1
MCQ (Single Correct Answer)

### AIPMT 2005

Application of a forward bias to a p-n junction
A
widens the depletion zone
B
increases the potential difference across the depletion zone
C
increases the number of donors on the n side
D
decreases the electric field in the depletion zone.

## Explanation

Number of donors is more because electrons from –ve terminal of the cell pushes (enters) the n side and decreases the number of uncompensated pentavalent ion due to which potential barrier is reduced. The neutralised pentavalent atom are again in position to donate electrons.
2
MCQ (Single Correct Answer)

### AIPMT 2005

Choose the only false statement from the following.
A
In conductors the valence and conduction bands overlap.
B
Substances with energy gap of the order of 10 eV are insulators.
C
The resistivity of a semiconductor increases with increase in temperature.
D
The conductivity of a semiconductor increases with increase in temperature.

## Explanation

Option (a) is correct as in conductor conduction and valance band overlap and the conduction band is partially filled

Option (b) is correct as insulators have energy gap of 5 - 10 eV.

Option (c) is incorrect as resistivity decreases with increase in temperature.

Option (d) is correct as with increase in temperature, more and more electrons jump to conduction band, hence conductivity increases.
3
MCQ (Single Correct Answer)

### AIPMT 2004

The output of OR gate is 1
A
if both inputs are zero
B
if either or both inputs are 1
C
only if both inputs are 1
D
if either inputs is zero

## Explanation

The truth table of OR gate is

$\matrix{ A & B & Y \cr 0 & 0 & 0 \cr 0 & 1 & 1 \cr 1 & 0 & 1 \cr 1 & 1 & 1 \cr }$

It is seen that output of OR gate is Y = A + B.
If both or any one has 1 input then output will be 1.
4
MCQ (Single Correct Answer)

### AIPMT 2004

Of the diodes shown in the following diagrams, which one is reverse biased ?
A
B
C
D

## Explanation

A diode is said to be reverse biased if p-type semiconductor of p-n junction is at low potential with respect to n-type semiconductor of p-n junction. It is so for circuit (c).

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