Influence of electron velocity overshoot on collector transit times of HBTs

Collector transit times of heterojunction bipolar transistors with a pronounced electron velocity overshoot effect are investigated using a simple analytical model. The effective carrier velocity, ν_eff , which is a measure for determining the transit time, is defined as π_C=W_C/2ν_eff. It is found that ν_eff is much different from the average velocity, ν_av, that is given by the traveling time through the whole collector depletion layer and the depletion width. With a higher overshoot peak velocity, the collector transit time is shorter than that estimated simply from the average velocity ν_av