Recombination Processes in Insulators and Semiconductors

The discrete states in the forbidden zone are divided into ground states and shallow trapping states. The major recombination traffic passes through the ground states. The shallow trapping states cause the observed decay time of free carrier concentrations to exceed the lifetime of a free carrier in the conduction (or valence) band. At low rates of excitation (free carrier concentrations less than ground state concentrations) the electron lifetime and hole lifetime are independent and, in general, significantly different. At high rates of excitation the free electron and hole lifetimes are equal. For an insulator having one class of ground states (a class being defined by the capture cross sections for electrons and holes) the high-light lifetime is bracketed by the two low-light lifetimes.The behavior of a model having one class of ground states can be described relatively simply and quantitatively. The behavior of a model having more than one class of ground states becomes sufficiently complex that only special cases can be treated easily. More than one class of ground states, however, is required to account for infrared quenching, "superlinearity" and the ability of added ground states to reduce the rate of recombination. These phenomena involve a redistribution of electrons and holes amongst the classes of ground states. Such redistributions can give some meaning to the phrases: "filling of traps" or "saturation of centers."The recombination behavior of a semiconductor is significantly different from that of an insulator. For example, superlinearity can occur in a semiconductor having only one class of ground states. Also, the photocurrents in a semiconductor can be intrinsically more noisy than the photocurrents in an insulator.