The NHa -plasma passivation has been performed on polycrystalline silicon (poly-Si) thin-film transistors (TFT's). It is found that the TFT's after the NHa-plasma passivation achieve better device performance, including the off-current below 0.1 pA/μm and the on/off current ratio higher than 108, and also better hot-carrier reliability than the H2 -plasma ones. Based on optical emission spectroscopy (OES) and secondary ion mass spectroscopy (SIMS) analysis, these improvements were attributed to not only the hydrogen passivation of the defect states, but also the nitrogen pile-up at SiO2 /poly-Si interface and the strong Si-N bond formation to terminate the dangling bonds at the grain boundaries of the polysilicon films. Furthermore, the gate-oxide leakage current significantly decreases and the oxide breakdown voltage slightly increases after applying NHa-plasma treatment. This novel process is of potential use for the fabrication of TFT/LCD's and TFT/SRAM's.