Let \( V_{0} \) be the built-in barrier potential of a p-n junction diode. It is forward biased by applying an external voltage \( V \). The effective barrier height under forward bias is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( V_{0}-V \)
In forward bias, the p-side is connected to the positive terminal, so the applied voltage opposes the built-in potential of the depletion layer and lowers the barrier.
\( V_{\mathrm{eff}} = V_{0}-V \)
Why the other options are incorrect
Option (2) \( V_{0}+V \) — Incorrect. \( V_{0}+V \) is the barrier height in reverse bias, where the applied voltage adds to the built-in potential.
Option (3) \( V-V_{0} \) — Incorrect. \( V-V_{0} \) is the negative of the barrier height; it would be negative for \( V \lt V_{0} \).
Option (4) \( V-2V_{0} \) — Incorrect. \( V-2V_{0} \) does not follow from the way the two voltages combine.
NCERT (2026-27):
p-n Junction Diode Under Forward Bias – “… As a result, the depletion layer width decreases and the barrier height is reduced [Fig. 14.13(b)]. The effective barrier height under forward bias is (V0 – V).”