In this brief, small-signal characterizations of selectively implanted collector (SIC) and non-SIC SiGe-heterojunction bipolar transistors (HBT) fT-doublers up to 120 GHz are measured, analyzed, and compared with those of the corresponding single devices. The measured results confirm a great improvement in transit frequency with almost no considerable decrease in the maximum stable gain/maximum available gain, and maximum oscillation frequency. Since the fT-doubler can be used easily instead of a single transistor, a much higher transit frequency and RF power can be achieved while using a technology with a moderate fT.

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