An analysis of transistor base spreading resistance and associated effects

For the circular disk type of transistor geometry, as commonly used in alloy junction transistors, base spreading 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 is in general complex. Comparison with measurement shows that this impedance, which includes the base spreading resistance, can be accurately calculated over a wide frequency range in terms of dimensions, physical constants, frequency, dc emitter bias, and effective minority carrier lifetime in the base region for small signal operation of low-power alloy junction transistors. Limitations and extensions of the analysis in its present form are discussed.