We compare the performance of heterojunction bipolar transistors with pure SiGe (SiGe HBTs) with those incorporating C-doped SiGe base layers (SiGe:C HBTs). The transistors were produced in a single-polysilicon technology with implanted, epi-free wells. Doping the SiGe layers with low C concentration (similar to 0.2%) allows us to use a higher base boron dose than for C-free HBTs, without B outdiffusion from the heteroepitaxial layer. As a result, the device RF performance can be significantly improved. The higher base doping of SiGe:C HBTs increases the peakf(max) from around 50 up to more than 80 GHz, and reduces the minimum noise figure (at 10 GHz) from >3 to 2 dB and ring oscillator delays from 21-22 to 12-14 ps. The SiGe:C HBTs also exhibit leakage currents, which are sufficiently low for reliable IC application.