The forward biased, Abrupt p- n junction

Abstract

The abrupt, forward biased pn-junction is solved very accurately for its terminal characteristics and injection efficiency for all values of forward bias. This is accomplished by using a three-region approach and formulating the transport problem in terms of the local pn-product. Analytic solutions are derived for low, medium and high injection conditions. The low injection regime is characterized by negligible majority carrier density increases. However, in this injection region bulk width modulations due to decreasing transition layer widths modify the standard diffusion model results. The medium injection condition prevails if majority carrier density changes due to injection are significant on the lightly doped side of the junction. This as well as an inreasing bulk voltage drop are taken into account to establish new relationships for injection efficiency, which is decreasing with injection, and the terminal characteristics. The high injection regime involves significant majority carrier modulation in both bulk regions. The injection efficiency becomes a constant which is determined by mobility differences since diffusion currents are negligible. The transition region vanishes in width and its total voltage drop goes to zero. The current through the device is governed by a power law because it is a conductivity modulated resistor.