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Objective Questions

Short notes

Objective questions (Answers are given below)

LEDs are commonly fabricated from gallium
compounds like gallium arsenide and gallium
phosphide because they
(a) are cheap
(b) are easily available
(c) emit more heat
(d) emit more light.
A LED is basically a ………………. P-N junction.
(a) forward-biased
(b) reverse-biased
(c) lightly-doped
(d) heavily-doped.
As compared to a LED display, the distinct advantage
of an LCD display is that it requires
(a) no illumination
(b) extremely low power
(c) no forward-bias
(d) a solid crystal
Before illuminating a P-N junction photodiode,
it has to be
(a) reverse-biased
(b) forward-biased
(c) switched ON
(d) switched OFF.
In a photoconductive cell, the resistance of the
semiconductor material varies …………. with the
intensity of incident light.
(a) directly
(b) inversely
(c) exponentially
(d) logarithmically.
A photoconductive cell is known as ……………..
cell.
(a) phototransistor
(b) photoresistor
(c) photovoltaic
(d) both (a) and (b).
A phototransistor excels a photodiode in the
matter of
(a) faster switching
(b) greater sensitivity
(c) higher current capacity
(d) both (a) and (b)
(e) both (b) and (c).
A photodarlington comprises of
(a) a phototransistor
(b) a transistor
(c) a photodiode
(d) both (a) and (b).
A solar cell operates on the principle of
(a) diffusion
(b) recombination
(c) photo voltaic action
(d) carrier flow.
Solar cells are used as source of power in earth
satellites because they have
(a) very high efficiency
(b) unlimited life
(c) higher power capacity per weight
(d) both (b) and (c)
(e) both (a) and (b).
The device possessing the highest sensitivity
is a
(a) photo conductive cell
(b) photovoltaic cell
(c) photodiode
(d) phototransistor
The unique characteristics of LASER light are
that it is
(a) coherent
(b) monochromatic
(c) collimated
(d) all of the above
The LASCR operates like a
(a) latch (b) LED
(c) photodiode (d) phototransistor.
Optical couplers are designed to …………. one
circuit from another.
(a) control (b) isolate
(c) disconnect (d) protect.
The main purpose of using optical isolators is
to provide protection to devices from
(a) high-voltage transients
(b) surge voltages
(c) low-level noise
(d) all of the above.

A LED emits visible light when its …………..
(a) P-N junction is reverse-biased
(b) depletion region widens
(c) holes and electrons recombine
(d) P-N junction becomes hot.
In LED, light is emitted because
(a) recombination of charge carriers takes
place
(b) diode gets heated up
(c) light falling on the diode gets amplified
(d) light gets reflected due to lens action.
GaAs, LEDs emit radiation in the
(a) ultraviolet region
(b) violet-blue green range of the visible region
(c) visible region
(d) infra-red region
Phototransistors respond much like a conventional
transistor except that, in their case, light
energy is used to ………
(a) alter leakage current
(b) change base voltage
(c) switch it ON
(d) alter emitter current.