Numerical Modeling Of Energy Transport Effects In AlGaAs/GaAs Heterojunction Bipolar Transistors

A new numerical model for AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is proposed in which the transport of electron energy is included using "hydrodynamic" equations derived from the Boltzmann equation. This model can analyze nonequilibrium transport effects such as velocity overshoot, in contrast to the conventional static model. It is shown that the importance of energy transport effects arises from the fact that the average electron energy deviates largely from the field-determined one. The velocity overshoot can occur in a graded-gap base and a collector depletion region, resulting in a much higher cutoff frequency than that predicted by the conventional model.