In this work metal/oxide/semiconductor (MOS) capacitors with different nitrogen content SiO xN y gate dielectric are fabricated and characterized. The dielectric films are deposited by the plasma enhanced chemical vapor deposition technique from N 2, N 2O and SiH 4 gaseous mixtures at low temperatures. The MOS capacitors were characterized by low and high frequency capacitance ( C– V) measurements, from where the interface state density ( D it), the effective charge density ( N eff) and the dielectric constant ( k) were extracted. The results show a dielectric constant varying linearly in function of the films nitrogen concentration, from a value of 3.9, corresponding to SiO 2 to 7.2, corresponding to Si 3N 4. We observed a variation of D it in function of the films nitrogen concentration, the smallest obtained value corresponding to the Si 3N 4 film (∼1 × 10 11 cm −2 eV −1), however this film presents higher leakage current density than others. In order to optimize both parameters a double dielectric layer is proposed, a first layer of Si 3N 4 film, which presents the highest dielectric constant and best interface properties, and a second layer of SiO x N y with high nitrogen concentration, in order to maintain the equivalent dielectric constant high but minimizing the leakage current problems.