Effects of hydroxyl group (OH) in the gate insulator (GI) of AlOx formed by atomic layer deposition on the negative bias illumination stress (NBIS) stability of amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) by changing the deposition temperature (T-dep) of the AlOx GI are studied. There are no significant differences in the electrical properties and stabilities of each device, such as field effect mobility (mu(FE)), subthreshold swing (S.S.), turn on voltage (V-on), positive bias temperature stress (PBTS), and negative bias temperature stress (NBTS). Meanwhile, the NBIS stability is improved, resulting in V-on shift from -4.34 V to -2.48 V as the T-dep of the GI increased. This suggests that as the amount of OH increases in GI, more hole/and or ionized V-o trapping sites are formed in the bulk GI. Based on the energy level in photoluminescence (PL) spectra of ALD AlOx, it is suggested OH related-bulk trapping sites in ALD AlOx as the origin of non-bridging oxygen hole center (NBOHC) in AlOx GI.