# Electronics and Communication Engineering - Electronic Devices and Circuits

46.
Which of the following is basically a voltage controlled capacitance?
Zener diode
Diode
Varactor diode
LED
Explanation:

Therefore, varactor is used for tuning in TV.

47.
When the i-v curve of a photodiode passes through origin the illumination is
maximum
minimum
zero
equal to rated value
Explanation:

i is plotted on y-axis and v on x-axis. When passing through origin current can be zero only.

48.
An n type silicon bar 0.1 cm long and 100 μm2 in cross-sectional area has a majority carrier concentration of 5 x 1020/m3 and the carrier mobility is 0.13 m0/V-s at 300k. If the charge of an electron is 1.6 x 10-19 coulomb, then the resistance of the bar is
106 ohm
104 ohm
10-1 ohm
10-4 ohm
Explanation:

Resistance of bar(R) =

l = 0.1 cm, A = 100 x 10-6 m2

σ = neμn + Peμp

But bar is of n type then it can be approximated to σ = neμn.

49.
The threshold voltage of a MOSFET can be lowered by
1. using thin gate oxide
2. reducing the substrate concentration
3. increasing the substrate concentration.
Of the above statement
3 alone is correct
1 and 2 are correct
1 and 3 are correct
2 alone is correct
Explanation:

Increasing the substrate concentration lowers threshold voltage. Gate oxide layer has no effect an threshold voltage.

50.
In which device does the extent of light controls the conduction
photovoltaic cell
photo electric relay
LED
photo sensitive device