We have investigated the temperature dependencies of charge carrier densities and Hall mobilities in tensile strained Si1-yCy and in compressively strained Si1-x-yGexCy layers. In both cases, the measured charge carrier densities at room temperature are not affected substantially by the addition of a small concentration of carbon (≪1%) under identical growth conditions and dopant fluxes. The measured Hall mobilities monotonically decrease with increasing carbon content for electrons in Si1-yCy, and for holes in Si1-x-yGexCy, respectively. Our results indicate that electrically active defects are formed with the addition of carbon. These defects are presumably connected with carbon/Si interstitials or other C-related complexes. It seems to be difficult to attribute the formation of those electrically active defects solely to contaminations originating from the used carbon evaporation source. We observed that donor- and acceptor-like defects are formed in Si1-yCy as well as in Si1-x-yGexCy layers with roughly a constant ratio, independent of source temperature. © 1997 American Institute of Physics.