We demonstrate that dopant diffusion coefficients in Si can be changed by more than one order of magnitude due to alloying with Ge and C, The observed suppression of boron diffusion in C-rich layers is explained on the basis of coupled diffusion of Si point defects and carbon. A model for B diffusion and segregation in SiGe heterostructures in presented. The consequences of the unique diffusion properties of SiGe:C alloys for heterojunction bipolar transistors are investigated by process and device simulation.