Thermal filling effects in capacitance experiments on deep levels in semiconductors

The interpretation of capacitance studies of deep levels in semiconductors is complicated by the presence of a Debye tail of free carriers, thermally excited from the bulk into the depletion region. The concentration of these carriers varies spatially within the depletion region and so produces a variation in the rate constant for capture by deep traps. Unless this effect is taken into account wrong values of concentration of the centre being studied will be obtained. A further consequence is that capacitance transients are in general not exponential functions of time, although in favourable cases an exponential function may be a good approximation. In the general case unreliable time constants may be deduced if capture effects are not eliminated in the reduction of data; this may be difficult to do satisfactorily when measuring initial rates of change of capacitance.