INFLUENCE OF ELECTRON VELOCITY OVERSHOOT ON COLLECTOR TRANSIT TIMES OF HBTS

Collector transit times of heterojunction bipolar transistors with pronounced electron velocity overshoot effect are investigated based on a simple analytical model. The “effectiveness carrier velocity” υeff, which is a measure in determining the transit time, is defined as τc = Wc/2 υeff. It is found that υeff is much different from the average velocity υav which 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 υav. © 1990 IEEE