The effect of ammonia gas on amorphous indium gallium zinc oxide thin film transistors is investigated. The ammonia is incorporated into the sputtered a-IGZO film during the deposition process. The results indicate that the sub-threshold swing of the NH3 incorporated TFTs is significantly improved from 2.8 to 1.0 V/decade, and the hysteresis phenomenon is also suppressed during the forward and reverse sweeping measurement. By X-ray photoelectron spectroscopy analyses. Zn-N and O-H bonds are observed in ammonia incorporated a-IGZO film. Therefore, the improvements in the electrical performance of TFTs are attributed to the passivation of dangling bonds and/or defects by ammonia. (C) 2011 Elsevier Ltd All rights reserved.