interface state properties and tunnel transport properties of ultrathin insulators formed on Si surfaces at low temperatures (LT) were characterized by XPS, contactless UHV C-V and I-V methods. Electron cyclotron resonance (ECR)-assisted N2O plasma process realized the formation of good interface with low interface state density. On the other hand, strong Fermi level pinning was observed at the interfaces formed by chemical oxidation and LT thermal oxidation, due to high density of interface states. In these interfaces, there was a large discrepancy between the measured tunnel I-V curves and the calculated ones using the direct tunnel theory. This discrepancy was explained in terms of the large band bending due to the high density of interface states, which causes depletion of electrons at the interface.