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

We revisit the theory of the transients in low-temperature extrinsic photoconductors, with emphasis on the initial fast component response. Application of a photon-generation step launches a traveling zone boundary between part of the device where, in the short term, a quasi-equilibrium has been reached and a part where the carrier concentration is still growing. As the boundary passes, the exponentially slowing growth of carriers is abruptly terminated. The time to reach this quasi-equilibrium depends on the sample length and the carrier drift velocity rather than on the carrier lifetime. We note restricting errors in earlier analyses and produce an improved formula for the subsequent slow fraction of the total current change by including carrier diffusion. Comparison of numerical and analytical results suggests that measurement of the fast or slow fraction of the response can provide a highly accurate measure of the mobility-lifetime (μτ) product.