HfO2-based oxide is a promising gate insulator for the next generation of the field effect transistors. Yet it is accompanied by a problematic effect, that is, the Fermi level of a metal electrode deposited on HfO2 is pinned at a certain position regardless of the type of the metal. On the other hand, this effect, the so-called Fremi level pinning, does not occur in a metal/La2/O3/Si gate stack. This is puzzling because both oxides have similar band gaps and dielectric constants. We suggest that the high solubility of Si in La2O3, and thereby the formation of a silicate layer LaxSiyO z between La2O3 and the Si substrate plays important role in the suppression of electron transfers from La 2O3 to the metal electrode: The charge neutrality is satisfied at the ionic/ionic interface La2O3/La xSiyOz, and thus there is no need to exchange electrons with outside. At the ionic/covalent interface HfO2/SiO 2, however, the charge neutrality is insufficient and electrons are readily transferred into the metal electrode causing the Fermi level pinning.