Concept: Reverse-biased $p$-$n$ junction

In a reverse-biased $p$-$n$ junction, the depletion region becomes wider, and an electric field exists across the depletion layer due to the uncovered immobile ions.

Electric field inside the depletion region

The electric field is not zero inside the depletion region. It varies across the depletion region:

• It is zero outside the depletion region.
• It becomes nonzero inside the depletion region.
• Its magnitude is maximum near the metallurgical junction, i.e. around the middle of the depletion layer.

Thus, in the middle of the depletion layer, the electric field is maximum.

Potential inside the depletion region

The electric potential changes continuously across the depletion region. It does not become zero specifically at the middle, and saying it is maximum at the middle is incorrect.

Evaluate the options

• A: electric field is zero — Incorrect
• B: potential is maximum — Incorrect
• C: electric field is maximum — Correct
• D: potential is zero — Incorrect

Final answer

Therefore, the correct option is: $\boxed{\text{C}}$

Comparison with stored correct answer

The stored correct answer is A, but from depletion-region field distribution, the correct answer should be C. Hence, I do not agree with the stored answer.