The crucial importance of surface recombination in many solid-state devices is well known. Therefore one needs to know the differences between recombination via a spectrum of states and recombination via a single level. The generalised Shockley-Read recombination rate U1via Ntstates located at a single energy level Etis compared with the analogous rate Uspobtained when the Ntstates are spread over a range of energies Etl, Etuin the forbidden gap. The same recombination coefficients, electron and hole concentrations, etc., are used in both cases. The ratio R=U1/Uspis studied. Among the conclusions are that R as a function of the location of Etis peaked at midgap if Usprefers to a uniform spectrum in the forbidden gap, and the capture cross sections for holes and electrons are of the same order. This arises from the efficient recombination for midgap states. For the same reason R drops below unity if Etlies near the band edges. For a strongly n-type or a strongly p-type surface the replacement of the spectrum by a single level is normally reasonable. The effects on R of energy-dependent capture coefficients and a non-uniform spectrum of trapping states are also considered. Although developed for surface recombination, the theory is also applicable to bulk recombination via an appropriate change of units for Nt.
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