Background impurities (carbon and oxygen) incorporation and the n-type doping of AlGaAs by gas-source molecular beam epitaxy (GSMBE) using triethylaluminum (TEAl) and trimethylamine alane as aluminum sources have been extensively studied; triethylgallium as the gallium source; cracked arsine as the arsenic source; and uncracked disilane as an n-type dopant source. A carbon-doped base (p = 4 × 10 19 cm −3 , 92.5-nm thick, trimethylgallium as the carbon source) GaAs/AlGaAs heterojunction bipolar transistor was grown (TEAl as the aluminum source). The dc current gain of 45 was obtained at a current density of 4 × 10 4 A/cm 2 (4 × 5 μm 2 emitter). Device characteristics under current stress were found to be stable