Abstract
This letter reports a temperature-dependent limit for the subthreshold swing in MOSFETs that deviates from the Boltzmann limit at deep-cryogenic temperatures. Below a critical temperature, the derived limit saturates to a value that is independent of temperature and proportional to the characteristic decay of a band tail. The proposed expression tends to the Boltzmann limit when the decay of the band tail tends to zero. Since the saturation is universally observed in different types of MOSFETs (regardless of dimension or semiconductor material), this suggests that an intrinsic mechanism is responsible for the band tail.
Highlights
INTRODUCTIONThe measurements in FETs at deep-cryogenic temperatures reach merely ≈ 11 instead of 0.8 mV/dec at 4.2 K [1]–[6], ≈ 9 mV/dec instead of 20 μV/dec at 100 mK [7], and ≈ 7 mV/dec instead of 4 μV/dec at 20 mK [8]
T HE Boltzmann limit of the subthreshold swing in FETs, SS = ln 10, predicts at room temperature the well-known ≈ 60 mV/dec, and at deep-cryogenic temperatures (
The additional power that the FET consumes at deep-cryogenic temperatures due to the excess SS is a crucial metric for the realization of Manuscript received December 2, 2019; revised December 23, 2019; accepted December 26, 2019
Summary
The measurements in FETs at deep-cryogenic temperatures reach merely ≈ 11 instead of 0.8 mV/dec at 4.2 K [1]–[6], ≈ 9 mV/dec instead of 20 μV/dec at 100 mK [7], and ≈ 7 mV/dec instead of 4 μV/dec at 20 mK [8]. Relying on numerical simulations Bohuslavskyi et al demonstrated that an exponential band tail and FermiDirac statistics leads to saturation of SS at deep-cryogenic temperatures [32], [33]. Since SS(T ) is fairly independent of technology, this suggests that the extent of the band tail is fairly independent of technology, which points in the direction of an intrinsic mechanism being responsible for blurring the band edges (e.g., electronphonon scattering, electron-electron and electron-hole interactions, finite crystalline periodicity, etc.) and to a lesser extent extrinsic mechanisms (impurities, disorder, defects, etc.) [34]
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