Self-consistent particle simulation of heterojunction bipolar transistors under high temperature operating conditions

The high-temperature operation of an AlGaAs-GaAs heterojunction bipolar transistor (HBT) has been investigated quantitatively using a one-dimensional self-consistent particle simulator. Emitter charging time is found to have little temperature dependence below 10/sup 5/ A/cm/sup 2/ current density, while it decreases with an increase in temperature above 10/sup 5/ A/cm/sup 2/ because of the decrease in transconductance. The base transit time is found to decrease slightly as temperature rises, because of the reduction in ionized impurity scattering for thermally excited high-energy electrons in the base. A drastic increase in collector transit time is observed with an increase in temperature, which is attributed not to LO phonon scattering but to intervalley scattering. Nonequilibrium electron transport effects, such as velocity overshoot, become less effective as temperature rises, especially in the p-collector, thus deteriorating the high-speed performance. Therefore, the choice of materials with larger Delta E/sub Gamma -L/, such as InP, is recommended to take advantage of the velocity overshoot in the collector even under high-temperature operation.<>