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Double-heterojunction bipolar transistors have been fabricated on InGaAs(P)/InP with current gains of up to 200. Transistors with a p+-InGaAs/N-InP base/collector junction exhibited drastic gain reduction at low collector bias voltages which is ascribed to the electron repelling effect of the conduction-band spike formed at the collector heterojunction. To overcome this complication a thin n-InGaAs transition layer was inserted between the ternary base and the InP wide-gap collector. The resulting nN double-layer collector structure leads to excellent current/voltage characteristics.

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Alumina- and silica- based spin-on sources for p+-doping of InGaAs contact layers are compared. Shallow diffusion profiles with hole concentrations up to 2×1020/cm3 were obtained. Higher doping levels and improved long-term stability of the spin-on solution were achieved with the alumina-based source.