Carbon doping of SiGe heterobipolar transistors

We demonstrate cutoff and maximum oscillation frequencies of more than 70 GHz, and delays per stage down to 15 ps for ring oscillators with integrated SiGe:C heterobipolar transistors (HBT). We also demonstrate almost ideal base current characteristics for these, even with carbon in the depleted regions. The incorporation of low carbon concentration (<10/sup 20/ cm/sup -3/) within the SiGe region of SiGe HBTs can significantly suppress boron outdiffusion caused by a variety of subsequent processing steps. For example, transient enhanced boron diffusion due to BF/sub 2/ implantation of the external base regions can be eliminated. Thus, the use of epitaxial SiGe:C instead of SiGe layers provides greater flexibility in process design, and wider latitude in process margin.