On adding a small amount of boron to a silicon crystal

The correct answer is Option (4) → it becomes a p-type semiconductor

Boron is trivalent (three valence electrons). In a silicon crystal it bonds with three neighbours and leaves one bond short of an electron, that is a hole. Boron therefore accepts an electron and is an acceptor impurity, and holes become the majority carriers.

Option (1) boron becomes a donor atom — Incorrect. A donor gives an electron; boron, with only three valence electrons, accepts one, so it is an acceptor.

Option (2) its resistance is increased — Incorrect. The added holes increase the number of charge carriers, so the conductivity rises and the resistance decreases.

Option (3) it becomes a n-type semiconductor — Incorrect. An n-type semiconductor is made with pentavalent (donor) atoms such as P or As, not boron.

Option (4) it becomes a p-type semiconductor — Correct. The trivalent acceptor creates holes as majority carriers, so the crystal becomes p-type.

NCERT (2026-27):
p-type Semiconductor – “This is obtained when Si or Ge is doped with a trivalent impurity like Al, B, In, etc. The dopant has one valence electron less than Si or Ge and, therefore, this atom can form covalent bonds with neighbouring three Si atoms but does not have any electron to offer to the fourth Si atom.”

p-type Semiconductor – “… Thus, for such a material, the holes are the majority carriers and electrons are minority carriers. Therefore, extrinsic semiconductors doped with trivalent impurity are called p-type semiconductors.”