Semiconducting Characteristics of Magnesium-Doped Al2O3 Single Crystals
  DC and AC electrical measurements were performed to investigate the electrical conductivity of alpha-Al2O3:Mg samples with different concentrations of [Mg]^0 centers .The concentration of [Mg]^0 centers was monitored by the opticalabsorption peak at 2.56 eV. At low electrical fields, DC measurements reveal blocking contacts. Steady electroluminescence is emitted at the negative electrodeindicating that the majority of carriers are holes. Low voltage AC measurements show that the equivalent circuit for the sample is the bulk resistance in series with the junction capacitance connected in parallel with a capacitance, which represents the dielectric constant of the sample. The values determined for the bulk resistance in both DC and AC experiments are in good agreement. The electrical conductivity of Al2O3:Mg crystals increases linearly with the concentration of [Mg]^0 centers, regardless of the amount of other impurities also present in the crystals, and is four times higher in the $c_{\perpendicular}$ than in the $c_{\parallel}$ direction. The conductivity is thermally activated with an activation energy of 0.68 eV, which is independent of: 1) the [Mg]^0 content, 2) the crystallographic orientation, and 3) the concentration of other impurities. These results favor the {\it small-polaron-motion} mechanism. 
M. Tardio, R. Ramirez, R. Gonzalez, Y.Chen, M.R.Kokta