Question

In forward and reverse biased silicon p-n junction, the dominant mechanisms for the motion of charge carriers are:

• A. diffusion in forward bias, drift in reverse bias
• B. drift in both forward bias and in reverse bias
• C. drift in forward and diffusion in reverse bias
• D. diffusion in both forward and reverse bias

Hint:

Forward bias lowers the barrier so majority carriers diffuse. Reverse bias lets only minority carriers drift.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
In a p-n junction, charge carriers move in two ways. Diffusion is motion from a region of high concentration to low concentration. Drift is motion under an electric field.

Step 2: Forward bias:
The external voltage opposes the built-in field, so the depletion layer becomes thin and the barrier becomes low.
Holes from the p-side and electrons from the n-side cross the junction in large numbers. This motion is by diffusion.
The forward current is therefore a diffusion current, and it rises quickly with voltage.

Step 3: Reverse bias:
The external voltage adds to the built-in field, so the barrier becomes higher and the depletion layer widens.
Majority carriers cannot cross. Only a few minority carriers are swept across by the strong field. This motion is by drift.
The reverse current is a small drift current of minority carriers.

Step 4: Check the options:
(B) says drift in both. Forward current is not a drift current, so (B) is wrong.
(C) reverses the correct statement, so it is wrong.
(D) says diffusion in both. Reverse current is carried by drift, so (D) is wrong.

Final Answer:
Diffusion dominates in forward bias and drift dominates in reverse bias. This is option (A). \[ \boxed{\text{diffusion (forward), drift (reverse)}} \]