Theory of transient photoconductivity in counter-doped semiconductors

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Abstract
A theory of impurity-related transient photoconductivity in semi-insulating semiconductors is developed. When a photoconductor is illuminated in an applied electric field, the initial sweep-out of carriers generates space charge at the active centres within the crystal. This space charge is neutralised by injection from contacts, but the injected carriers are partially trapped leading to the possibility of current oscillation with frequency determined by a dielectric relaxation time and a trapping time. The theory of this effect is developed for a simple model involving a deep donor and compensating shallow acceptor. Effects of additional traps on the transients are also discussed. If extra traps with large capture cross sections are present near the quasi-Fermi energy, they can damp the oscillations. Such traps can also strongly affect the transient shape, introducing dependence on optical chopping frequency and temperature.