Relationship between transient response and output characteristics of avalanche transistors

Abstract

The output characteristics of avalanche transistors are investigated under high-current conditions and compared with their transient response. A strict connection can be demonstrated between the switching time and that branch of the output characteristics where the transient process ends. There are two different kinds of switching processes, “slow” ones, approximately showing the switching times of the “classical” theory for avalanche transistors (e.g. 10 ns), and “fast” ones with switching times nearly one or two orders of magnitude lower (e.g. 0.2 ns). A fast switching process will always end on a branch of the output characteristics with low series resistance and comparably low collector-emitter voltage (less than VB CEO ). This branch is nearly independent of the circuitry. Slow switching will end on branches close to the I B = 0-characteristic. It will be shown that for the generation of steep pulses transistors with a low product of impurity concentration and effective thickness of the epitaxial layer must be used. Due to the dominating role of the pinch-in effect which leads to the desired high current density, the results are interpreted by a simple two-dimensional transistor model. The quantitatively exact description of the transient response of avalanche transistors is very complicated and therefore it has not yet been dealt with in this paper.