GAAS ALGAAS HETEROJUNCTION BIPOLAR-TRANSISTORS WITH A BASE DOPING 10(20) CM(-3) GROWN BY SOLID-SOURCE MOLECULAR-BEAM EPITAXY USING CBR4/

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 epitax y using CBr4 as a doping precursor. We have observed that the gain of devices fabricated from these layers is improved using an abrupt condu ction-band discontinuity at the emitter-base heterointerface. The obse rved relationship between current gain and the base width Wb Of these devices deviates from the 1/W-b(2) dependence predicted by diffusive e lectron transport for base widths that are shorter than 60 nm. The rel ationship is better approximated by a 1/W-b dependence, which is in be tter agreement with the theories of ballistic or quasiballistic electr on transport. Current gain of the devices drops rapidly as the base th ickness exceeds 60 nm. (C) 1998 American Vacuum Society.