Thin film transistors (TFTs) incorporating high mobility In-Ga-Zn-O (IGZO) semiconductors were fabricated and evaluated. The Ar:O-2 gas flow rates were varied during the sputter growth of IGZO layers, which had a substantial influence on the device performance and stability under negative bias stress (NBS) and negative bias illumination stress (NBIS). As the Ar gas flow rate is increased, the TFTs exhibit inferior performance and stability. Thin film analyses indicate that the relative content of oxygen deficient sites in IGZO increases with increasing Ar gas flow rate, which is accompanied with more pronounced device degradation under bias stress. Relatively high oxygen deficiency usually leads to the generation of excess free carriers in oxide semiconductors, which normally results in high field effect mobility. However a counter-intuitive decrease in mobility is observed in this work, which is suggested to originate from the overall decrease in IGZO film density as a result of elevated sputtering pressure at high Ar gas flow rates. It is conjectured that disruptions of conduction paths occur in low density IGZO layers, owing to the lack of overlap between metal 5s orbitals. (C) 2017 Elsevier B.V. All rights reserved.