Dear Syed
If we join the N and P materials together (create a diode) the electrons in the N material diffuse (move or spread out) across the junction into the P material and fill some of the holes. At the same time, the holes in the P material diffuse across the junction into the N material and are filled by N material electrons. This process, called JUNCTION RECOMBINATION, reduces the number of free electrons and holes in the vicinity of the junction. Because there is a depletion, or lack of free electrons and holes in this area, it is known as the DEPLETION REGION.
The loss of an electron from the N-type material created a positive ion in the N material, while the loss of a hole from the P material created a negative ion in that material. These ions are fixed in place in the crystal lattice structure and cannot move. Thus, they make up a layer of fixed charges on the two sides of the junction.
On the N side of the junction, there is a layer of positively charged ions; on the P side of the junction, there is a layer of negatively charged ions. An electrostatic field is established across the junction between the oppositely charged ions. The diffusion of electrons from the P-type material towards the positively charged N-type material and holes from N-type towards the negatively charged P-type is favored.
In between the p and n side, the built-in voltage is created and because of this reasons, there is also the drift current besides with the diffusion current.
At zero, both Diffusion and drift component of carriers are equal and opposite. reason are well mentioned in above answers.
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