The thermal stability of amorphous InGaZnO thin film transistors (a-IGZO TFTs) with various ambient gases was investigated. The a-IGZO TFTs in air were more thermally stable than the devices in the ambient argon. Oxygen, rather than nitrogen and moisture, was responsible for this improvement. Furthermore, the thermal stability of the a-IGZO TFTs improved with the increasing oxygen content in the surrounding atmosphere. The related physical mechanism was examined, indicating that the higher ambient oxygen content induced more combinations of the oxygen vacancies and adsorbed oxygen ions in the a-IGZO, which resulted in the larger defect formation energy. This larger defect formation energy led to the smaller variation in the threshold voltage for the corresponding TFT devices. (C) 2016 Elsevier Ltd. All rights reserved.