In this paper the growth and characterization of heavily carbon-doped GaAs for heterojunction bipolar transistors is reported. The GaAs:C layers were grown by MOVPE at 650 degrees C using carbon tetrabromide as dopant source. The lattice mismatch of carbon-doped GaAs epilayers was analyzed using symmetric planes of double crystal X-ray diffraction. Hall effect measurements were used to study the hole mobilities and hole concentrations. Double crystal X-ray diffraction measurements show that the lattice contraction of carbon-doped GaAs epilayers is less than 0.03% for the carbon doping levels used in the base region of HBTs. Hall effect measurements show that the hole mobilities of heavily carbon-doped GaAs are higher than those observed when zinc and beryllium are used as dopant, indicating less compensation in our samples.