Nanoscale device technology is driving intense study of thin dielectric layers on semiconductors. The aggressive scaling of Si CMOS technology calls for identifying high κ dielectrics to replace SiO2 and oxynitrides in gate related applications. The material, requirements for the alternative gate dielectric are very challenging in order to achieve performance comparable to SiO2. Furthermore, there are demanding issues for process integration compatibility. Among several binary oxides proposed the rare earth oxides are attractive candidates based on thermodynamic energy considerations and a high conduction band offset over 2eV. The interest in the rare earth oxide stems from our earlier work on GaAs passivation. The Ga2-xGdxO3 mixed oxides (κ =12) and the Gd2O3 oxides (κ =14) films grown by ultrahigh vacuum deposition from an oxide source formed an excellent insulating barrier with low interfacial state density Dit on the GaAs surface. This discovery has led to the first GaAs based inversion channel MOSFET devices. These dielectrics were also successfully applied to other III-V semiconductors including InGaAs, AlGaAs, InP, and GaN producing MOS diodes and MOSFETs.