superconducting material it induces some magnetic field in the material and because of this magnetic field, the superconducting property of the material is destroyed.

47
Q

Superconductors can be used as a memory or storage elements in computers.
a) True
b) False

A

True
Since the current in superconducting ring can flow without any change in its value, it can be used as a memory or storage element in computers.

48
Q

What is the energy level below which all levels are completely occupied at Zero Kelvin called?
a) Boson Energy
b) Fermi Energy
c) Stable Energy
d) Ground Energy

A

Fermi Energy

49
Q

The concentration of doping is kept below ______________
a) 1 %
b) 5 %
c) 10 %
d) 50 %

A

1 %

50
Q

Which one of the following is not an intrinsic semiconductor?
a) Carbon
b) Silicon
c) Germanium
d) Lead

A

Carbon

51
Q

Which of the following is n-type semiconductor?
a) CaO
b) MgO
c) ZnO
d) BaO

A

ZnO

II-VI semiconductors are generally p-type semiconductors except for ZnO and ZnTe.

52
Q

P-Type semiconductor has a lower electrical conductivity than N-Type semiconductor.
a) True
b) False

A

True

Due to comparatively lower mobility of holes than electrons for the same level of doing as in an N-Type semiconductor, it has lower electrical conductivity.

53
Q

Pure Si at 300 K has equal electron (ni) and hole concentration (p) of 1.5 X 1016 m-3. Doping by indium increases p to 4.5 X 1022 m-3. What is n in the doped silicon?

A

np*ne = ni^2

5*10^9

54
Q

In a semiconductor it is observed that three-quarters of the current is carried by electrons and one quarters by holes. If the drift speed is three times that of the holes, what is the ratio of electrons to holes?

A

1 : 1

Ie = 3⁄4 I and Ih = 1⁄4 I
Ie/Ih = nv_e/nv_h

55
Q

If the number of electrons (majority carrier) in a semiconductor is 5 X 10^20 m-3 and μe is 0.135 mho, find the resistivity of the semiconductor.
a) 0.0926 Ωm
b) 0.0945 Ωm
c) 0.0912 Ωm
d) 0.0978 Ωm

A

conductivity = ne*mu
Resistivity = 1/σ
= 0.0926 Ωm.

56
Q

n a P-N Junction, the depletion region is reduced when _________
a) P side is connected to the negative side of the terminal
b) P side is connected to the positive side of the terminal
c) N side is connected to the positive side of the terminal
d) Never reduced

A

P side is connected to the positive side of the terminal

57
Q

The voltage at which forward bias current increases rapidly is called as ___________
a) Breakdown Voltage
b) Forward Voltage
c) Knee Voltage
d) Voltage barrier

A

Knee Voltage

Till the knee voltage, the current in a semiconductor increases slowly. After Knee voltage, the current increases rapidly for a small change in the voltage.

58
Q

The Knee Voltage for germanium is _________
a) 0.1 V
b) 0.3 V
c) 0.7 V
d) 1.4 V

A

b) 0.3 V

for silicon it is 0.7 V.

59
Q

The resistance of the semiconductor decreases in forward biased.
a) True
b) False

A

a) True

60
Q

The current produced in reverse-bias is called as __________
a) Reverse Current
b) Breakdown Current
c) Negative Current
d) Leakage Current

A

Leakage Current

61
Q

Which diode is designed to work under breakdown region?
a) Photodiode
b) Light Emitting Diode
c) Solar Cell
d) Zener diode

A

Zener diode

Zener Diode is designed specifically to operate in the breakdown region. It is mostly used as a voltage regulator in various circuits.

62
Q

Zener Diode is designed specifically to operate in the breakdown region. It is mostly used as a voltage regulator in various circuits.

A

True

The current-voltage curve of a P-N junction diode is not a straight line. Thus, it does not obey Ohm’s law and is a non-ohmic device.

63
Q

The leakage current is measured in ________
a) A
b) mA
c) μA
d) nA

A

μA

64
Q

Zener diode is designed to specifically work in which region without getting damaged?
a) Active region
b) Breakdown region
c) Forward bias
d) Reverse bias

A

Breakdown region

65
Q

What is the level of doping in Zener Diode?
a) Lightly Doped
b) Heavily Doped
c) Moderately Doped
d) No doping

A

Heavily Doped

he breakdown voltage occurs at a lower voltage. If it were lightly/moderately doped, it would breakdown at a comparatively high voltage and, thus, would not be able to serve its purpose.

66
Q

Zener Diode is mostly used as ____________
a) Half-wave rectifier
b) Full-wave rectifier
c) Voltage Regulator
d) LED

A

Voltage Regulator

The Zener diode, once in the breakdown region, keeps the voltage in the circuit to which it is connected as constant. Thus it is widely used as a voltage regulator.

67
Q

In normal junctions, the breakdown is same as Zener breakdown.
a) True
b) False

A

False

In normal p-n unction diodes, the breakdown takes place by avalanche breakdown which is different than the Zener breakdown. Zener diode is specifically made to operate in that region.

68
Q

The depletion region of the Zener diode is ____________
a) Thick
b) Normal
c) Very Thin
d) Very thick

A

c) Very Thin

Zener diode is fabricated by heavily doping both p- and n-sides of the junction, which results in an extremely thin depletion region.

69
Q

he electric field required for the field ionization is of what order?
a) 10^4 V/m
b) 10^5 V/m
c) 10^6 V/m
d) 10^7 V/m

A

c) 10^6 V/m

In a Zener diode, a very high electric field is produced for even a very small voltage. The electric field required for field ionization is of the order 106 V/m.

70
Q

A light emitting diode is _________
a) Heavily doped
b) Lightly doped
c) Intrinsic semiconductor
d) Zener diode

A

Heavily doped

LED, is heavily doped. It works under forward biased conditions. When the electrons recombine with holes, the energy released in the form of photons causes the production of light.

71
Q

Which of the following materials can be used to produce infrared LED?
a) Si
b) GaAs
c) CdS
d) PbS

A

GaAs

GaAs has an energy band gap of 1.4 eV. It can be used to produce infrared LED. Various other combinations can be used to produce LED of different colors.

72
Q

The reverse breakdown voltage of LED is very low.
a) True
b) False

A

True

The reverse breakdown voltages of LEDs are very low, typically around 5 V. So, if Eccess voltage is provided, they will get fused.

73
Q

What should be the band gap of the semiconductors to be used as LED?
a) 0.5 eV
b) 1 eV
c) 1.5 eV
d) 1.8 eV

A

1.8 eV

Semiconductors with band gap close to 1.8 eV are ideal materials for LED. They are made with semiconductors like GaAs, GaAsP etc.

74
Q

What should be the biasing of the LED?
a) Forward bias
b) Reverse bias
c) Forward bias than Reverse bias
d) No biasing required

A

Forward bias

75
Q

IV Characteristics of LED and Its frequency

A

When the Voltage required for same current increases (Slope of I-V graph Decreases) -> Frequency of light increases

76
Q

Increase in the forward current always increases the intensity of an LED.
a) True
b) False

A

False

As the forward current is increased for an LED, the intensity of the light increases up to a certain maximum value. After that, the intensity starts decreasing.

77
Q

Which process of the Electron-hole pair is responsible for emitting of light?
a) Generation
b) Movement
c) Recombination
d) Diffusion

A

Recombination

78
Q

What is the bandwidth of the emitted light in an LED?
a) 1 nm to 10 nm
b) 10 nm to 50 nm
c) 50 nm to 100 nm
d) 100 nm to 500 nm

A

10 nm to 50 nm

The bandwidth of the emitted light is 10 nm to 50 nm. Thus, the emitted light is nearly (but not exactly) monochromatic.

79
Q

Which of the following is not a characteristic of LED?
a) Fast action
b) High Warm-up time
c) Low operational voltage
d) Long life

A

High Warm-up time

80
Q

A solar cell is a ___________
a) P-type semiconductor
b) N-type semiconductor
c) Intrinsic semiconductor
d) P-N Junction

A

P-N Junction

A p-n junction which generated EMF when solar radiation is incident on it is called a solar cell. The material used for fabrication of solar cell should have a band gap of around 1.5 eV.

81
Q

Which of the following materials cannot be used as solar cells materials?
a) Si
b) GaAs
c) CdS
d) PbS

A

PbS

If we use PbS as the solar cell material, then most of the solar radiation will be absorbed on the top-layer of the solar cell and will not reach in the depletion zone.

82
Q

The principle of a solar cell is same as the photodiode.
a) True
b) False

A

True

The solar cell works on the same principle as the photodiode, except that no external bias is applied and the junction area is kept much larger.

83
Q

What is the difference between Photodiode and Solar cell?
a) No External Bias in Photodiode
b) No External Bias in Solar cell
c) Larger surface area in photodiode
d) No difference

A

No External Bias in Solar cell

84
Q

During the collection of e-h pairs, holes are collected by _________
a) Front contact
b) Back contact
c) Si-wafer
d) Finger electrodes

A

Back contact

As the electron-hole pairs move, the electrons are collected by the front contact and the holes reaching p-side are collected by the back contact.

85
Q

The I-V characteristics of a solar cell are drawn in the fourth quadrant.
a) True
b) False

A

True

The I-V characteristics of a solar cell is drawn in the fourth quadrant of the coordinate axis because a solar cell does not draw current but supplies the same to the load.

86
Q

What should be the band gap of the semiconductors to be used as solar cell materials?
a) 0.5 eV
b) 1 eV
c) 1.5 eV
d) 1.9 eV

A

1.5 eV

87
Q

Which of the following should not be the characteristic of the solar cell material?
a) High Absorption
b) High Conductivity
c) High Energy Band
d) High Availability

A

High Energy Band

The Energy Band of the semiconductor should not be too high. It should be around 1.5 eV so that the incident solar radiation can cause the generation of e-h pairs.

88
Q
A

Physics (5 decks)

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