GaAs/AlGaAs heterojunction bipolar transistors with a base doping 10[sup 20] cm[sup −3] grown by solid-source molecular beam epitaxy using CBr[sub 4]

We show that epitaxial layers suitable for fabrication of AlGaAs/GaAs heterojunction bipolar transistors with a carbon base doping level of up to 10(20) cm(-3) can be grown by solid-source molecular beam epitaxy using CBr4 as 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 width Wb Of these devices deviates from the 1/W-b(2) dependence predicted by diffusive electron transport for base widths that are shorter than 60 nm. The relationship is better approximated by a 1/W-b dependence, 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. (C) 1998 American Vacuum Society.