An analysis of base resistance for alloy junction transistors

For the circular disk type of transistor geometry, as commonly used in alloy junction transistors, base resistance is determined by treating it as a boundary value problem. This treatment results from consideration of the over-all behavior of both minority and majority charge carriers in the base region and leads to an expression for base spreading resistance in terms of alpha, frequency, resistivity, and transistor dimensions. Further consideration of this over-all charge carrier behavior leads to a determination of the entire common-emitter short-circuit input impedance, which in general is complex. Comparison with measurement shows that this impedance, which includes the base resistance, can be calculated accurately over a wide frequency range in terms of physical constants, dimensions, frequency, dc emitter bias, and effective minority carrier lifetime in the base region for small-signal operation of low power alloy junction transistors. Application to other types, such as power transistors and diffused-base transistors, may require extension of the present analysis with a considerable increase in complexity. Limitations and extensions of the analysis in its present form are discussed.