During the formation of a p-n junction, diffusion of carriers goes on and the space-charge regions on both sides of the junction grow wider. What happens next?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The electric field grows, and the drift current rises until it equals the diffusion current.
Diffusion leaves fixed ions on both sides of the junction. These space charges set up the electric field that causes drift.
Option (1) The electric field grows, and the drift current rises until it equals the diffusion current. — Correct. Wider space-charge regions hold more fixed charge, so the field across the junction becomes stronger. The stronger field drives a larger drift current. The build-up stops when drift balances diffusion, and the junction is then in equilibrium.
Option (2) The electric field weakens, and the drift current stops. — Incorrect. More fixed charge on each side makes the field stronger. A stronger field increases the drift current.
Option (3) The diffusion current keeps rising without any limit. — Incorrect. The potential across the junction opposes further flow of carriers. So diffusion slows down until it equals the drift current.
Option (4) The drift current reverses and flows in the same direction as the diffusion current. — Incorrect. Drift is caused by the junction field. It always flows opposite to the diffusion current.
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.”