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A new and interesting InGaP/AlxGa1-xAs/GaAs composite-emitter heterojunction bipolar transistor (CEHBT) is fabricated and studied. Based on the insertion of a compositionally linear graded AlxGa1-xAs layer, a near-continuous conduction band structure between the InGaP emitter and the GaAs base is developed. Simulation results reveal that a potential spike at the emitter/base heterointerface is completely eliminated. Experimental results show that the CEHBT exhibits good dc performances with dc current gain of 280 and greater than unity at collector current densities of JC=21kA/cm2 and 2.70×10-5 A/cm2, respectively. A small collector/emitter offset voltage ΔVCE of 80 meV is also obtained. The studied CEHBT exhibits transistor action under an extremely low collector current density (2.7×10-5 A/cm2) and useful current gains over nine decades of magnitude of collector current density. In microwave characteristics, the unity current gain cutoff frequency fT=43.2GHz and the maximum oscillation frequency fmax=35.1GHz are achieved for a 3×20 μm2 device. Consequently, the studied device shows promise for low supply voltage and low-power circuit applications.