How p-n junction diode is formed ? And explain depletion layer and barrier potential.

Explanation

When in one region of Si wafer acceptor impurity (Al) is added the p-type semiconductor and another region donor impurity (As) is added the n-type semiconductor is form. Thus, p-n junction is formed and there is metallic junction between p and n region.
Important processes occur during the formation of a p-n junction. Thus, is diffusion and drift.
The number of electrons or holes per unit volume is called number density of electron or holes.
During formation of p-n junction difference in number densities of p and n semiconductor diffusion of hole takes place from p-semiconductor to n-semiconductor `(p to n)` and diffusion of electron takes place from n-semiconductor to p-semiconductor `(n to p)`.
This motion of charge carriers gives rise to diffusion current across the junction.
When an electron diffuses from `n to p` it leaves behind an ionised donor on n-side. This ionised donor (positive charge) is immobile as it is bonded to the surrounding atoms.
As the electrons continue to diffuse from `n to p`, a layer of positive charge (or positive space charge region) on n-side of the junction is developed.
Similarly, when a hole diffuses from `p to n` due to the concentration gradient, it leaves behind an ionised acceptor (negative charge) which is immobile.
These ionized charges are immobile means they cannot move.
The space-charge region on either side of the junction together is known as deplection region or depletion layer.
The n-region of depletion layer does not have majority charge carrier electron and in p-region does not have charge carrier holes. These regions are empty in terms of their majority charge carriers.

The thickness of region is of the order of one-tenth of a micrometer in p-n junction. (Means approximately 0.5 `mum`).
Due to the positive space-charge region on n-side of the junction and negative space charge region on p-side of the junction an electric field directed from positive charge towards negative charge develops. Due to this field, an electron on p-side of the junction moves to n-side and a hole on n-side of the junction moves to p-side. The motion of charge carriers due to the electric field is called drift.
Initially, diffusion current is large and drift current is small.
As the diffusion process continues, the space charge regions on either side of the junction extend thus increasing the electric field strength and hence drift current.
This process continues until the diffusion current equals the drift current.
Thus a p-n junction is formed. Since p-n junction has two connection terminals, it is called p-n junction diode.
In p-n junction under equilibrium there is no net current.