Diffusive electrical conduction in high-speed p-i-n photodetectors.

We report on time-resolved photocurrent measurements in InGaAs/GaAs-quantum well p-i-n photodetector structures in which the photocurrent is optically excited and electrically detected at different locations on the sample. As the electrical pulse propagates out from the point of excitation, a temporal broadening of the signal is induced by the in-plane resistance of the doped contact layers. This temporal broadening agrees quantitatively with analytical solutions obtained from the diffusion equation used to describe the lateral voltage propagation. We discuss the significance of this temporal behavior for the optimization of high-speed photodetectors. Finally, we point out the relationship between our optical-pump/electrical- probe experiments and the all-optical pump/probe experiments reported by Livescu et al.