Concept:An Avalanche Photodiode (APD) operates under high reverse bias near breakdown, where photo-generated carriers accelerate through a high-field region and free additional electron-hole pairs via impact ionization, providing internal current gain \( M \).
Formula:$$I_{\text{out}} = M \cdot I_{\text{primary}} = M \cdot (\mathcal{R}_0 P_{\text{opt}}) \quad (\text{where typical gain } M \approx 50\text{ to } 200)$$
Solution:- In a standard PIN photodiode, each absorbed photon generates at most one electron-hole pair (\( M = 1 \)).
- In an APD, high reverse bias accelerates primary photo-carriers to high kinetic energies.
- These carriers collide with lattice atoms, generating secondary electron-hole pairs (avalanche multiplication) that provide internal gain (\( M = 50\text{–}200 \)) and improve detection sensitivity for weak optical signals.
Why other options are incorrect:- Option A: APDs detect light electronically; they do not generate acoustic waves.
- Option C: All semiconductor junctions have finite junction capacitance.
- Option D: APDs require high reverse bias voltages (typically \( 50\text{–}200\text{ V} \)) to create the avalanche multiplication field.
Quality & Fidelity Assurance:
Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.