1

### AIPMT 2007

For a cubic crystal structure which one of the following relations indicating the cell characteristics is correct ?
A
a $\ne$ b $\ne$ c  and  $\alpha$ = $\beta$ = $\gamma$ = 90o
B
a = b = c  and  $\alpha$ $\ne$ $\beta$ $\ne$ $\gamma$ = 90o
C
a = b = c  and  $\alpha$ = $\beta$ = $\gamma$ = 90o
D
a $\ne$ b $\ne$ c  and  ;$\alpha$ $\ne$ $\beta$ $\ne$  and  $\gamma$ = 90o

## Explanation

In a cubic crystal structure
a = b = c  and  $\alpha$ = $\beta$ = $\gamma$ = 90o
2

### AIPMT 2007

In the energy band diagram of a material shown below, the open circles and filled circles denote holes and electrons respectively. The material is

A
an insulator
B
a metal
C
an n-type semiconductor
D
a p-type semiconductor.

## Explanation

For a p-type semiconductor, the acceptor energy level, as shown in the diagram, is slightly above the top Ev of the valence band. With very small supply of energy an electron from the valence band can jump to the level EC and ionise acceptor negatively.
3

### AIPMT 2007

In the following circuit, the output Y for all possible inputs A and B is expressed by the truth table.

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

## Explanation

Y' = $\overline {A + B}$

Y = $\overline {Y'}$

Y = $\overline{\overline {A + B}}$

Y = A + B which is OR-gate.
4

### AIPMT 2007

Which one of the following represents forward bias diode ?
A
B
C
D

## Explanation

A diode is said to be forward biased if p-side is at higher potential than n-side of p-n junction.