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