The conductivity of electrons in the inversion layer of silicon has been measured from 0 to 40 ${\mathrm{cm}}^{-1\mathrm{}}$ at 1.2°K in the metallic and localized regimes. The correlation between $\sigma \mathrm{}\left(T\right)\mathrm{}$ and $\sigma \left(\omega \right)$ in the localized regime suggests that the drop in conductivity at low electron concentrations is caused by the appearance of a gap at the Fermi level.

• Received 5 June 1975

DOI:https://doi.org/10.1103/PhysRevLett.35.1359