For an unbiased \( p-n \) junction at equilibrium, which of the following statement is true?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Diffusion current is equal to drift current
At a p-n junction, holes diffuse from p to n and electrons from n to p, giving a diffusion current. The ions left behind set up a field in the depletion region that sweeps minority carriers across the other way, giving a drift current. The depletion region grows until the two currents are equal, so in an unbiased junction at equilibrium the net current is zero.
\( I_{\mathrm{diffusion}} = I_{\mathrm{drift}} \)
\( I_{\mathrm{net}} = 0 \)
Why the other options are incorrect
Option (2) Drift current exists, while diffusion current is zero — Incorrect. Diffusion continues at equilibrium; it is balanced by drift, not stopped.
Option (3) Diffusion current exists, while drift current is zero — Incorrect. The field of the depletion region always drives a drift current; without it nothing would balance the diffusion.
Option (4) Neither diffusion current nor drift current exists — Incorrect. Both currents flow; only their sum is zero.
NCERT (2026-27):
p-n Junction Formation – “… Initially, diffusion current is large and drift current is small. As the diffusion process continues, the space-charge regions on either side of the junction extend, thus increasing the electric field strength and hence drift current. This process continues until the diffusion current equals the drift current. Thus a p-n junction is formed. In a p-n junction under equilibrium there is no net current.”