Effects of transverse diffusion and transverse stored charge in alloy transistor base

Transverse diffusion of minority carriers under the action of a transverse concentration gradient in the base, which will exist even in the absence of surface recombination, is shown to make an appreciable contribution to the base current and is to be taken into account in considering transistor action. The dependence of transistor current gain on base geometry is traced to this factor. The role hitherto assigned exclusively to surface recombination in this regard and in several areas of disagreement between the predictions of the one-dimensional theory and experimental observations now appears to be largely assignable to transverse diffusion. Transverse diffusion processes in alloy junction type and grown junction type base geometries are contrasted. The adverse effect of transverse stored charge on the high frequency performance of transistors is analysed and their effect on increasing the ' effective base transit time ' discussed. The possibility of localized reduction of volume lifetime in the base in the vicinity of the junctions during the alloying process, despite precautions taken to preserve the volume lifetime of the base wafer as a whole, is pointed out.