CBSE 2025 · Region 6 · Set 1 · Q30 · 4 marks

Extrinsic semiconductors are made by doping pure or intrinsic semiconductors with suitable impurity. There are two type of dopants used in doping, Si or Ge, and using them p-type and n-type semiconductors can be obtained. A p-n junction is the basic building block of many semiconductor devices. Two important processes occur during the formation of a p-n junction : diffusion and drift. When such a junction is formed, a 'depletion layer' is created consisting of immobile ion-cores. This is responsible for a junction potential barrier. The width of a depletion layer and the height of potential barrier changes when a junction is forward-biased or reverse-biased. A semiconductor diode is basically a p-n junction with metallic contacts provided at the ends for application of an external voltage. Using diodes, alternating voltages can be rectified.

(i)

Which of the following is a donor impurity atom for Ge ?

(A)

Boron

(B)

Antimony

(C)

Aluminium

(D)

Indium

(ii)

When a pentavalent atom occupies the position of an atom in the crystal lattice of Si , four of its electrons form covalent bonds with four silicon neighbours, while the fifth remains bound to the parent atom. The energy required to set this electron free is about : ) $\displaystyle \mathrm{C}_{0}$

(A)

0.$\displaystyle 5$ eV

(B)

0.$\displaystyle 1$ eV

(C)

0.$\displaystyle 05$ eV

(D)

0.$\displaystyle 01$ eV

(iii)

During formation of a p-n junction :

(A)

a layer of negative charge on n -side and a layer of positive charge on p -side appear.

(B)

a layer of positive charge on $\displaystyle n$-side and a layer of negative charge on p -side appear.

(C)

the electrons on p -side of the junction move to n -side initially.

(D)

initially diffusion current is small and drift current is large.

(iv)

(a)OR(b)

In reverse-biased p-n junction :

(A)

the drift current is of the order of few mA .

(B)

the applied voltage mostly drops across the depletion region.

(C)

the depletion region width decreases.

(D)

the current increases with increase in applied voltage.

CBSE Class 12 Physics past-paper question from the 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.