What causes the charge carriers generated in the depletion region to leave the depletion region in a photodiode absorbing light?
A. The depletion region in a photodiode is set-up when charge carriers from each side of the p-n junction diffuse across the ‘interface’ of the p-n junction. Thus consider the n-type material – some of the electrons have diffused out of the material and some holes have diffused into the material. This generates a net +ve charge in the n-type material and vice-versa for the p-type material. This then sets up an electric field across each side of the junction extending into the material. An equilibrium position is reached where the diffusion of charge carriers is balanced against the electric field that diffusion sets up which tries to ‘pull’ the native charge carriers back into their ‘own’ host material. When light of high enough energy is absorbed in the depletion region an electron-hole pair is generated where the electron has sufficient energy to be promoted to the conduction band. The equilibrium setup by diffusion and electric field no longer holds and the ‘excess’ charge carriers
