It is fabricated with a transparent window to allow light to fall on diode.
When the photodiode is illuminated with photons of energy (hv > Eg) greater than the energy gap of the semiconductor, electron - holes pairs are generated. These gets separated due to the junction electric field (before they they recombine) which produced an emf.
Reason : It is easier to observe the change in the current, with change in light intensity, if a reverse bias is applied.
The fractional change in the minority carrier current, obtained under reverse bias, is much more than the corresponding fraction change in majority 5 carrier current obtained under forward bias.