Question

What is the depletion region in pn junction?

Answer 1

Hint
Think of the depletion region as something of a region where electrons and holes interact in a certain manner. This region exists in the space between P and N type semiconductors; thus, it is easy to reach at.

Complete step by step answer
Depletion region is a region in a P-N Junction diode where no mobile charge carriers, i.e. electrons and holes, are present. Depletion layer does act like a wall that opposes or restricts, to a limit, the flow of electrons from n-side and holes from p-side.

Additional information
A depletion region is a natural feature of doping a semiconductor n-type and right next to it p-type. At the p-n junction, the mobile electrons in the n-type diffuse across the junction, into the p-type area. This then leaves the n-type area with a net positive charge, because the electron left. Holes can be said to 'diffuse' toward the n-type.
So, we have already established that the n-type is left with a positive charge. The electrons which moved to the p-type area from the n-type combined with the holes in the p-type area, and thus leave the p-type with a negative charge.
When the electron combined with the hole in the p-type, the outer shell of the atom which previously had the hole is now filled, and thus the atom becomes very unreactive, as it has a stable full outer shell. Therefore, when many electrons fill holes, this happens, and so the area becomes very reactive. Also, since the p-type is negative, electrons are repelled away.
In the n-type, atoms have always had a full outer shell, but have gotten rid of their free electron. So now, the p-type region and the n-type region both are charged, repel charge-carriers, and also are unreactive because of their full outer shells. This -freezes- this region, and no mobile charge-carriers exist here. This is called the depletion zone.

Note
This only happens towards close to the boundary of the p-n junction, and so the depletion zone is not infinite, it has a defined width/size. Also, many students confuse the depletion region as a region where it depletes the properties of p-n junction diodes, which is completely wrong.