Characteristics of two-region saturation phenomena

Diffused base bipolar transistors, especially high-voltage types, often exhibit two pronounced saturation regions. This paper elucidates the qualitative and quantitative features of this behavior which is due to conductivity modulation of the lightly doped collector region. The transport equations for ambipolar conduction in the collector region are solved with a minimum of simplifying assumptions. It is shown how this portion of the solution explains the general features of the phenomenon. A complete solution for the collector characteristics depends on an explicit knowledge of the recombination statistics in both the base and collector regions. A simple case is considered to demonstrate the detailed dependence of the phenomena on the physical structure. Design tradeoffs involved in the control of the characteristics are discussed in the light of the results obtained from the analysis.