The Effective Surface Recombination of a Germaniun Surface with a Floating Barrier

Abstract

The effect of heavily doped (alloyed) p-type and n-type surface layers on n-type base, and of metallic plating on n-type base, on the surface recombination velocity s has been computed on the basis of one-dimensional junction theory. The results indicate that s should be of the order 1 cm/sec for the heavily doped surfaces, and several thousand cm/sec for the electroplated surface. The low s comes about for the same reason that the injection efficiency of alloy junctions is high; the alloy junction is a very efficient emitter of minority carriers into the base and a poor acceptor of majority carriers from the base because of the high doping level in the alloyed region. Since recombination in the surface layer of minority carriers from the base requires both majority and minority carriers, the restriction of the flow of either reduces the surface recombination. However, measurements of s by diffusion and pulse methods on alloy junction surfaces indicate that their apparent recombination is almost the same as adjacent untreated surface, e.g., 300-500 cm/sec. It is shown that lateral current flow, due to minority carrier gradients parallel to the junction interface, and neglected in one-dimensional theory, gives rise to circulating currents which translate the minority carriers to the nearest high recombination surface. This hole translation property of the floating p-layer is used to explain the erroneously high lifetimes often observed by diffusion measurements on silicon and p-type germanium, and certain discrepancies in effective life measurement on completed transistors.