Effects of Impurity Deionization and Temperature on Carrier Equivalent Lifetime in Diffused Semiconductor Structures

Abstract

In an inhomogeneously doped semiconductor built-in electrostatic field and mobility gradients result in a minority-carrier equivalent lifetime which is of a few orders lower in magnitude compared to normal lifetime in a homogeneous semiconductor. To find the built-in field it is commonly assumed that all the impurity atoms are fully ionized at room temperature. Since this assumption is not valid in some cases, expressions have been derived for the built-in field and the equivalent lifetime taking deionization into account. It is shown that the consideration of deionization for an exponential distribution results in a nonconstant built-in field. Second, the temperature variation of minority-carrier equivalent lifetime has been studied. To make an analytical study possible, an empirical relation for the minority-carrier mobility in silicon as a function of both temperature and impurity concentration has been proposed. Some typical inhomogeneous silicon layers having exponential, Gaussian, and complementary error-function impurity distributions have been considered. It is shown that the equivalent lifetime may decrease with temperature in some regions of an inhomogeneous layer, contrary to the deductions obtained from the Shockley-Read theory for normal lifetime.