Evidence for multiple impurity bands in sodium-doped silicon MOSFETs
  We report measurements of the temperature-dependent conductivity in a silicon metal-oxide-semiconductor field-effect transistor that contains sodium impurities in the oxide layer. We explain the variation of conductivity in terms of Coulomb interactions that are partially screened by the proximity of the metal gate. The study of the conductivity exponential prefactor and the localization length as a function of gate voltage have allowed us to determine the electronic density of states and has provided arguments for the presence of two distinct bands and a soft gap at low temperature. 
T. Ferrus, R. George, C. H. W. Barnes, N. Lumpkin, D. J. Paul and M.   Pepper