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The authors describe a novel method for fabricating collector-up AlGaAs/GaAs heterojunction bipolar transistors. An AlAs layer is inserted into the emitter layer and is oxidised in water vapour at 450°C. The resulting AlAs-oxide serves as a current confining layer that constricts collector current flow to the intrinsic portion of the device. DC devices with typical current gains of 50 are observed.
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Massengale et al. (1996) studied this question.
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