Reduction of the base-collector capacitance in InP/GaInAs heterojunction bipolar transistors due to electron velocity modulation

The base-collector capacitance of an InP/GaInAs heterojunction bipolar transistor (HBT) was measured as a function of collector current and base-collector voltage, The experimentally obtained results were considerably smaller than the expected dielectric capacitance. For example, at a collector current density of 50 kA/cm(2) the value of the intrinsic C-bc was 33% less than the expected dielectric capacitance. A model that takes into account modulation of electron velocity in the collector depletion region by the base-collector voltage was employed to account for the experimental results. An arbitrary profile of the electron velocity in the collector, which accounts for the velocity overshoot effect, was assumed in developing this model. Excellent agreement was obtained with no fitting parameters. The model relates the change in C-bc to the variation of the collector delay time with base-collector voltage.