Assertion (A): The motion of an electron from the p-side to the n-side of a p-n junction due to the electric field of the junction is called drift.
Reason (R): During the formation of a p-n junction, the diffusion current is initially large and the drift current is small.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both A and R are true, but R is NOT the correct explanation of A.
Assertion (A) The motion of an electron from the p-side to the n-side of a p-n junction due to the electric field of the junction is called drift. — True. The positive space charge on the n-side and the negative space charge on the p-side set up a field directed from n to p. This field moves an electron on the p-side across to the n-side. The motion of charge carriers due to an electric field is called drift.
Reason (R) During the formation of a p-n junction, the diffusion current is initially large and the drift current is small. — True. At first, the difference in concentration drives a large diffusion current. The space-charge layers and their field are still small, so the drift current is small. As the layers extend, the drift current grows until it equals the diffusion current.
Why R does not explain A?
(A) is a definition. It names the field-driven motion of a carrier as drift, whatever the size of the drift current.
(R) describes how the sizes of the two currents change while the junction forms. It does not say why the motion caused by the junction field is called drift, so it is not the explanation of (A).
NCERT (2026-27):
p-n Junction Formation – “… 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. The motion of charge carriers due to the electric field is called drift.”
p-n Junction Formation – “Initially, diffusion current is large and drift current is small. As the diffusion process continues, the space-charge regions on either side of the junction extend, thus increasing the electric field strength and hence drift current. This process continues until the diffusion current equals the drift current.”