This letter investigates carbon doping in AlGaAs using flow-rate modulation epitaxy (FME). The tendency of hole concentration increasing with Al composition is explained by the thermal decomposition of trimethylaluminum molecules on the substrate surface. The highest hole concentration obtained is 3×1020 cm-3 in Al0.4Ga0.6As layers. An AlGaAs/GaAs heterojunction bipolar transistor with an FME-grown compositionally graded carbon-doped base layer was fabricated for the first time. It exhibits a current gain of 25 with 2×1019 cm -3 base doping.
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