In this study, the role of nitrogen in the dc-sputtered amorphous indium gallium zinc oxide (a-IGZO):N are explored extensively with a series of nitrogen gas flow rates during IGZO film deposition. The amorphous film structure and the evolution of chemical bondings were confirmed by X-ray diffractometer and X-ray photoelectron spectroscopy spectra analysis. Also, electrical performance and reliability of a-IGZO:N thin-film transistors (TFTs) formed with different nitrogen gas flow rates were analyzed to study the effects of nitrogen on TFT devices. The device performance of a-IGZO:N TFTs can be enhanced with a proper nitrogen doping concentration. However, with excess nitrogen incorporation in the a-IGZO:N channel layer, both electric characteristic and reliability are degraded due to the extra creation of oxygen deficiencies in a-IGZO:N film and easy formation of unstable interface between gate insulator and channel layer, which were confirmed by low-frequency noise measurement. This potential issue of a-IGZO:N TFT characteristics can be effectively released by introducing a post-treatment on the surface of gate dielectric layer. The optimized electrical characteristics of a-IGZO:N TFT can exhibit a carrier mobility of 19.21 cm 2 /V⋅s, subthreshold swing of 0. 26 V/decade and threshold voltage (V th of -0.74 V in this study.