The figure below illustrates the transient response of a photodiode to a square pulse of radiation. When a photodiode is operated in the unbiased mode, the slow diffusion component dominates, giving risetimes on the order of 0.5 microseconds. For a fast response time, silicon resistivity and operating voltage must be chosen to produce a depletion layer within which the majority of the carriers are generated. In this case the transit time will be dependent on both the electron and hole drift velocities. The depletion depth necessary for full absorbtion increases rapidly with operationg wavelength.
Response times increase correspondingly.
In this structure silicon thickness is reduced to just contain the required depletion depth, and a heavily doped back layer is used to supply the necessary charge to support the depletion region at higher voltage. In this way the operating field, and hence the carrier drift drift velocities, may be increased without a significant increase in depletion depth. Further increase in speed may be obtained at the expense of overall sensitivity by using silicon which is not thick enough to allow full absorbtion of incident radiation. The equivalent circuit of a photodiode is shown in the figure. Fundamentally a photodiode is a current generator.