A Note on the Extended Theory of the Junction Transistor

In obtaining the current voltage relation of a junction transistor, one solves continuity equations for minority carriers with appropriate boundary conditions. There are two kinds of boundary condition which have been extended so as to apply to the high injection level operation of a junction transistor. One is in terms of the electrostatic potential and the other is in terms of the quasi-Fermi levels. Taking into account the effect of the base spreading resistance, it is shown that they can be equivalently used in describing the dc and the low frequency performances of a junction transistor but at medium frequencies the quasi-Fermi level approach is preferable. The small signal ac conductances and capacitances which are related to the emitter junction have another correction factor that saturates to 1/2 at high injection levels. Conductances and capacitances are calculated as functions of the emitter current by using the latter boundary condition.