Question

Write briefly the important processes that occur during the formation of p−n junction. With the help of necessary diagrams, explain the term 'barrier potential'.

Answer 1

A thin p-type semiconductor wafer is taken and a part of it is converted into n-si by adding a small quantity of pentavalent impurity. The wafer now contains a p-region and an n-region and a metallurgical junction between p and n region. 
The n-type semiconductor has more concentration of electrons than hole and p-type semiconductor has more concentration of holes than electrons. So, the holes diffuse from the p-side to the n-side and electrons diffuse from the n-side to the p-side. When an electron diffuses from the n-side to the p-side, it leaves an ionised donor behind it in the n-region. The ionised donor, which is positively charged, is immobile as it is bounded by surrounding atoms. Therefore, a layer of positive charge is developed on n-side of the junction. Similarly, when holes diffuse from p-side to n-side, they leave an ionised acceptor behind them in the p-region. The ionised acceptor, which is negatively charged, is also immobile due to surrounding atoms. Thus, a layer of negative charge is developed on the p-side of the junction. 
This space region on both the sides of p-n junction, devoid of any charge carrier, will form a region called depletion region.

Due to the positive charge region on n-side of the junction and negative charge on p-side of the junction, an electric electric field is set up across the junction from the positive charge towards the negative charge. This electric field sets up a barrier potential at the junction that opposes diffusion of majority charge carriers into opposite regions. As the diffusion process continues, the space-charge regions across the junction extend. The strength of the electric field increases; thereby, increasing the drift current . This process continues until the diffusion current becomes equal to the drift current. When this stage is reached, the movement of majority carriers across the junction stops. Now the junction is in equilibrium state. At this stage, the potential barrier across the p-n junction has maximum value \[V_B\].

It means that he barrier voltage stops the diffusion of majority charge carriers from p- to n-region across the junction and vice versa. This potential acts as a barrier and, hence, is known as barrier potential.