CBSE Class 12 · 2008 · SemiconductorsVery Short Answer

State the reason why a photodiode is usually operated at reverse bias.

Solution

On illumination a photodiode generates electron-hole pairs that change the current.
In reverse bias the dark current is a small minority-carrier current, so the extra light-generated carriers produce a large fractional change that is easy to measure accurately.
In forward bias the current is already large, so the same light produces only a tiny fractional change.

Reverse bias gives a larger, measurable fractional change in current.

CBSE Class 12 Board · 2008 · Subjective

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