On the post-injection voltage decay of p- s- n rectifiers at high injection levels

Abstract

The post-injection voltage decay of p +- s- n + rectifiers is analysed at high carrier levels where the recombination in the heavily-doped end regions is significant. On interrupting the current, the drop in ohmic voltage is followed by a very rapid initial decay of the post-injection voltage, which becomes increasingly important with growing injection level. Hence the initial voltage drop appears larger than the ohmic voltage; from the post-injection voltage, values for the injected carriers concentration are obtained which are too small. The rapid initial decay is followed by a slow and almost linear decay for large base widths. The lifetime determined from this slow voltage decay portion can be equated to the carrier lifetime in the base for an important range of w/ L ( w base width, L diffusion length), although the recombination in the end regions may be significant. Hence in contrast to the method of stored charge, the post-injection voltage method is suited for a lifetime determination also at these high injection levels. Using the theory, measurements made by both methods are compared. in silicon, a high-level lifetime in the order of 10 μsec is observed to be independent of the injected carrier concentration up to 10 17 cm −3.