Question

Depletion layer consists of:
A) Electrons
B) Protons
C) Mobile ions
D) Immobile ions

Answer 1

Hint: The depletion layer is an insulating region in a conductive and doped semiconductor material where the mobile charge carriers are forced away by an electric field. Although the $PN$ semiconductor consists of mobile ions but in the depletion layer the case is different.

Complete answer:
Depletion layer is formed by diffusion of holes and electrons from the $P$ side and the $N$ side and vice versa in a $PN$ junction. By definition, $N$ type semiconductor has an excess of free electrons while the $P$ side has an excess of holes. When these two sides are placed together to form a junction, the free electrons from the $N$ side conduction band diffuses or migrates into the $P$ side conduction band, and the holes from the $P$ side valence band diffuses into the $N$ side valence band. During this process the diffused electrons come in contact with the holes and are eliminated and the same happens when the diffused holes come in contact with the electrons.
As a result, the majority of the charge carriers are depleted in the region near the interface of the junction and this region is known as depletion layer. Depletion layer is also known as depletion region, depletion zone, junction region, space charge region or space charge layer. It is named so because it is formed by removal of all free charge carriers and thus this leads to a depleted layer consisting of no charge carriers.

The correct answer is (D), immobile ions.

Note: The depletion layer consists of both electrons and protons but they are immobile in nature therefore the correct answer would be immobile ions. The donor and acceptor atom get a positive and negative charge due to which the $PN$ junction forms.