Which of the following is true for a p-n junction diode in forward bias?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → All of the above
In forward bias, the p-side is joined to the positive terminal and the n-side to the negative terminal. So the applied voltage \( V \) opposes the built-in potential \( V_0 \).
Option (1) It offers a low resistance. — Incorrect. This is true, because a large forward current flows even at a small voltage. But it is only one of the three true statements.
Option (2) The width of the depletion layer decreases. — Incorrect. This is true, as the applied voltage opposes the barrier potential. On its own, it is not the full answer.
Option (3) Electrons from the n-side cross the depletion region and reach the p-side. — Incorrect. The lowered barrier does allow the electrons to cross. Still, this option gives only one of the three true statements.
Option (4) All of the above — Correct. All three statements hold:
The barrier height falls to \( V_0-V \) and the depletion layer width decreases, so (2) holds.
With the lower barrier, electrons from the n-side cross the depletion region and reach the p-side. There they are minority carriers (holes cross the other way), so (3) holds.
These injected carriers give a forward current of the order of mA. This is far more than the reverse current, so the diode offers a low resistance and (1) holds.
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 …. The effective barrier height under forward bias is (V0 – V).”
p-n Junction Diode under Forward Bias – “Due to the applied voltage, electrons from n-side cross the depletion region and reach p-side (where they are minority carries).”
p-n Junction Diode under Reverse Bias – “… The forward bias resistance is low as compared to the reverse bias resistance.”