Arrange the following steps in sequence during the process of formation of p-n junction.
A. Due to the concentration gradient, holes diffuse from p-side to n-side and electrons diffuse from n-side to p-side.
B. Due to junction field, an electron on the p-side of the junction moves to n-side and a hole on n-side of the junction moves to p-side
C. A small quantity of pentavalent impurity is added to the p-Si wafer.
D. Positive space charge region on n-side and negative space region on p-side of the junction are developed.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → C, A, D, B
A p-n junction forms when part of a p-type wafer is made n-type; diffusion then builds a space-charge layer whose field causes drift.
(C) A small quantity of pentavalent impurity is added to the p-Si wafer. — First: part of the wafer becomes n-type, next to the p-region.
(A) Due to the concentration gradient, holes diffuse from p-side to n-side and electrons diffuse from n-side to p-side. — Second: diffusion starts across the new junction.
(D) Positive space charge region on n-side and negative space region on p-side of the junction are developed. — Third: the diffusing carriers leave immobile donor and acceptor ions behind, the space-charge (depletion) region.
(B) Due to junction field, an electron on the p-side of the junction moves to n-side and a hole on n-side of the junction moves to p-side — Last: the field of the space charge drives this drift.
The sequence is C, A, D, B.
Note: It seems that there is a mistake in the original NTA paper, which prints “negative space region” in statement D when it should read “negative space charge region”. The first half of the statement makes the meaning clear, so the question can still be solved.
NCERT (2026-27):
p-n Junction Formation – “… By adding precisely a small quantity of pentavelent impurity, part of the p-Si wafer can be converted into n-Si. … During the formation of p-n junction, and due to the concentration gradient across p-, and n- sides, holes diffuse from p-side to n-side (p → n) and electrons diffuse from n-side to p-side (n → p). … As the electrons continue to diffuse from n → p, a layer of positive charge (or positive space-charge region) on n-side of the junction is developed. … As the holes continue to diffuse, a layer of negative charge (or negative space-charge region) on the p-side of the junction is developed. … Due to this field, an electron on p-side of the junction moves to n-side and a hole on n-side of the junction moves to p-side.”