Origin of the hook effect in extrinsic photoconductors

The response of extrinsic photoconductors to a step change in incident phot on flux has long been known to exhibit a sharp transient feature, particula rly at higher signal levels, known as the hook effect. We demonstrate exper imentally and theoretically that the hook effect can be due to reduced illu mination adjacent to the injecting contact. This nonuniformity can be produ ced by the transverse illumination of the detector that is common for far-i nfrared Ge:Ga devices. The hook effect has been demonstrated to be either p resent or absent in the same Ge:Ga photoconductor, at comparable signal siz e, depending on the nature of the contact illumination. Numerical finite-di fference calculations of the transient response support this explanation an d produce features that replicate the. experimental results. (C) 2001 Optic al Society of America.