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.
Assertion (A): An electric field directed from the n-side to the p-side is set up across the junction.
Reason (R): A positive space-charge region forms on the n-side and a negative space-charge region on the p-side of the junction.
Which of the following is best suited?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A) is correctly explained by (R).
Assertion (A) An electric field directed from the n-side to the p-side is set up across the junction. — True. After diffusion, the n-side of the junction carries a positive charge and the p-side a negative charge. The field of these charges points from the n-side to the p-side.
Reason (R) A positive space-charge region forms on the n-side and a negative space-charge region on the p-side of the junction. — True. An electron that diffuses from n to p leaves behind an immobile ionised donor (positive) on the n-side. A hole that diffuses from p to n leaves behind an immobile ionised acceptor (negative) on the p-side.
Why R explains A?
An electric field is directed from positive charge towards negative charge.
The positive layer lies on the n-side and the negative layer on the p-side, as R states. Hence the field across the junction points from the n-side to the p-side, and R is the correct explanation of A.
NCERT (2026-27):
p-n Junction Formation – “… Due to the positive space-charge region on n-side of the junction and negative space charge region on p-side of the junction, an electric field directed from positive charge towards negative charge develops.”