Effects of a Compositionally-Graded InxGa1-xAs Base in Abrupt-Emitter InP/InGaAs Heterojunction Bipolar Transistors

A compositionally-graded In x Ga1- x As base is experimentally shown to improve electron transport properties in abrupt-emitter InP/InGaAs heterojunction bipolar transistors (HBTs). The built-in field 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 200 GHz even at a collector bias voltage V CE as low as 1 V.