Doping and electrical characteristics of in-situ heavily B-doped Si 1− x− y Ge x C y (0.22< x<0.6, 0< y<0.02) films epitaxially grown on Si(100) were investigated. The epitaxial growth was carried out at 550°C in a SiH 4–GeH 4–CH 3SiH 3–B 2H 6–H 2 gas mixture using an ultraclean hot-wall low-pressure chemical vapor deposition (LPCVD) system. It was found that the deposition rate increased with increasing GeH 4 partial pressure, and only at high GeH 4 partial pressure did it decrease with increasing B 2H 6 as well as CH 3SiH 3 partial pressures. With the B 2H 6 addition, the Ge and C fractions scarcely changed and the B concentration ( C B) increased proportionally. The C fraction increased proportionally with increasing CH 3SiH 3 partial pressures. These results can be explained by the modified Langmuir-type adsorption and reaction scheme. In B-doped Si 1− x− y Ge x C y with y=0.0054 or below, the carrier concentration was nearly equal to C B up to approximately 2×10 20 cm −3 and was saturated at approximately 5×10 20 cm −3, regardless of the Ge fraction. The B-doped Si 1− x− y Ge x C y with high Ge and C fractions contained some electrically inactive B even at the lower C B region. Resistivity measurements show that the existence of C in the film enhances alloy scattering. The discrepancy between the observed lattice constant and the calculated value at the higher Ge and C fraction suggests that the B and C atoms exist at the interstitial site more preferentially.