STABILITY OF CARBON AND BERYLLIUM-DOPED BASE GAAS/ALGAAS HETEROJUNCTION BIPOLAR-TRANSISTORS

GaAs/AlGaAs heterojunction bipolar transitors (HBTs) utilizing highly Be-doped base layers display a rapid degradation of dc current gain and junction ideality factors during bias application at elevated temperature. For example, the gain of a 2 X 10-mu-m2 device with a 4 X 10(19) cm-3 Be-doped base layer operated at 200-degrees-C with a collector current of 2.5 X 10(4) A cm-2 falls from 16 to 1.5 within 2 h. Both the base emitter and base collector junction ideality factors also rise rapidly during device operation, and this current-induced degradation is consistent with recombination-enhanced diffusion of Be interstitials producing graded junctions. By sharp contrast, devices with highly C-doped (p = 7 X 10(19) cm-3) base layers operated under the same conditions show no measurable degradation over much longer periods (12 h). This high degree of stability is most likely a result of the fact that C occupies the As sublattice, rather than the Ga sublattice as in the case of Be, and also has a higher solubility than Be. The effect of nearby implant isolated regions in promoting Be diffusion is also reported.