In this paper, the defects of Al doped ZnO films generated by magnetron sputtering in the deposition processes are comprehensively investigated. It is found that oxygen ion bombardment deteriorates the crystallinity and generates oxygen related defects, such as oxygen interstitials (O-i), chemisorbed oxygen at the grain boundaries (O-GB), and oxygen vacancies (V-O). O-i and OGB decrease the carrier concentration and mobility of the pristine films remarkably, but they can be removed by hydrogen annealing. However, the grain boundary scattering originated from poor crystallinity cannot be reduced by the annealing below 450 degrees C. Moreover, the in-situ temperature-dependent resistivity measurement under hydrogen atmosphere suggests that hydrogen atoms are incorporated into the ZnO: Al films and interact with V-O. We propose that there are two energetically favorable states for the incorporated hydrogen. The defect configurations of (H-2)(i) and H-O +H-i are assigned to the high resistivity state (HRS) and low resistivity state (LRS) respectively and the switching between these two states is activated by V-O and mediated by a metastable state (H-2)*(0). The transformation between these two resistivity states leads to a hysteresis loop during the heating and cooling process. (C) 2015 Elsevier B.V. All rights reserved.