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, it is found that velocity overshoot effects are difficult to observe in the normal operating range of base-collector bias. Impulse response analysis predicts that velocity overshoot should reduce the collector delay in AlGaAs/GaAs HBTs but the effect should rapidly diminish with increasing bias. It is also shown that impulse response is sensitive to the field profile. Intentional, unintentional, or current-induced variations in the collector field profile may have substantial effects on collector delay.<>