We systematically investigated the role of the SiOx and/or SiONx passivation layer in the amorphous hafnium indium zinc oxide (HIZO) thin film transistors (TFTs) under a negative bias temperature illumination stress (NBTIS) condition. The device instability of the TFTs with a SiOx passivation layer [threshold voltage shift (Delta V-th) similar to -6.5 V] is better than that of the TFTs with a SiONx passivation layer (Delta V-th similar to -8.5 V) in the atmosphere. However, the devices with the SiOx passivation layer showed different instabilities in the atmosphere (-6.5 V) and N-2 ambient (-5.5 V). The film analysis demonstrated the higher water permeability of the SiOx film and higher hydrogen content of the SiONx films, suggesting the existence of not only water related positive charge traps but also hydrogen related positive charge traps under NBTIS conditions. After including the SiOx (inner)/SiONx (outer) passivation layers, the instability of the amorphous HIZO device was drastically improved by the suppression of the positive charge trapping sites under the NBTIS conditions. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3519987] All rights reserved.