Electron transport in AIGaAs/GaAs heterojunction bipolar transistors with compositionally abrupt emitter-base junctions is examined. Transport across the abrupt emitter-base heterojunction is treated quantum mechanically, and the Monte Carlo technique is used to study transport through the base. Although there is a sizeable population of upper-valley electrons in the bulk emitter, the AlGaAs/GaAs heterojunction is found to favor the injection of r -valley electrons into the base. This valley

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... tering effect enhances device performance by reducing base transit time, but quantum mechanical tunneling lowers the average energy of the injected flux which increases base transit time. The design of a heterojunction bipolar transistor for minimum base transit time involves a careful tradeoff between these competing factors. We examine the influence of varying aluminum fraction and bias on base transit time. The results suggest that a moderately doped emitter with high aluminum mole fraction produces the shortest base transit time.