Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): In a compound semiconductor such as GaAs, a change in the ratio of Ga atoms to As atoms can change the type of the semiconductor.
Reason (R): In elemental semiconductors such as Si and Ge, n-type or p-type material is obtained by adding dopants as defects.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
Assertion (A) In a compound semiconductor such as GaAs, a change in the ratio of Ga atoms to As atoms can change the type of the semiconductor. — True. In ideal GaAs, the ratio Ga:As is 1:1. A crystal with a little extra Ga, such as \( \mathrm{Ga_{1.1}As_{0.9}} \), or a little extra As, such as \( \mathrm{Ga_{0.9}As_{1.1}} \), can be of a different type.
Reason (R) In elemental semiconductors such as Si and Ge, n-type or p-type material is obtained by adding dopants as defects. — True. A pentavalent dopant gives n-type material and a trivalent dopant gives p-type material.
Why Reason (R) does not explain Assertion (A)?
R is about Si and Ge. There, atoms of another element are added to the crystal.
In A, no new element is added. The type changes because the ratio of the two elements of the compound moves away from 1:1. So R does not give the reason for A.
NCERT (2026-27):
Points to Ponder – “… In compound semiconductors, the change in relative stoichiometric ratio can also change the type of semiconductor. …”
Points to Ponder – “2. In elemental semiconductors (Si or Ge), the n-type or p-type semiconductors are obtained by introducing ‘dopants’ as defects. …”