We show that epitaxial layers suitable for fabrication of AlGaAs/GaAs heterojunction bipolar transistors with a carbon base doping level of up to1020cm−3can be grown by solid-source molecular beam epitaxy usingCBr4as a doping precursor. We have observed that the gain of devices fabricated from these layers is improved using an abrupt conduction-band discontinuity at the emitter–base heterointerface. The observed relationship between current gain and the base widthWbof these devices deviates from the1/Wb2dependence predicted by diffusive electron transport for base widths that are shorter than 60 nm. The relationship is better approximated by a1/Wbdependence, which is in better agreement with the theories of ballistic or quasiballistic electron transport. Current gain of the devices drops rapidly as the base thickness exceeds 60 nm.