Impurity bands in inversion layers

Impurity bands have been created and observed in inversion layers on silicon by drifting Na+ ions close to the SiSiO2 interface. The resulting hydrogen-like states can be made dense enough so that a peak in the conductivity can be observed at gate fields below the threshold for conduction in the conduction band. Three modes of conduction were observed: activation to the mobility edge, nearest neighbor hopping, and variable range hopping. From these measurements and by varying the density of ions information about binding energy, impurity band width, and the extent of the bound wave functions as a function of the electron density, ion concentration and substrate bias may be inferred. © 1978.