The de and microwave characteristics of In-0.3(Al0.5Ga0.5)(0.7)As/In0.3Ga0.7As and In0.29Al0.71As/In0.3Ga0.7As heterojunction bipolar transistors (HBTs) grown on GaAs are investigated. A step-graded InXGa1-XAs buffer is employed to effectively suppress the threading dislocations resulting from the lattice mismatch between In0.3Ga0.7As and GaAs. These devices exhibit a high collector-emitter breakdown voltage (BVCEO > 11 V) and a collector offset voltage as small as 66 mV, demonstrating the excellent quality of the base-emitter and base-collector junctions. The typical common-emitter current gain at a collector density of 16 kA/cm(2) is 25 for the In-0.3(Al0.5Ga0.5)(0.7)As/In0.3Ga0.7As HBTs with a base doping concentration of 1x10(19) cm(-3). The Ft and Fmax for the non-self-aligned device with an emitter size of 4x4 mu m(2) is 33 GHz and 20.5 GHz, respectively.
