High injection in epitaxial transistors

Abstract

An analysis of base widening and the current dependence of the cutoff frequency f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</inf> has been given previously [1]. The analysis is approximate and is based on the location of the edges of transition regions. Definition of transition regions becomes problematic in transistors having nonuniform base doping and for high-current densities. Such difficulties are avoided in this paper by use of the charge control approach. Numerical solutions of delay time ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">= 1/2\pi f_{T}</tex> ) as a function of current density are given for low reverse collector bias. Whereas transition regions can be defined only approximately, the electric field is a well-defined quantity, and changes in the electric field that accompany base widening are shown in detail. At low injection levels, a high-field region exists near the transition between base and epitaxial layer. This high-field region is relocated to the interface between epitaxial layer and substrate under high injection conditions. When this "high-field relocation" has occurred, the epitaxial layer acts as an extension of the base with an attendant increase in the delay time [1].