When an intrinsic semi conductor is doped with an acceptor impurity:(…

The correct answer is Option (3) → (A) and (D) only

An acceptor impurity is a trivalent atom (boron, aluminium, indium). It has one electron less than needed for the four covalent bonds, so each such atom creates a hole and the crystal becomes a p-type semiconductor. The acceptor energy level lies just above the top of the valence band.

(A) The hole concentration increases — Correct. Every acceptor atom adds a hole, so the holes become the majority carriers, far more numerous than in the pure crystal.

(B) The conductivity of semiconductor decreases — Incorrect. Doping adds a very large number of charge carriers, so the conductivity increases.

(C) The electron concentration increases — Incorrect. At equilibrium \( n_e n_h = n_i^{2} \). With many more holes, recombination is faster, so the electron concentration falls below its intrinsic value.

(D) The energy gap decreases — Correct. An electron of the valence band needs only about 0.01 to 0.05 eV to jump to the acceptor level, instead of the full band gap (about 1.1 eV for silicon). So the energy needed to create a free carrier is much smaller after doping; the band gap of the host crystal itself stays the same.

NCERT (2026-27):
Extrinsic Semiconductor – “… Thus, for such a material, the holes are the majority carriers and electrons are minority carriers.”

Extrinsic Semiconductor – “… Similarly, for p-type semiconductor, the acceptor energy level E_A is slightly above the top E_V of the valence band as shown in Fig. 14.9(b). With very small supply of energy an electron from the valence band can jump to the level E_A and ionise the acceptor negatively.”