A new InP/InGaAs double heterojunction bipolar transistor with a step-graded InAlGaAs collector structure

An interesting InP/InGaAs double heterojunction bipolar transistor with a step-graded InAlGaAs layer at the base-collector (B-C) heterojunction is fabricated and studied. Simulated results reveal that the potential spike at the B-C heterointerface is completely eliminated. Experimentally, the operation regime is wider than 11 decades in magnitude of the collector current (I-C = 10(-12) A to I-C = 10(-1) A). Furthermore, the studied device exhibits a relatively high common-emitter breakdown voltage and low output conductance even at high temperature. In the microwave characteristics, the unity current gain cutoff frequency f(T) = 72.7 GHz and the maximum oscillation frequency f(max) = 50 GHz are achieved for a nonoptimized device (A(E) = 6 x 6 mu m(2)).