Dielectric behavior of SiNx films, fabricated by microwave electron cyclotron resonance discharge, has been studied as a function of film thickness on the basis of the current–voltage and the capacitance–voltage characteristics. In the thickness range (20 nm≪d≪80 nm), the resistivity and the critical field for SiNx were found not to be sensitive to the film thickness (d) and which was opposite to strong dependence of the dynamic dielectric constant Єd on thickness. To explain the Єd behavior as a function of d, a model based on trapped space charge effects is proposed. The dominant mode of electronic conduction, determined from J–E1/2 curves and Arrhenius plots of leakage current, appears to be Poole–Frenkel emission only for thicker films (d≫20 nm). Finally, the spatial profile of fixed charges reveals that SiNx/Si interface has a much greater concentration of defects than the bulk film. © 1999 American Vacuum Society.