Solid-State Thermal Diffusion: A Contributor to Degradation of Semiconductor Junction Devices

THIS paper is based upon the premise that metallic impurities diffuse into the bulk semiconductor regions of semiconductor junction devices, within the limit imposed by the solid solubility of these impurities. It will be shown that the subsequent ionization of these "unwanted" impurities causes certain electronic parameters of the junction to be altered in a predictable way; for example, the addition of these ionizable impurities near the physical interface at the junction causes a change in the electronic structure of the depletion layer which is clearly evidenced by shifts in reverse-biased junction capacitance. The capacitive technique of measuring solid-state diffusion is examined in detail, and its application to the problem of describing failure in semiconductor junction devices is set forth. The diffusion process is assumed to be governed by Fick's diffusion equation. As a first approximation, a simple one-dimensional diffusion model has been proposed, and the equations (Fick, Poisson, and capacitance) consistent with such a model are solved, simultaneously, to yield an expression for junction capacitance as a function of initial resistivities, diffusion constants, bias voltages, and diffusion time. The Burroughs 220 computer has been used to provide numerical results for a large number of sets of variables. Successive models which have been proposed seem to be more comprehensive, since they take into consideration additional ways in which impurities may be "moved about" in the vicinity of the junction. One such way is the transport of ionic impurities under the influence of an applied external field.