We have studied the electrical and optical properties of light emitting MOS devices based on Si nanostructures. In these devices the dielectric layer consists of a SiOx thin film prepared by plasma enhanced chemical vapor deposition. As-deposited SiOx films were annealed at high temperature to induce the separation of the Si and SiO2 phases with the formation of Si nanocrystals (nc) embedded in the insulating matrix. Devices based on Si nc present a strong light emission at room temperature at a wavelength of about 900 nm. Devices emitting at different wavelengths have been fabricated by implanting SiOx films with rare earth ions. Devices based on Er, Tm and Yb-doped Si nanoclusters exhibit an intense room temperature electroluminescence (EL) respectively at 1.54 μm, 0.78 μm and 0.98 μm. The EL properties of the devices have been optimized through the study of the structural properties of their active …