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 μm showed near-ideal Gummel plots, DC current gains of up to 485, an S₂₁ gain of 20 dB at 2 GHz and an f_max of 33 GHz. A problem to be solved is the large parasitic C_BE capacitance which degrades the transit frequency