The effect of vacuum annealing on the amorphous oxide semiconductor Cd-Ga-O system with a tunable band gap was examined. While the amorphous halo peak in XRD patterns of the films with Cd concentration of similar to 70% started sharpening after annealing at a temperature >= 200 degrees C. In contrast, the films with Cd content of similar to 50% and similar to 20% were not crystallized for annealing temperatures up to 700 degrees C. Carrier concentrations and Hall mobilities of similar to 70% and similar to 50% films were maximized by annealing at 400 degrees C and 500 degrees C, respectively, regardless of the properties of the as-deposited films, which were varied because of the unintended variation of deposition conditions such as the residual water vapor pressure in the deposition chamber. The initial electrical conductivity of similar to 20% films widely varied from <10(-8) to 11 S.cm(-1), and annealing at 500 degrees C was required for obtaining conductive films from the insulating as-deposited films. The maximum Hall mobility for films with Cd content of similar to 70% and similar to 50% films was 10 cm(2) V-1 s(-1) and that for the film with-20% Cd content was >= 3 cm(2) V-1. (C) 2017 Elsevier B.V. All rights reserved.