Some aspects of current gain variations in bipolar transistors

Abstract

This paper deals with the study of the main mechanisms involved in the bipolar transistor current-gain variations. In the first part, we describe some properties of characteristic parameters N S and I SR of the surface “diode”; an attempt is made to explain the strong correlation between N S and I SR. In a second part, we analyse the influence of emitter current crowding, high injection in the base region, and base widening effects on the high level bias current gain value. Critical current densities beyond which each of these effects takes place are defined, allowing us to evaluate their comparative importance at a given collector current value. Our study shows that, on the one hand, for transistors affected by emitter-current crowding, the surface currents play an essential part in the high level bias current gain fall off and, on the other hand, the base widening effect has a very great influence on the static ( I C, V EB) characteristic and on the emitter injection efficiency value. In the third part, the experimental results obtained are reported and compared with previously proposed analytical relations.