An intrinsic semiconductor (Si) is doped with indium (In). What do you expect?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A hole is generated.
Indium is a trivalent atom. In the Si lattice, it forms covalent bonds with only three neighbours. It has no electron for the fourth bond, so that bond is left with a vacancy, a hole.
Option (1) A hole is generated. — Correct. Each indium atom acts as an acceptor. One acceptor atom gives one hole, which is available for conduction. The doped Si becomes a p-type semiconductor.
Option (2) An electron is generated. — Incorrect. A free electron comes from a pentavalent donor such as As or P. Indium has one valence electron fewer than Si.
Option (3) One hole and one electron are generated. — Incorrect. An electron-hole pair is produced when thermal energy breaks a bond between Si atoms. The acceptor gives only a hole. The conduction electrons still come only from bonds broken by heat (intrinsic generation).
Option (4) The resistivity rises. — Incorrect. Adding a small amount of a suitable impurity increases the conductivity of the semiconductor many times, so the resistivity falls.
NCERT (2026-27):
Extrinsic Semiconductor – “(ii) p-type semiconductor
This is obtained when Si or Ge is doped with a trivalent impurity like Al, B, In, etc. … It is obvious that one acceptor atom gives one hole.”