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Sputtered zinc oxide (ZnO) without intentional doping was thermally annealed and the dependence of its resistivity on different sample configurations and heat-treatment conditions was studied. The ZnO was either exposed to the ambience or sealed with an impermeable cover during the annealing. The resistivity resulting from the sealed configuration was found to be lower. The possible origins of the charge carriers responsible for the conductivity were investigated, with the most plausible one studied in greater detail using photoluminescence. The leakage current in the OFF-state of a field-effect thin-film transistor is largely controlled by the residual conductivity of the channel region of the transistor. For a ZnO transistor, this region is typically undoped and subjected to a series of thermal processes under a variety of coverage configurations during the course of the fabrication of the transistor. The implied correlation between the aggregate thermal treatment and the characteristics of a transistor in its OFF-state was investigated and demonstrated.