Base-width modulation and the high-frequency equivalent circuit of junction transistors

The effect of base-width modulation on the exact small-signal, high-frequency equivalent circuit of p-n-p (and n-p-n) junction transistors is examined. It is found that (except for the effect on the base spreading resistance) base-width modulation only helps to determine the magnitude of one element. Failure to recognize this fact has resulted in the past in the "discovery" of "new" elements and the creation of a multiplicity of equivalent circuits. Through the use of an engineering approach, the most important of these circuits are related to the exact circuit, and hence their degree of approximation becomes apparent. The effect of a high injection level of minority carriers is also mentioned. Finally, the equivalent circuit of p-n-i-p (and n-p-i-n) transistors is discussed.