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Asked in RS Academy GUJCET booklet · Photodiode and solar cell
Idea: detecting light means turning it into a measurable change of current, so the current flowing in the dark must be small enough for the light-generated carriers to stand out against it.
A photodiode is therefore operated in reverse bias. In the dark only the tiny reverse saturation current flows.
Light whose photon energy exceeds the band gap creates electron-hole pairs in and near the depletion layer, and the reverse field sweeps them straight out as a photocurrent.
Because the dark current is so small, that photocurrent is a large fractional change, and it stays very nearly proportional to the light intensity -- which is what makes the reading useful.
An LED is the reverse process: a forward biased junction that gives out light rather than responding to it.
So for detecting light the photodiode has to be reverse biased.
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