1

### AIPMT 2011 Prelims

If a small amount of antimony is added to germanium crystal
A
it becomes a p-type semiconductor
B
the antimony becomes an acceptor atom
C
there will be more free electrons than holes in the semiconductor
D
its resistance is increased

## Explanation

If a small amount of antimony is added to germanium crystal, crystal becomes n-type semiconductor. Hence, there will be more free electrons than holes.
2

### AIPMT 2011 Prelims

Symbolic representation of four logic gates are shown as

Pick out which ones are for AND, NAND and NOT gates, respectively
A
(ii), (iii) and (iv)
B
(iii), (ii) and (i)
C
(iii), (ii) and (iv)
D
(ii), (iv) and (iii)

## Explanation

AND, NAND and NOT gates are (ii), (iv) and (iii) respectively.
3

### AIPMT 2011 Prelims

A transistor is operated in common emitter configuration at VC = 2 V such that a change in the base current from 100 $\mu$A to 300 $\mu$A produces a change in the collector current from 10 mA to 20 mA. The current gain is
A
50
B
75
C
100
D
25

## Explanation

Current gain, $\beta$ = ${{\Delta {I_C}} \over {\Delta {I_B}}}$

= ${{\left( {20 - 10} \right) \times {{10}^{ - 3}}} \over {\left( {300 - 100} \right) \times {{10}^{ - 6}}}}$ = 50
4

### AIPMT 2010 Mains

For transistor action
(1)  Base, emitter and collector regions should have similar size and doping concentrations.
(2)  The base region must be vety thin and lightly doped.
(3)  The emitter-base junction is forward biased and base-collector junction is reverse biased.
(4)  Both the emitter-base junction as well as the base-collector junction are forward biased.

Which one of the following pairs of statements is correct?
A
(4) and (1)
B
(1) and (2)
C
(2) and (3)
D
(3) and (4)

## Explanation

For transistor action, the base region must be very thin and lightly doped. Also, the emitter-base junction is forward biased and base-collector junction is reverse biased.