In this study, Al-doped zinc oxide (AZO) thin films with different Al concentrations fabricated by atomic layer deposition are investigated to determine the Al doping effect for AZO/ZnO double-stacked active layer thin-film transistor (TFT) applications. The AZO films are analyzed by X-ray diffraction, photoluminescence, and X-ray photoelectron spectroscopy, which show that the Al dopants affect the crystallinity, including the crystal direction and grain size, and reduce the deep trap sites such as oxygen vacancies (V0). The optimized Al doping concentration is about 2%. TFTs with an AZO (2%)/ZnO double-stacked active layer are fabricated and shown to exhibit a lower threshold voltage (V-th), subthreshold slope, and Vth shift under a positive gate-bias stress compared to ZnO single-layer devices. In the case of the on-current, however, the AZO stacked devices exhibit a smaller value. These electrical characteristics can be explained by Vo suppression and altered crystal properties due to Al doping. For the field-effect mobility, the temperature dependence also reveals that the main transport mechanisms are thermionic and thermal field emission over the grain boundary in the AZO stacked devices. These results indicate that the AZO film properties depend strongly on the Al concentration and hence the ZnO-based devices can be optimized for specific application by Al doping. (C) 2017 Elsevier B.V. All rights reserved.