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50. When a battery is connected to the junction
diode with p-section connected to the negative pole
and n-section connected to the positive pole, the
junction is said to be reverse biased.
51. If the reverse bias is made very high, the covalent
bonds near the junction break down and a large
number of electron hole pairs are liberated. The
reverse current then increases abruptly to a relatively
large value. This is known as Avalanche break down.
52. The maximum voltage that a junction diode
can bear without break is called zener voltage and
the junction diodes possessing this voltage is known
as Zener diode.
53. When a junction diode is forward biased, energy
is released at the junction due to recombination of
electrons and holes. In case of silicon and germanium
diodes the energy released is in infra red region,
while in the junction diode made of gallium arsenide
or indium phosphide, the energy is released in visible
region. Such a junction diode is called light emitting
diode or LED.
54. When a metal is subjected to bombardment of
highly energetic electron, they strike out the free
electrons of metal and electrons come out of the
metal surface. This is known as secondary emission.
55. The electrons as well as hole mobility decreases
with the increase in temperature.
56. The value of potential barrier depends on the
doping of the semiconductor.
57. The width of the potential barrier is of the order
of 0.01 mm. It opposes the forward current and
also opposes the reverse current.
58. The depletion region widens during the reverse
bias and so it offer high resistance, while becomes
thin during forward bias and offer less resistance.
59. In an ideal diode the forward bias offers zero
resistance and acts as a short circuit. Voltage drop
across such a diode is also zero.
60. A signal which can have only discrete value is
called digital signal.
61. In the digital electronics, we make use of two
types of logic system : Positive logic and negative
logic.
62. In the positive logic 0 stands for lower (say
voltage level) and 1 stands for higher level.

63. In the negative logic 0 stands for higher (say
voltage) level and 1 stands for lower level.
64. A diode as a rectifier converts ac to dc.
65. Modulation is the process of superimposing signal
frequency (i.e audiowave) on the carrier frequency.
66. The ratio of change in collection current to the
change in emitter current for a constant value of
collection voltage in a common base arrangement
is current gain.
67. A transistor is preferable to triode valve because
it does not require a heater and hence efficiency
is higher.
68. The current gain of common base transistor is
less than 1.
69. In pnp transistor the p type crystal acts as either
emitter or collector.
70. Tow pnp transistors in series give a positive
AND circuit.
71. In npn transistor, the p-type crystal acts as base
only.
72. The emitter of a transistor is doped the heaviest
because it is a supplier of charge carriers.
73. Radar is a device for detecting distant enemy
planes.
74. Digital circuit can be made by respective use
of NAND gates.
75. To invert an input signal, complement a signal
or to change the logic in a digital circuit are the
functions of a NOT gate.
AND gate is equivalent to a series switching
76.
circuit.
77. Diffusion if holes and electrons causes the
depletion layer across the p-n junction.
78. The fermi energy is the maximum energy
possessed by the conduction electron at OK.
79. The conduction electrons have higher mobility
than the holes because they need less energy to
move.
80. The hole is created when valence band electrons
jump to conduction band.
81. There is no hole current in good conductors.
82. Forbidden band is absent in conductors small
in semiconductors and very large in insulators.

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Continued in last part 4

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