Transient establishment of the Dember field in fast photoconductive circuit elements

The time τeefrequired to establish the Dember electric fieldE_{D}(t)following the sudden application or removal of incident light or other radiation is examined in general for large surface recombination velocity and illustrated for fast photoconductive circuit elements. The influence of surface scattering on mobility depends strongly onE_{D}(t)because EDresults in electrons and holes locating themselves away from the surface. The analysis is subject to the assumptions underlying the ambipolar transport equation and to a single time-constant characterization of the recombination rate. The results show that if τeffexceeds other times characteristic of the turn-on or turn-off transient, then it enters importantly into determining the transient observed. Specifically, if the product of the steady-state high injection lifetime, the ambipolar diffusion coefficient, and the square of the absorption coefficient of the incident light greatly exceeds unity, then τeffenters importantly. Otherwise it does not.