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, nu /sub eff/, which is a measure for determining the transit time, is defined as pi /sub C/=W/sub C//2 nu /sub eff/. It is found that nu /sub eff/ is much different from the average velocity, nu /sub 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 nu /sub av/.<
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