DOES VELOCITY OVERSHOOT REDUCE COLLECTOR DELAY TIME IN ALGAAS/GAAS HBTS

The effect of velocity overshoot on the collector delay in an AlGaAs/GaAs heterojunction bipolar transistor (HBT) is examined. A new impulse response technique to rigorously evaluate the total transit-time delay using transient Monte-Carlo simulation is introduced. By applying the technique to a conventional HBT and comparing the result with a similar calculation ignoring the effects of velocity overshoot, we find that velocity overshoot effects are difficult to observe in the normal operating range of base-collector bias. Although velocity overshoot reduces collector delay significantly at V(cb) = 0 V, the improvement diminishes rapidly as the base-collector bias increases. Enhanced velocity overshoot effects, however, could be achievable with proper tailoring of the electric field profile in the base-collector depletion region.