Incorporation of C in the SiGe base of a heterojunction bipolar transistor (HBT) prevents outdiffusion of B across the junctions, improves film stability by reducing strain, and leads to better process integration. This paper investigates theoretically how the values of figure of merit such as current gain (beta), cut-off frequency (f(T)) related inversely to the base transit time (tau (b)), and the Early voltage (V-A) change with a small amount (< 1%) of C incorporation in the base. For this purpose, we have derived generalized expressions for these quantities using a box-type, trapezoidal and triangular material (Ge and C) profile and an exponential doping profile. The variation of saturation-drift velocity with Ge mole fraction has been used to calculate Tb. A comparison between the calculated values, keeping the strain the same in both SiGe and SiGeC bases, indicates that all values of figure of merit are enhanced with C incorporation. The value of VA increases in SiGeC HBTs with small amounts of C because of prevention of B outdiffusion across the collector-base junction and sharp fall off of the C profile at the same junction.