What is the principle of a p-n junction diode?

The correct answer is Option (1) → The resistance of a p-n junction is low when forward biased and high when reverse biased.

A p-n junction diode allows current mainly in one direction, and this one-way conduction is the basis of its use as a rectifier.

Option (1) The resistance of a p-n junction is low when forward biased and high when reverse biased. — Correct. In forward bias the barrier height falls to \( V_0-V \) and a current of some mA flows, so the forward resistance is low. In reverse bias the barrier rises to \( V_0+V \) and only a few μA flow, so the reverse resistance is high.

Option (2) The resistance of a p-n junction is high when forward biased and low when reverse biased. — Incorrect. The large current, and so the low resistance, is in forward bias. The reverse current is tiny.

Option (3) The resistance of a p-n junction is the same in forward and reverse bias. — Incorrect. The forward current is in mA at a small voltage. The reverse current is only a few μA even at a large voltage. So the two resistances differ greatly.

Option (4) The current in reverse bias is due to diffusion of majority carriers. — Incorrect. The raised barrier holds back the diffusion of majority carriers. The reverse current is the drift of minority carriers swept across by the junction field.

NCERT (2026-27):
p-n Junction Diode under Reverse Bias – “… The forward bias resistance is low as compared to the reverse bias resistance. This property is used for rectification of ac voltages as discussed in the next section.”