Collector capacitance and high-level injection effects in bipolar transistors

A simple model for the behavior of the collector capacitance of bipolar transistors has been developed with the aim of studying high-level injection phenomena in epitaxial collectors. The Collector capacitance (Cc) calculated from the results of a dynamic small-signal measurement. It is Observed that Ccincreases by more than an order of magnitude as the collector current is increased from a low value into the quasi-saturation regime at a fixed collector-emitter voltage of 1 V. The collector capacitance is composed of a transition capacitance, which is due to the presence of unneutralized charges in the collector region, and of a diffusion capacitance, which is due to the presence of neutralized charge in transit across the base and the collector regions. The transition capacitance is the dominant component at low-current levels. However, at high-current levels, the diffusion capacitance predominates if the one, dimensional "base-widening" model (Kirk effect) becomes operative and the total capacitance becomes very large. This capacitance is found to be about an order of magnitude larger than that expected if the two-dimensional "lateral-spreading" model were dominant. Good agreement is observed between the experimental data and the theoretical Ccestimates using the one-dimensional model. Thus it is concluded that the base-widening model controls the behavior of our devices at high levels of injection in the collector.