In this work metal/oxide/semiconductor (MOS) capacitors with different nitrogen content SiO x N y gate dielectric are fabricated and characterized. The dielectric films are deposited by the plasma enhanced chemical vapor deposition technique from N 2 , N 2 O 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 3 N 4 .We observed a variation of D it in function of the films nitrogen concentration, the smallest obtained value corresponding to the Si 3 N 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 3 N 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.