We have investigated the limitation of the active area ofphyphen;nGe/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 450thinsp;deg;C with a growth rate of about 10 nm/min. The Ge/GaAs heterojunction diode, fabricated withphyphen;nGe/GaAs structure which was grown at the substrate temperature of 500thinsp;deg;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 thenhyphen;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 highhyphen;resistivity region was formed in the GaAs substrate at Ge/GaAs interface. The forward current of the B ion implanted junction was suppressed 4ndash;5 orders of magnitude in comparison to that of a non B implanted junction.
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