Photoconductive and Photoelectromagnetic Lifetime Determination in Presence of Trapping. I. Small Signals

Abstract

Impurities which are located in the forbidden energy gap of a semiconductor are classified as traps or as recombination and generation levels, according to their capture cross sections and their proximity to the quasi-Fermi levels of the carriers. Their influence upon the photoconductance and the photoelectromagnetic effect is considered; noted in particular are their effects on the values of carrier lifetimes deduced from these photosignals. The lifetimes deduced are always too high for that type carrier of which some are trapped, and they are too small for the other type carrier. In extrinsic material the photoeffects are augmented by trapping of minority carriers, and diminished by trapping of majority carriers. In general the effects of minority carrier trapping are more severe than those of majority carrier trapping. Further-more, the photoelectromagnetic effect is much less sensitive to trapping than is the photoconductance and may often yield the correct minority carrier lifetime in extrinsic material. Consequently, the method of deducing carrier lifetimes by combining the photoconductance and the photoelectromagnetic effects may lead to very misleading results indeed. Yet separate measurements of the two effects over a range of temperatures will yield the carrier lifetimes, the energy level of the traps, and their density. Moreover, concomitant measurements of the spectral dependence of photoconductance and the photoelectromagnetic effect in an extrinsic semiconductor would serve to classify the impurity centers which are found, because an impurity photoelectromagnetic effect occurs only if the carriers generated from the centers are minority carriers.