Active area limitation of Ge/GaAs heterojunctions by means of B ion implantation

We have investigated the limitation of the active area of p-n Ge/GaAs heterojunctions by means of B ion implantation into the interface of Ge/GaAs structure. Germanium epilayers were grown on (100) GaAs substrates by molecular beam epitaxy at a substrate temperature above 450 °C with a growth rate of about 10 nm/min. The Ge/GaAs heterojunction diode, fabricated with p-n Ge/GaAs structure which was grown at the substrate temperature of 500 °C, represented a forward current voltage shoulder of 0.44 V and a breakdown voltage of 44 V. The ideality factor and the backward step recovery time of this diode was 1.09 and about 10 ns, respectively, similar to a Schottky barrier diode with same active area. In order to use the n-GaAs substrate as an emitter of a heterobipolar transistor, it is necessary to limit the current flow into the same area of the collector through the base region. To realize this idea, B ions were implanted through the Ge epilayer, and the high-resistivity region was formed in the GaAs substrate at Ge/GaAs interface. The forward current of the B ion implanted junction was suppressed 4–5 orders of magnitude in comparison to that of a non B implanted junction.