Collector Signal Delay Time And Collector Transit-Time Of Hbts Including Velocity Overshoot

The collector signal delay time tau\u27(CT) and collector transit timer tau(CT) in the heterojunction bipolar transistor (HBT) are studied and discussed using the delay time concept. Based on the assumption that the free carrier travels with the saturation velocity upsilon(s), in the base-collector space-charge region, tau\u27(CT) is conventionally modeled as 0.5 tau(CT), which equals W(c)/2-upsilon(s), where W(c) is the thickness of the collector depletion region. For an HBT in which velocity overshoot can occur in the collector layer, the constant velocity profile used conventionally is questionable. In addition to the constant velocity profile and the step-like velocity profile proposed more recently, the present study considers also a more realistic velocity profile consisting of a linear-dependent velocity in the velocity overshoot region and an average velocity in the rest of the collector space-charge region. This piecewise-linear velocity profile resembles closely that predicted by Monte-Carlo simulations reported in the literature. It is shown that tau\u27(CT) can be notably smaller than 0.5 tau-(CT), particularly when velocity overshoot is significant. Cutoff frequencies calculated from the present approach using the piecewise-linear velocity profile are in better agreement with Monte-Carlo simulation than that calculated from the constant and step-like velocity profiles