In this study, the role of many-body interactions, Pauli blocking, and impurity-carrier interactions on the band gap variation were used to analyze the observed blue shift of the Al doped ZnO thin films. The optical constants and the band gap variations of our samples were obtained using spectroscopic ellipsometry method. Tauc-Lorentz model and effective medium approximation were used to model the dielectric function. We demonstrated for the first time that the combination of the Tauc formalism for absorption and the proposed model by Jain et al. for the band gap variation, can be used to determine both electron and hole effective masses. Reasonable results were obtained for carriers effective mass and their dependence on carrier concentration. (C) 2016 Elsevier B.V. All rights reserved.