Question

Describe briefly, with the help of a diagram, the role of the two important processes involved in the formation of a p-n junction ?

Answer 1

 Two important process involved in the formation of a p-n junction are:

Diffusion and,

Drift

In n-type semiconductor electrons are the majority carriers and holes are minority carriers. In the same way, in p-type semiconductor holes are majority and electrons are minority carriers. During the formation of p-n junction, due to concentration gradient across p- and n-side, holes diffuses from p-side to n-side and electrons diffuses from n-side to p-side. This motion gives rise to diffusion current across the junction.

When an electron diffuses from n- to p-side, it leaves behind a positive charge. In such a manner a positively charged layer forms on n-side of the junction.

Similarly, when a hole diffuses from p- to n-side, it leaves behind a negative charge and a negatively charged layer forms on p-side of the junction.

This space-charge region is known as depletion legion. An electric field directed from positive charge towards negative charge develops. Due to this field, electrons on p-side of the junction move to n-side and holes on n-side of the junction move to p-side. This motion of charge carriers due to the electric field is called drift.

Drift current is opposite in direction to the diffusion current.

Initially, diffusion current is large and drift current is small. Space-charge region on either side increases as the diffusion process continues. This increases the electric field and hence the drift current. This process continues until the diffusion current equals the drift current. Thus a p-n junction is formed.