Question

Why photodiodes are used in reverse bias not in forward bais?

Answer 1

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Photoconductive mode. In this mode the diode is often reverse biased (with the cathode driven positive with respect to the anode). This reduces the response time because the additional reverse bias increases the width of the depletion layer, which decreases the junction's capacitance.

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Answer 2

The fractional changes due to photo effects on the minority carrier dominated reverse bias current is more easily measurable than the fractional change in majority carrier. So photo diodes are preferable used in reverse bais