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 SiO x thin film prepared by plasma enhanced chemical vapor deposition. As-deposited SiO x films were annealed at high temperature to induce the separation of the Si and SiO 2 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 SiO x 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 lm, 0.78 lm and 0.98 lm. The EL properties of the devices have been optimized through the study of the structural properties of their active layers by means of the energy filtered transmission electron microscopy (EFTEM) technique. EFTEM has evidenced the presence of a relevant contribution of amorphous nanostructures, and has given a very reliable quantitative estimation of the size and density of the Si nanostructures, as well as of the concentration of Si atoms remaining dissolved in the matrix. In the case of REdoped SiO x films, the role of amorphous nanostructures in sensitizing the RE luminescence has been clearly demonstrated. These results open the way to new interesting applications in optoelectronics, such as electrically driven optical amplifiers or light sources.