Effets de la defocalisation (c.c. et c.a.) sur le comportement des transistors a jonctions

Abstract

This paper deals with a two dimensional analysis of junction transistor electrical behavior. Direct current and a.c. crowding effects are studied and compared to the observed experimental results. At first, and for a circular geometry transistor, d.c. crowding laws are set up, from which static and dynamic base resistance expressions are derived. Afterwards, the definition of transistor active base impedance is given and an electrical model representing conduction phenomena in the base region is proposed. The equivalent circuit of the input impedance which is obtained fits, at any bias level and for all frequencies, with the experimental results. Furthermore the crowding influence on surface conduction is proved and a theoretical expression of the current gain is derived. This expression is consistent with experiments in a large range of bias current values. Finally, a two dimensional analysis of the collector zone conduction phenomena is presented taking into account collector current crowding resulting from the transversal base current. A general expression of the collector impedance is given, from which specific effects outlined by experiments on transistors operating in the saturation mode can be explained.