Quasineutrality in semiconductors

Abstract

A general analysis of the semiconductor transport equations is performed in order to establish under which conditions the often used, but rarely verified, quasineutrality assumption of negligible space charge in the so-called ‘‘neutral regions’’ in a semiconductor holds. The small-signal deviation from exact neutrality is calculated for all four nonequilibrium states, that is, injection, accumulation, extraction and exclusion, for one-dimensional current flow in a semi-infinite homogeneous semiconductor. It is shown how the deviation from neutrality depends on the material parameters. Contrary to injection and exclusion, the small-signal deviation from neutrality for accumulation and extraction depends strongly on the current density. A range of current densities for which quasineutrality holds is given. Significant new results are obtained for the intermediate- and large-signal regime. For high-level injection and accumulation in n-type material, quasineutrality is shown to hold whenever small-signal quasineutrality holds at the same material parameters and current density. For high-level extraction, the deviation from neutrality is found to increase compared to the small-signal case. The corresponding current density range for which quasineutrality holds is given. The high-level exclusion case is still an open issue.