When an aluminium atom takes the place of a silicon atom in a silicon…

The correct answer is Option (2) → completes covalent bonds with three neighbouring silicon atoms, leaving a vacancy in the bond with the fourth

Aluminium is a trivalent atom. It has three valence electrons, one fewer than silicon. But each lattice site has four neighbours to bond with.

Option (1) completes covalent bonds with all four neighbouring silicon atoms and gives one electron for conduction — Incorrect. With three valence electrons aluminium cannot complete four bonds. It also has no spare electron to give. A free electron for conduction comes from a pentavalent donor such as arsenic.

Option (2) completes covalent bonds with three neighbouring silicon atoms, leaving a vacancy in the bond with the fourth — Correct. The three valence electrons bond with three silicon neighbours. The bond with the fourth neighbour is one electron short, and this vacancy is the hole that makes the material p-type.

Option (3) completes covalent bonds with three neighbouring silicon atoms and sets its fourth valence electron free for conduction — Incorrect. Aluminium has no fourth valence electron. All three of its valence electrons are used up in the three bonds, so nothing is left over to be set free.

Option (4) completes covalent bonds with two neighbouring silicon atoms, leaving two vacancies — Incorrect. Three valence electrons complete three bonds. One aluminium atom leaves one vacancy and so gives one hole.

NCERT (2026-27):
Extrinsic Semiconductor – “… 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. So the bond between the fourth neighbour and the trivalent atom has a vacancy or hole ….”