The development of silicon-germanium alloys to extend the range of silicon high-frequency circuits has highlighted the need to understand diffusion mechanisms in this important material. To optimise performance, it is necessary to minimise the diffusion of dopants such as boron, in these very narrow width devices. This paper discusses recent progress in understanding the role of carbon doping in retarding boron diffusion. Much progress in understanding the diffusion mechanisms in silicon has been gained using selective defect injection, building on the discovery in 1972 that broadening of marker layers could monitor the injection of defects. The technique has now been used successfully in silicon-germanium alloys, and has shown that interstitial type defects are responsible for boron diffusion both in SiGe and in SiGe:C. The effects of carbon in retarding boron diffusion in as-grown structures, as well as ion-implanted structures, are discussed. (c) 2006 Springer Science + Business Media, Inc.