The width of depletion layer increases when a diodes is reversed biased and a small reverse current flows through the diode.When light is incident on the junction, electronhole pairs are generated in depletion layer in a large amount due to wide depletion layer. So, in reverse bias, diode changes the incident light to current, more significantly i.e., photocurrent is significant in reverse bias as compared to the forward bias current.
Current in forward bias is more than reverse bias but the fractional change in current in reverse bias is more easily measurable than fractional change in forward bias current. For this reason, photo diode is used in reverse bias.Uses: In smoke detectors, in bar code scanners.