In this letter, we investigated Ti (20 nm)/Al (30 nm)/Au (60 nm) metallic contacts to Al-doped ZnO thin films prepared by the sol–gel method. In order to measure specific contact resistance, circular transfer length method structures were patterned on the samples by a lithography process. The samples were annealed at the different temperatures and electrical characterization ( $I$ – $V$ ) was performed. The annealing process was optimized to produce ohmic contacts with the lowest specific contact resistance. The results showed that the sample annealed at 350 °C had the lowest specific contact resistance of $\sim 3.7\times 10^{-7}~\Omega \cdot \textrm {cm}^{2}$ . Surface and cross section energy dispersive X-ray analysis was used to determine the distribution of different elements during the annealing process. The results indicated effective inter-diffusion of Al and Ti as well as out-diffusion of oxygen, resulting in an increase in the n-type carrier concentration close to the surface of the ZnO. Scanning electron microscopy and atomic force microscopy analysis were used to determine the effect of annealing on the surface morphology.