Self-aligned structured oxide thin-film transistors (TFTs) are appropriate candidates for use in the backplanes of high-end displays. Although SiNx is an appropriate candidate for use in the gate insulators (GIs) of high-performance driving TFTs, direct deposition of SiNx on top of high-mobility oxide semiconductors is impossible due to significant hydrogen (H) incorporation. In this study, we used AlOx deposited by thermal atomic layer deposition (T-ALD) as the first GI, as it has good H barrier characteristics. During the T-ALD, however, a small amount of H from H2O can also be incorporated into the adjacent active layer. In here, we performed O-2 or N2O plasma treatment just prior to the T-ALD process to control the carrier density, and utilized H to passivate the defects rather than generate free carriers. While the TFT fabricated without plasma treatment exhibited conductive characteristics, both O-2 and N2O plasma-treated TFTs exhibited good transfer characteristics, with a V-th of 2 V and high mobility (similar to 30 cm(2) V-1 s(-1)). Although the TFT with a plasma-enhanced atomic layer deposited (PE-ALD) GI exhibited reasonable on/off characteristics, even without any plasma treatment, it exhibited poor stability. In contrast, the O-2 plasma-treated TFT with T-ALD GI exhibited outstanding stability, i.e., a V-th shift of 0.23 V under positive-bias temperature stress for 10 ks and a current decay of 1.2% under current stress for 3 ks. Therefore, the T-ALD process for GI deposition can be adopted to yield high-mobility, high-stability top-gate-structured oxide TFTs under O-2 or N2O plasma treatment.