Question

The diffusion current in a p-n junction is greater than the drift current when the junction is-
(A) Forward biased
(B) Reverse biased
(C) Unbiased
(D) Both forward and reverse biased

Answer 1

A p-n junction consists of a p-type and n-type semiconductors joined together end to end. The n-type semiconductor is rich in electrons while the p-type semiconductor is rich in holes. These, when joined together, try to neutralize each other by diffusing. But when the junction is kept in an electric- field, they try to move towards the terminals of a battery, this is known as drift current.

Complete Step by step solution:
When a semiconductor is doped, a p-type is rich in holes, the positive charge careers and an n-type is rich in electrons, the negative charge careers. A p-n junction is the end-to-end connection of a p-type and an n-type semiconductor.
When physically joined, the p-side has a net positive charge while the n-side has a net negative charge. This creates an electric field in the p-n junction. And the electrons move towards the p-type part while the holes move towards the n-type part. This current is known as diffusion current.
After a point when many electrons get collected near the junction, they start repelling any new electrons trying to move due to the diffusion current, forming a depletion region.
Now if the ends of a p-n junction are connected to a battery, an external electric field, created by the battery is introduced. The current caused due to this is known as Drift current.
If the positive terminal of the battery is connected to the p-type while the negative terminal of the battery is connected to the n-type, the movement of electrons is eased as they can travel to the positive terminal of the battery. This causes an increase in the diffusion current.
This is the configuration of a forward bias.
Thus option (A) is the correct answer.

Note:
The direction of electron flow, the direction of current and the terminal of a battery must not be confused with each other. The direction of current is always the direction of flow of positive charge, which is opposite to the direction of flow of electrons. The positive charge (current) in a battery always travels from positive terminal to the negative terminal.