Transit time effect in high-frequency characteristics of HBTs

Multiple negative resistance bands are predicted at frequencies beyond f/sub max/ in the conventional mesa-type emitter-up AlGaAs-GaAs HBTs (heterojunction bipolar transistors) with careful design and reduced parasitics. Thus, HBT power gain may be obtained in these frequency bands with the negative resistance even above f/sub max/. To obtain the usable transit time effect with large negative resistance, the increase of collector transit time is preferable to increasing base transit time. Therefore, for transit time HBTs, the base should be designed to be as thin as possible, and the collector should be thick enough to obtain the large transit time in the frequency range of interest, while keeping parasitics small.<>