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N-type Schottky-gated Si:SiGe heterostructure field-effect transistors with physical gate lengths between 70 and 450nm are characterized over a wide temperature range (T=10 K...300 K) for low electric fields. The room-temperature maximum low-field transconductance increases 61% to 440 mS/mm at T=10 K for the 70-nm device. The minimum subthreshold slope is 14...19 mV/dec at T=10 K. The off-state currents IOFF are limited by parallel conduction at high temperatures and by the gate leakage current at low temperatures. Substrate leakage currents are found to be due to generation of carriers within the drain/substrate depletion layer and only make a minor contribution to IOFF. Operation of the devices at the lowest temperature is found to result in the occurrence of the floating-body kink effect, as a consequence of substrate freeze-out and subsequent self-biasing by impact ionization currents. Low temperature characteristics exhibit a nonlinear low-field drain current dependence on the drain voltage, due to the presence of parasitic Schottky source/drain contacts. An extraction method for access resistance consistent with this phenomenon is presented.
Date of Publication: Sept. 2005