Questionability of drift-diffusion transport in the analysis of small semiconductor devices

Abstract Analysis and design of such semiconductor devices as bipolar and field-effect transistors now relies, in part, on the assumption that current flows by drift and diffusion. The minute size of existing devices raises questions about the appropriateness of this assumption, however, because an internal region critical to device performance may become so small that carriers crossing it fail to experience many collisions. To demonstrate possible consequences of continuing to base analysis on assumed drift and diffusion, we contrast the predictions of two models for the electrical base region of a bipolar transistor. The first model assumes that carriers in transit across the base will experience many collisions, which requires a base region of sufficiently large dimensions. The second model assumes that a carrier in transit will suffer no collisions, displaying what tends to occur in the limiting case of small dimensions. A comparison of saturation current, temperature dependence, and base transit time shows that significant differences exist.