Consequences of valley filtering on abrupt junction AlGaAs/GaAs heterojunction bipolar transistors

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Abstract
Electron transport in AlGaAs/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 Γ-valley electrons into the base. This valley filtering 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.