The theory of transient photoconductive processes in II–VI compounds with three deep trapping levels

Abstract

The transient photoconductive rise and decay processes were studied quantitatively in a typical II–VI compound by solving the rate equations on a digital computer. The calculations were based on a model with three deep trapping levels, as proposed by Rose and by Bube. A study was made of the effect of changing the concentration, position, and capture asymmetry fo these levels. We have also considered the way in which the defect centers change their role from trapping to recombination (or vice versa) as a result of free carrier concentration changes. Our work confirms previous explanations of the relation between sensitizing level properties and the steady-state superlinear behavior exhibited by II–VI photoconductors. On the other hand, a different approach appears to be necessary to account for transient phenomena. We do not believe that the position or asymmetry of the trapping levels is crucial in determining photoconductive sensitivity and response times. Instead, we find that these material properties can be predicted from a knowledge of the equilibrium and the steady-state charge concentrations. In addition, we show that the dynamic changes in the function of the deep levels from trapping to recombination, or vice versa, are the consequence—rather than the cause—of the specific way in which the photoconductive rise and decay processes occur.