Collector-up SiGe heterojunction bipolar transistors

SiGe heterojunction bipolar transistors (HBTs) in a collector-up version have been built. This configuration has the advantage of a very low collector-base capacitance. In addition, because the substrate is the emitter, extremely low emitter series inductances may be achieved, e.g., in the case of packaged discrete devices. This is particularly interesting for RF power amplifiers. Transistors fabricated on MBE-grown layers with an emitter size of 1.6 /spl mu/m showed near-ideal Gummel plots, DC current gains of up to 485, an S/sub 21/ gain of 20 dB at 2 GHz and an f/sub max/ of 33 GHz. A problem to be solved is the large parasitic C/sub BE/ capacitance which degrades the transit frequency.