High-Frequency Noise in Germanium-Silicon n-n Heterojunctions

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Abstract
Germanium‐silicon heterojunctions have been fabricated by placing bulk n‐type germanium on a germanium film that has been vacuum deposited on an n‐type epitaxial silicon substrate. The germanium is alloyed to the silicon in a hydrogen atmosphere with the resulting heterojunctions showing either a current saturation when forward and reverse biased or for reverse bias only, depending upon the resistivity of the bulk n‐type germanium. The forward‐biased conductance and capacitance is measured over a range of frequencies from 1 to 50 MHz. The forward‐biased conductance and capacitance of the n‐n heterojunction have variations with bias current and with frequency similar to those of a Schottky barrier device. The excess noise temperature of the device is measured over a range of bias current at 30 MHz. The theoretical excess noise temperature of the n‐n heterojunction is modeled with a mean‐square noise current generator in parallel with the junction conductance and capacitance; a series resistance is added to complete the model. Comparison between the theoretical and measured excess noise temperatures indicates that the junction shows nearly full shot noise at high current levels. At low current levels traps at the interface cause the junction to show less than full shot noise. In the case of the double saturating diode, a minimum excess noise temperature is observed at a current level slightly less than the saturation current. Beyond the saturating current, the onset of breakdown is evidenced by a large increase in the excess noise temperature.

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