On the determination of the minority carrier lifetime from the reverse recovery transient of pnR diodes

Abstract

The determination of the minority carrier lifetime of pn diodes with abrupt junctions and recombination ( R) contacts is discussed. It is confirmed that the base transit time of diodes with sufficiently small base widths (less than approximately three minority carrier diffusion lengths) affects the reverse recovery transient and that large error can be introduced if the minority carrier lifetime of a diode with a small base width is computed using an expression which was derived for diodes with an infinite base width. Approximations to the general expression for the minority carrier lifetime as a function of the storage time and base width are developed, and the errors associated with their use are analyzed. In addition the effect of measurement errors on the determination of lifetime in short base diodes is discussed. A procedure is described which can be used to compute the minority carrier lifetime of pnR diodes which the ratio of the base width to the minority carrier diffusion length is larger than some small number. The applicability of the procedure and the approximations which are developed are a function of the ratio of the diode base width to the minority carrier diffusion length ( W L ) , and the forward to reverse current ratio ( I f I r ) at which the experimental measurements are made. An example is given which shows that less than 1 per cent error is introduced in the computed lifetime by the approximations if the lifetime is computed from measurements made at I f I r = 4 and 8 for diodes such that W L > 2 × 10 −2 .