Formation of depletion region: In the p-type semiconductor, holes are the majority carrier and in the n-type semiconductor, electrons are the majority carrier.

When a p−n junction is formed, some of the electrons from the n-region which have reached the conduction band are free to diffuse across the junction and combine with holes.

Filling a hole, makes a negative ion in p-side and a positive ion in the n-side. Thus, free charges get depleted and a depletion region is formed, which inhibits any further electron transfer.
Applying forward bias, the depletion region reduces and again electrons can diffuse.
OR
Photodiode is light sensitive electronic device that converts light into voltage signal or current signal. It carries two terminals-anode and cathode showing Light rays reflection in Form of photo detector. It works normally on principle of photo generation.
Working: In this, the photons hit the diode resulting in electron-hole pairs where intensity of photon absorption is based on photon energy where less in the PN junction shown, in case of absorption in depletion region, the hole pairs are removed from the p-n junction due to inside electric field in depletion region that makes the holes to move towards anode with electrons toward cathode forming photo current. Here, total photo currents and dark currents flowing with/without light is sum of current through photodiode.