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: Look at the size of the current in each bias:
A forward-biased silicon junction carries a large current, of the order of mA.
A reverse-biased junction carries a tiny current, of the order of $\mu$A or less.
The two currents must come from different mechanisms.

Step 2: Forward bias:
The barrier height falls to $V_0 - V$. Many majority carriers now have enough energy to cross it.
They enter the other side where their concentration is low, then spread out. This is diffusion.

Step 3: Reverse bias:
The barrier height rises to $V_0 + V$. Majority carriers are blocked completely.
Thermally generated minority carriers near the junction feel the strong field and are pushed across. This is drift.
The current is small and almost constant because the number of minority carriers is small.

Step 4: Match with the options:
Only "diffusion in forward, drift in reverse" fits both cases.
The other three options each give at least one mechanism wrongly.

Final Answer:
Option (A) is correct. \[ \boxed{\text{diffusion in forward bias, drift in reverse bias}} \]