What is the dominant mechanism for the motion of charge carriers in a forward-biased and a reverse-biased silicon p-n junction diode, respectively?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → diffusion, drift
Charge carriers move across a p-n junction in two ways. Majority carriers diffuse across because their concentration is different on the two sides. Minority carriers drift across because the electric field of the junction sweeps them.
Option (1) drift, diffusion — Incorrect. Under forward bias, the drift current (\( \mu\mathrm{A} \)) is tiny beside the diffusion current (mA). Under reverse bias, the raised barrier stops most of the diffusion.
Option (2) diffusion, drift — Correct. The bias changes the barrier height:
Forward bias lowers the barrier to \( V_0-V \). Electrons and holes cross the junction and diffuse away from it. So the forward current is the sum of the hole and electron diffusion currents.
Reverse bias raises the barrier to \( V_0+V \), and the diffusion current falls enormously. The small current of a few \( \mu\mathrm{A} \) that remains is the drift of minority carriers swept across by the junction field.
Option (3) diffusion, diffusion — Incorrect. Under reverse bias, the higher barrier stops majority carriers from crossing the junction. So diffusion cannot carry the reverse current.
Option (4) drift, drift — Incorrect. Under forward bias the drift current stays at the \( \mu\mathrm{A} \) level, negligible beside the mA diffusion current of the injected carriers.
NCERT (2026-27):
p-n Junction Diode Under Forward Bias – “… The total diode forward current is sum of hole diffusion current and conventional current due to electron diffusion. The magnitude of this current is usually in mA.”
p-n Junction Diode Under Reverse Bias – “… This suppresses the flow of electrons from n → p and holes from p → n. Thus, diffusion current, decreases enormously compared to the diode under forward bias.”
p-n Junction Diode Under Reverse Bias – “The electric field direction of the junction is such that if electrons on p-side or holes on n-side in their random motion come close to the junction, they will be swept to its majority zone. This drift of carriers gives rise to current. The drift current is of the order of a few μA.”