The cause of the depletion layer in the p-n junction region is

The correct answer is Option (4) → diffusion of charge carriers

When a p-n junction forms, electrons diffuse from the n-side to the p-side and holes diffuse from the p-side to the n-side, because of the concentration difference. Near the junction they recombine and leave behind the fixed immobile ions (positive donor ions on the n-side, negative acceptor ions on the p-side). This region, free of mobile carriers, is the depletion layer.

Option (1) drift of holes — Incorrect. Drift of holes is the later motion under the junction field; it opposes diffusion and does not create the layer.

Option (2) drift of electrons — Incorrect. Drift of electrons is likewise the later current under the junction field.

Option (3) migration of impurity ions — Incorrect. The impurity ions are fixed in the crystal and cannot migrate.

Option (4) diffusion of charge carriers — Correct. Diffusion of carriers across the junction uncovers the immobile ions that form the layer.

Note: The same question was also asked in CUET (UG) 2026 on 26-05-2026 (Slot 1).

NCERT (2026-27):
p-n Junction Formation – “… During the formation of p-n junction, and due to the concentration gradient across p-, and n- sides, holes diffuse from p-side to n-side (p → n) and electrons diffuse from n-side to p-side (n → p). This motion of charge carries gives rise to diffusion current across the junction. … This space-charge region on either side of the junction together is known as depletion region as the electrons and holes taking part in the initial movement across the junction depleted the region of its free charges ….”