1

### AIPMT 2005

Copper has face centered cubic (fcc) lattice with interatomic spacing equal to 2.54 $\mathop A\limits^ \circ$. The value of lattice constant for this lattice is
A
2.54 $\mathop A\limits^ \circ$
B
3.59 $\mathop A\limits^ \circ$
C
1.27 $\mathop A\limits^ \circ$
D
5.08 $\mathop A\limits^ \circ$

## Explanation

Lattice constant for (f.c.c.)

= a = interatomic spacing $\times$ $\sqrt 2$ = 3.59 $\mathop A\limits^ \circ$
2

### AIPMT 2005

Carbon, silicon and germanium atoms have four valence electrons each. Their valence and conduction bands are separated by energy band gaps represented by (Eg)C, (Eg)si and (Eg)Ge respectively. Which one of the following relationships is true in their case?
A
(Eg)C > (Eg)Si
B
(Eg)C < (Eg)Si
C
(Eg)C = (Eg)Si
D
(Eg)C < (Eg)Ge.

## Explanation

Band gap of carbon is 5.5 eV while that of silicon is 1.1 eV

$\therefore$ (Eg)C $>$ (Eg)Si
3

### AIPMT 2005

Zener diode is used for
A
amplification
B
rectification
C
stabilisation
D
producing oscillations in an oscillator.

## Explanation

Zener diode is used for stabilisation while p-n junction diode is used for rectification.
4

### 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.