Self-consistent particle simulation for (AlGa)As/GaAs HBTs underhigh bias conditions

Nonequilibrium electron transport phenomena in the emitter and collector regions under high bias conditions were investigated for standard N-p-n (AlGa)As/GaAs heterojunction bipolar transistors (HBTs) by utilizing a previously developed one-dimensional self-consistent particle simulator. A dramatic increase in the cutoff frequency was observed for a lightly doped collector HBT as the current density increased over 10⁵ A/cm², where the collector transit time was reduced due to the extension of the velocity overshoot region in the collector corresponding to the decrease in electric field near onset of the Kirk effect. A saturation tendency was seen in the collector current versus base-to-emitter bias voltage (V_BE ) characteristic for high V_BE, where V_BE exceeded the base-to-emitter built-in voltage of the conduction band. Simulations indicate that this feature is caused by electron velocity saturation in the neutral n-type (AlGa)As emitter region