The barrier potential of an unbiased silicon p-n junction is 0.7 V. The p-side of the junction is now connected to a point at \( -1.5\,\mathrm{V} \) and the n-side to a point at \( +0.5\,\mathrm{V} \). Consider the following statements:
(A) The junction is reverse biased.
(B) The effective barrier height across the junction is 2.7 V.
(C) The depletion region is narrower than in the unbiased junction.
(D) The current across the junction is mainly due to the drift of minority carriers.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (B) and (D) only
A junction is reverse biased when its n-side is at a higher potential than its p-side. The applied voltage then acts in the same direction as the barrier potential. So the barrier height rises to \( V_0+V \), and the depletion region widens.
The voltage of the n-side with respect to the p-side is:
\( \begin{aligned} V &= V_n-V_p \\ &= (+0.5\,\mathrm{V})-(-1.5\,\mathrm{V}) \\ &= 2.0\,\mathrm{V} \end{aligned} \)
(A) The junction is reverse biased. — True. The n-side is 2.0 V above the p-side, so the n-side is positive and the p-side negative.
(B) The effective barrier height across the junction is 2.7 V. — True. Under reverse bias the applied voltage adds to the barrier potential:
\( \begin{aligned} V_0+V &= 0.7\,\mathrm{V}+2.0\,\mathrm{V} \\ &= 2.7\,\mathrm{V} \end{aligned} \)
(C) The depletion region is narrower than in the unbiased junction. — False. A reverse bias widens the depletion region. It becomes narrower under forward bias.
(D) The current across the junction is mainly due to the drift of minority carriers. — True. The higher barrier greatly reduces the diffusion of majority carriers. Only a small current of a few \( \mu\mathrm{A} \) remains. It comes from electrons on the p-side and holes on the n-side, which the junction field sweeps across.
NCERT (2026-27):
p-n Junction Diode under Reverse Bias – “When an external voltage (V ) is applied across the diode such that n-side is positive and p-side is negative, it is said to be reverse biased …. The applied voltage mostly drops across the depletion region. The direction of applied voltage is same as the direction of barrier potential. As a result, the barrier height increases and the depletion region widens due to the change in the electric field. The effective barrier height under reverse bias is (V0 + V )….”
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. This is quite low because it is due to the motion of carriers from their minority side to their majority side across the junction.”