Activation mechanisms in sodium-doped Silicon MOSFETs
  We have studied the temperature dependence of the conductivity of a silicon MOSFET containing sodium ions in the oxide above 20 K. We find the impurity band resulting from the presence of charges at the silicon-oxide interface is split into a lower and an upper band. We have observed activation of electrons from the upper band to the conduction band edge as well as from the lower to the upper band. A possible explanation implying the presence of Hubbard bands is given. 
T. Ferrus, R. George, C. H. W. Barnes, N. Lumpkin, D. J. Paul and M.   Pepper