Current transients in extrinsic photoconductors: comprehensive analytical description of initial response

We revisit the theory of the transients in low-temperature extrinsic photoc onductors, with emphasis on the initial fast component response. Applicatio n of a photon-generation step launches a traveling zone boundary between pa rt of the device where, in the short term, a quasiequilibrium has been reac hed and a part where the carrier concentration is still growing. As the bou ndary passes, the exponentially slowing growth of carriers is abruptly term inated. The time to reach this quasi-equilibrium depends on the sample leng th and the carrier drift velocity rather than on the carrier lifetime. We n ote restricting errors in earlier analyses and produce an improved formula for the subsequent slow fraction of the total current change by including c arrier diffusion. Comparison of numerical and analytical results suggests t hat measurement of the fast or slow fraction of the response can provide a highly accurate measure of the mobility-lifetime (mu tau) product.