In the process of doping,
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → dopant and semiconductor atoms should nearly be of the same size.
In doping, a small number of impurity atoms replace atoms in the crystal lattice. For this to happen without distorting the lattice, the dopant atom must be of nearly the same size as the host atom (e.g. As or B in Si).
Why the other options are incorrect
Option (1) the conductivity of the semiconductor is decreased. — Incorrect. Doping adds free electrons or holes, so the conductivity increases sharply.
Option (2) adding trivalent impurity to a pure semiconductor, a net positive charge is developed. — Incorrect. The trivalent atom and the crystal are both neutral, so a p-type semiconductor stays electrically neutral; holes are the majority carriers but there is no net charge.
Option (3) adding a pentavalent impurity to a pure semiconductor, a net negative charge is developed. — Incorrect. For the same reason an n-type semiconductor is also neutral; the extra electron is balanced by the extra proton in the pentavalent nucleus.
NCERT (2026-27):
Extrinsic Semiconductor – …
