Polar gating was the first mechanism used to explain the conductivity at LaAlO 3 /SrTiO 3 interfaces. It postulates that the LaAlO 3, which is polar in the 001 direction (with alternating sheets of positive and negative charge), acts as an electrostatic gate on the semiconducting SrTiO 3. When the LaAlO 3 layer grows thicker than three unit cells, its valence band energy rises above the Fermi level, causing holes (or positively charged oxygen vacancies ) to form on the outer surface of the LaAlO 3. The positive charge on the surface of the LaAlO 3 attracts negative charge to nearby available states. In the case of the LaAlO 3 /SrTiO 3 interface, this means electrons accumulate in the surface of the SrTiO 3, in the Ti d bands.