EFFECTS OF A COMPOSITIONALLY-GRADED INXGA1-XAS BASE IN ABRUPT-EMITTER INP/INGAAS HETEROJUNCTION BIPOLAR-TRANSISTORS

A compositionally-graded InxGa1-xAs base is experimentally shown to improve electron transport properties in abrupt-emitter InP/InGaAs heterojunction bipolar transistors (HBTs). The built-infield in the base of 6 kV/cm enables a more than 50% improvement in current gains, compared to a uniform-base structure. The peak current-gain cutoff frequency f(T) for the graded-base HBT is 143 GHz versus 121 GHz for the uniform-base HBT. It is also shown that the graded-base structure is effective in suppressing the space charge in the vicinity of the base-collector junction. The built-in field in the base accelerates low-speed energy-relaxed electrons and thereby increases the velocity of electrons injected into the collector. The minimized base widening effect, combined with low base resistance, yields a maximum oscillation frequency f(max) over 2 00 GHz even at a collector bias voltage V-CE as low as 1 V.