  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 extent of a
band tail. Since the saturation is universally observed in different types of
MOSFETs (regardless of dimension or semiconductor material), the band tail is
attributed to the finite periodicity of the lattice in a semiconductor volume,
and to a lesser extent to additional lattice perturbations such as defects or
disorder.
