1

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
2

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.
3

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.
4

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.