The study of thin film transistors (TFTs) and MIS structures using a variety of amorphous silicon nitride thin films (a-SiNx:H) as the gate dielectric is presented. These nitride films were deposited with N/Si ratios between 0.64 and 1.74, in order to evaluate the dependence of the TFT performance near the stoichiometric a-SiNX:H composition (1.33). These films were produced in a magnetically confined PECVD system. A set of samples was prepared for capacitance measurements (epsilon(r) determination), FTIR measurements (Si-H and N-H bonds relative abundance), Rutherford backscattering spectrometry and elastic recoil detection measurements (elemental concentration and density calculation). TFT results show a close relationship between the field effect mobility (mu(FE)) and the hydrogen concentration in the nitride films. Also, nitride films with the highest densities lead to superior electrical performance. These films are over-stoichiometric (N-rich), showing that, unlike CVD thermal nitride, the best a-SiNX:H are not necessarily stoichiometric. TFTs with N-poor a-SiNX:H films present a high degree of I(V) hysteresis due to charge trapping on the a-Si:H/a-SiNX:H interface. This effect is also confirmed by C(V) measurements performed on MIS structures.