Electro luminescence from MOS structures is investigated. Broadband visible light emission is observed from Au/SiO2/n-type Si MOS tunnel junctions. Their spectra show the same peak position for both polarities of the bias voltage but the detailed shape is different due to the involvement of different surface plasmon modes. A peak at around 1.9eV appears in the EL spectrum, and the emission cuts off at around 3.0eV. We will show the light emission characteristics of Au/SiO2/Si MOS tunnel junctions, including stability, uniformity and breakdown voltage, that are all better than those of the common Au/A12O3/A1 MIM junctions. We attribute these improvements to the usage of the electronically and structurally stable materials Si and SiO2 in the MOS junctions. After considering different possible emission mechanisms, we conclude that the visible light emission mechanism is that the surface plasmon polaritons that are excited by the tunneling current radiate light by scattering from residual surface roughness. This emission mechanism is well known in MIM light emitting tunnel junctions, but rarely applied to the case of MOS junctions.