1

### 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.
2

### 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.
3

### 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).
4

### AIPMT 2004

In semiconductors at a room temperature
A
the valence band is partially empty and the conduction band is partially filled
B
the valence band is completely filled and the conduction band is partially filled
C
the valence band is completely filled
D
the conduction band is completely empty

## Explanation

In semiconductors at room temperature the electrons get enough energy so that they are able to over come the forbidden gap. Thus at room temperature the valence band is partially empty and conduction band is partially filled. Conduction band in semiconductor is completely empty only at 0 K.