In this study, we attempt to find the origin of atypical doping concentration dependant electrical properties of group III element (Al, Ga, In) doped ZnO thin films. The carrier density and mobility of these films are measured and compared for a wide range of doping concentrations. It is found that donor density determines the electrical properties of the films for a low level of doping, whereas lattice defects play a dominant role beyond a certain level of doping concentration. We also demonstrate an interrelationship between the electrical properties and the structural/optical properties of the films. We find 2% as optimum doping concentration for Al-, Ga- or In-doped ZnO for obtaining highest conducting layers and Ga emerges as the most effective dopant among all. (C) 2016 Elsevier B.V. All rights reserved.