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The small signal ac transmission characteristics of junction transistors are derivedfrom physical structure and bias conditions. Effects of minority carrier flow and of depletion layer capacitances are analyzed for a one dimensional model. The ohmic spreading resistance of the base region of a three dimensional model is then approximated. Short circuit admittancesrepresenting minority carrier flow, depletion layer capacitances, and ohmic base resistance elements are then combined into an equivalent circuit. Theoretical calculations are compared to observations for two typical designs. INTRODUCTION GeneralJunction transistors have been in commercial production for nearly a year. A detailed understanding of their behavior is necessary both for the increasingly exacting requirements of modern circuit engineering and for the wise design of improved types. Design theory, by relating function to structure, can serve both these needs.The principal object of this paper is to develop in logical fashion a design theory for junction transistors. The product of the development is an equivalent circuit, founded on device physics, which predicts the circuit characteristics of junction devices in a simple and intelligible fashion. Although attention is concentrated on small signal transmission performance, some large signal aspects are also examined. 12il 1272 THE BELL SYSTEM TECHNICAL JOURKAL, NOVEMBER 1953 Method and AssumptionsThe usefulness of the j unction transistor derives primarily from the flow of holes or electrons across two closely-spaced p-n junctions, one of which is biased in the forward or conducting direction while the other is biased in the reverse or non-conducting direction. Development of design theory begins quite properly with analysis of this mechanism, which is considered, for simplicity, as a problem in the flow of holes and electrons in one dimension, at right angles to the p-n junctions. In the analysis, it is assumed that these carriers are controlled largely by the voltages applied to the junctions and that they move principally by diffusion. The dependence of the diffusion currents on the junction voltages is reduced to a set of two terminal-pair short-circuit admittances, which form the initial and most important segment of the equivalent circuit model for the junction transistor.Practical transistors have not only the very useful transisting mechanism mentioned above, but also passive capacitances across the charge depletion layers which separate the p and n regions at each junction. These capacitances limit the useful frequency range of transistors and must be considered in any practical theory. In the synthesis of the equivalent circuit, these capacitances are placed in parallel with the short-circuit input and output admittances which represent the flow of diffusing holes and electrons.A further limitation on performance is imposed by the ohmic or body spreading resistance of the base region. The base current of the transistor, in flowing from the region between the emitter and collector to t...