1

### AIPMT 2009

A p-n photodiode is fabricated from a semiconductor with a band gap of 2.5 eV. It can detect a signal of wavelength
A
4000 nm
B
6000 nm
C
4000 $\mathop A\limits^ \circ$
D
6000 $\mathop A\limits^ \circ$

## Explanation

$\lambda$max = ${{hc} \over E}$

= ${{6.6 \times {{10}^{ - 34}} \times 3 \times {{10}^8}} \over {2.5 \times 1.6 \times {{10}^{ - 19}}}}$

= 5000 $\mathop A\limits^o$

Now the wavelength detected by photodiode be less than $\lambda$max, hence it can detect a signal of wave length 4000 $\mathop A\limits^o$.
2

### AIPMT 2009

A transistor is operated in common-emitter configuration at VC = 2V such that a change in the base current from 100 $\mu$A to 200 $\mu$A produces a change in the collector current from 5 mA to 10 mA. The current gain is
A
100
B
150
C
50
D
75

## Explanation

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

= ${{\left( {10 - 5} \right) \times {{10}^{ - 3}}} \over {\left( {200 - 100} \right) \times {{10}^{ - 6}}}}$ = 50
3

### AIPMT 2009

The symbolic representation of four logic gates are given below

The logic symbols for OR, NOT and NAND gates are respectively
A
(iv), (i), (iii)
B
(iv), (ii), (i)
C
(i), (iii), (iv)
D
(iii), (iv), (ii)

## Explanation

OR gate, NOT gate and NAND gates are (iv), (ii) and (i) respectively.
4

### AIPMT 2008

If the lattice parameter for a crystalline structure is 3.6 $\mathop A\limits^ \circ$, then the atomic radius in fcc crystal is
A
2.92 $\mathop A\limits^ \circ$
B
1.27 $\mathop A\limits^ \circ$
C
1.81 $\mathop A\limits^ \circ$
D
2.10 $\mathop A\limits^ \circ$

## Explanation

The atomic radius in a f.c.c. crystal is = ${a \over {2\sqrt 2 }}$

$\therefore$ Atomic radius = ${{3.6} \over {2\sqrt 2 }}$ = 1.27 $\mathop A\limits^ \circ$