Stored charge and the Ebers-Moll ON voltage

Usual analyses of the current-saturated condition for the junction triode transistor have related storage time and the emitter-to-collector ON voltage to current density and the forward and inverse alpha. It is shown here that in the current-saturated condition the stored charge is proportional to current and to base transit time, and is reciprocal to the emitter-to-collector ON voltage. This reciprocity is linear when the collector-to base junction is significantly forward biased. As current injected at the collector becomes small in comparison to that which it receives from the emitter, the stored charge approaches a minimum for the given emitter-to-collector current and collector-to-emitter voltage rises without appreciable decrease of stored charge. These results can also be obtained by regarding the conductivity modulated base layer as a resistor whose resistance is reciprocal to the stored charge. An obvious conclusion is that extremely low Ebers-Moll electronic ON voltage is undesirable in fast switching transistors designed for maximum speed. An optimum value of the ON voltage appears to be around 0.06 volts for homogeneous base transistors and perhaps 0.12 volts for diffused base structures.