Concept:A reverse-biased guard ring is a concentric diffusion ring placed around the active photodiode area that intercepts surface leakage currents and routes them away from the signal amplifier.
Formula:$$I_{\text{measured}} = I_{\text{bulk,photo}} + I_{\text{bulk,dark}} \quad [\text{Surface leakage } I_{\text{surface}} \text{ shunted to ground}]$$
Solution:- Surface states at the semiconductor-oxide interface contribute significant leakage current.
- A biased guard ring isolates the active junction, diverting surface leakage current directly to ground.
- This reduces dark current and improves detector sensitivity.
Why other options are incorrect:- Option B: Forward bias causes heavy majority carrier current that obscures optical signals.
- Option C: High temperatures increase thermal carrier generation, dramatically worsening dark current.
- Option D: A PN junction is required for photodiode operation.
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.