Choose the only false statement from the following.

In conductors the valence and conduction bands may overlap.
Substances with energy gap of the order of 10 eV are insulators.
The resistivity of a semiconductor increases within crease in temperature.
The conductivity of a semiconductor increases within crease in temperature.

Solution

Semiconductors have –ve temperature coefficient of resistivity.

Of the diodes shown in the following diagrams, which one is reverse biased ?

Solution

Positive terminal is at lower potential (0V) and negative terminal is at higher potential 5V.

In a p-n junction photo cell, the value of the photo-electromotive force produced by monochromatic light is proportional to

the voltage applied at the p-n junction
the barrier voltage at the p-n junction
the intensity of the light falling on the cell
the frequency of the light falling on the cell

Solution

Electromotive force depends upon intensity of light falling, it does not depend on frequency of barrier voltage.

The peak voltage in the output of a half-wave diode rectifier fed with a sinusoidal signal without filter is 10V.The d.c.component of the output voltage is

20/π V
10/π√2 V
10/π V
10V

Solution

In semiconductors at a room temperature

the conduction band is completely empty
the valence band is partially empty and the conduction band is partially filled
the valence band is completely filled and the conduction band is partially filled
the valence band is completely filled

Solution

In semiconductors, the conduction band is empty and the valence band is completely filled at 0 K.No electron from valence band can cross over to conduction band at 0 K.But at room temperature some electrons in the valence band jump over to the conduction band due to the small forbidden gap, i.e.,1 eV.

Reverse bias applied to a junction diode

increases the minority carrier current
lowers the potential barrier
raises the potential barrier
increases the majority carrier current

Solution

In reverse biasing, the conduction across the p-n junction does not take place due to majority carriers, but takes place due to minority carriers if the voltage of external battery is large. The size of the depletion region increases there by increasing the potential barrier.

Following diagram performs the logic function of

XOR gate
AND gate
NAND gate
OR gate

Solution

A n-p-n transistor conducts when

both collector and emitter are negative with respect to the base
both collector and emitter are positive with respect to the base
collector is positive and emitter is negative with respect to the base
collector is positive and emitter is at same potentialas the base

Solution

When the collector is positive and emitter is negative w.r.t. base it causes the forward biasing for each junction,which causes conduction of current.

Barrier potential of a P-N junction diode does not depend on

doping density
diode design
temperature
forward bias

Solution

[Hint : Barrier potential depends on, doping density,temperature, forward/reverse bias but does not depend on diode design.]

In the case of a common emitter transistor amplifier the ratio of the collector current to the emitter current I_c/I_e is 0.96. The current gain of the amplifier is

Solution

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