Speed of Response
In many applications it is the dynamic performance which is of greatest interest. The photodiode response time is the quadrature sum of the charge collection time and the RC time constant effect arising from the series plus load resistances and the junction plus stray capacitances.
Charge collection time is voltage dependent and is made up of a fast and a slow component. The fast component is produced by transit of electrons and holes through the depletion region, at their respective drift velocities, under the influence of the electric field.
Photon energy absorbed outside the depletion region will produce carriers that are collected by diffusion and the response time of these carriers will be relatively slow. Figure 4 illustrates the transient response of a photodiode to a square pulse of radiation.