Question

In the depletion region of a diode
A. there are no mobile charges
B. equal number of holes and electrons exist, making the region neutral
C. recombination of holes and electrons has taken place.
D. immobile charges ions exist

Answer 1

Hint: As a very first step, one could read the question well along with the given options. Then one could recall the properties of a depletion region along with its formation process. Then you could look for the given properties in those properties and hence get the answer to the question.

Complete step-by-step solution:
In the question we are supposed to find the correct among the given conditions for the depletion region of a diode. In order to answer this question, one could recall the properties as well as the formation process of the depletion region of a p-n junction.
We could define depletion region as the region of a pn junction where no mobile charge carriers such as electrons and holes are present and it acts like a wall that opposes the flow of electrons from n-side and holes from p-side. The formation of depletion regions begins with diffusion of charges which occurs as the result of the presence of concentration gradient. However, while the diffusion takes place, the recombination of holes and electrons takes place. Hence, this region that is completely depleted of mobile charge carriers called depletion region is formed.
Therefore, we found that in the depletion region:
(1) There are no mobile charge carriers.
(2) The presence of equal number of holes and electrons makes the region neutral
(3) The recombination that has already taken place causes its very formation.
(4) Now we have the recombined immobile charges here.
Hence, all four options A, B, C and D are true.

Note: One should always keep in his or her mind that the thickness of this region is very small and is known to be of one-tenth of a micrometer. There would be an electric field that is directed from positive charge to negative charge as the result of the positive space charge region on the n-side of the junction and the negative space charge region on the p-side of the junction.