Consider a photodiode with a p+i-p-n+ configuration. This construction is similar to a PIN photodiode containing two heavily doped and two lightly doped regions. In the avalanche photodiode (APD), which of the following are the heavily and lightly regions in the device?

Correct option is D

​$$\text{A typical PIN or APD structure has:} \\[6pt]\bullet \ \text{p}^+ \text{ region} \rightarrow \text{ heavily doped} \\[2pt]\bullet \ \text{n}^+ \text{ region} \rightarrow \text{ heavily doped} \\[2pt]\bullet \ \text{i-region} \rightarrow \text{ intrinsic or very lightly doped} \\[2pt]\bullet \ \text{p-region} \rightarrow \text{ lightly doped compared to p}^+ \\[8pt]\textbf{In an APD (Avalanche Photodiode):} \\[4pt]\bullet \ \text{The p}^+ \text{ and n}^+ \text{ layers act as heavily doped contact regions.} \\[2pt]\bullet \ \text{The i-region is lightly doped (nearly intrinsic) to allow high electric field and carrier multiplication.} \\[2pt]\bullet \ \text{The p-region is also lightly doped to support depletion and avalanche action.} \\[8pt]\text{Therefore, the correct classification is:} \\[6pt]\text{Heavily doped: } p^+, \, n^+ \qquad \text{Lightly doped: } i, \, p$$​​