When a forward bias is applied to a p-n junction diode, then:

The correct answer is Option (4) → The width of depletion layer reduces

In forward bias the p-side is connected to the positive terminal. The applied voltage opposes the built-in barrier, so the barrier height falls from \( V_0 \) to \( V_0 - V \). Fewer immobile ions are needed to support this smaller barrier, so the depletion layer becomes narrower, and many majority carriers diffuse across the junction.

Option (1) The majority carrier current becomes zero — Incorrect. The diffusion current of majority carriers becomes large; it is this current that flows in forward bias.

Option (2) The potential barrier is raised — Incorrect. The barrier is lowered in forward bias; it is raised in reverse bias.

Option (3) The junction resistance increases — Incorrect. With a lower barrier and a large current, the junction resistance becomes small.

Option (4) The width of depletion layer reduces — Correct. The lower barrier needs a thinner layer of uncovered ions, so the depletion layer narrows.

NCERT (2026-27):
p-n Junction Diode Under Forward Bias – “… The direction of the applied voltage (V) is opposite to the built-in potential V₀. As a result, the depletion layer width decreases and the barrier height is reduced ….”