We review some important material properties of Si1-yCy and Si1-x-yGexCy layers grown pseudomorphically on Si(001). This new material might overcome some of the limitations of strained Si1-xGey and open new fields for device applications of heteroepitaxial Si-based systems. In addition, we demonstrated incorporating low carbon concentrations (<10(20) cm(-3)) into the SiGe region of a heterobipolar transistor (HBT) can significantly suppress boron outdiffusion caused by later processing steps. The static characteristics demonstrate that the transistors should be suitable for circuit applications. Comparing the high-frequency performance of MBE-grown SiGe:C HBTs with identically SiGe HBTs we found an increase in f(T) and f(max) by a factor of more than 2 for our chosen SiGe profile. This indicates that adding carbon enabled one to use implantation steps without affecting the boron profile, that is, it offers wider latitude in process margins.