We show that the incorporation of low carbon concentration(<1020cm−3)within the SiGe region of SiGe heterobipolar transistors (HBT) can significantly suppress boron outdiffusion caused by later processing steps. We were able to obtainfT/fmax⩾50 GHz for a simple SiGe:C transistor with a box-shaped Ge profile. Comparing the high frequency performance of molecular beam epitaxy grown SiGe:C HBTs with identical SiGe HBTs, we found an increase infTandfmaxby a factor of more than 2. The static characteristics for SiGe:C HBTs demonstrate that the transistors should be suitable for circuit applications. Process margins for SiGe HBT technology are shown to be relaxed due to C incorporation. The dramatic reduction of B diffusion in C-rich Si under conditions of point defect equilibrium is attributed to a reduction of the concentration of interstitials available for the B diffusion.