Read the following passage and answer the questions.
A thin wafer of p-type silicon is made by adding boron, a trivalent element, to pure silicon. About one boron atom is added for every million silicon atoms. Pure silicon has about \(5\times10^{28}\) atoms per cubic metre. Its intrinsic carrier concentration \(n_i\) at room temperature is about \(1.5\times10^{16}\ \mathrm{m^{-3}}\). Next, a larger and carefully measured amount of phosphorus, a pentavalent element, is added to one part of the wafer. This part becomes n-type. The wafer now has a p-region and an n-region with a junction between them. As soon as the junction forms, charge carriers start to cross it. After a short time, the net flow of carriers stops and a potential difference appears across the junction. Finally, metal contacts are fixed at the two ends, and the device is joined to a dc supply.
A p-n junction diode is connected to a potential difference such that the p-side is at \( -5\,\mathrm{V} \) and the n-side is at \( -3\,\mathrm{V} \). What can you say about this?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The junction is reverse biased.
The bias of a p-n junction depends only on which side is at the higher potential. The signs of the two potentials do not matter. Here the n-side is at \( -3\,\mathrm{V} \) and the p-side at \( -5\,\mathrm{V} \), so the n-side is 2 V higher than the p-side.
Option (1) The junction is unbiased because both sides are at a negative potential. — Incorrect. A junction is unbiased only when there is no potential difference across it. Here a potential difference of 2 V acts across the diode.
Option (2) The junction is forward biased. — Incorrect. Forward bias needs the p-side at the higher potential, connected to the positive terminal. Here the p-side is the lower one.
Option (3) The junction is reverse biased. — Correct. The n-side is positive with respect to the p-side. So the applied voltage acts in the same direction as the barrier potential, and the barrier height rises to \( V_0+V \).
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.”