This study examined the effects of the Zn ratio on the microstructure of InGaZnO (IGZO) films and the device performance of their transistors. IGZO films with various Zn ratios were produced by co-sputtering InGaO and ZnO at different ZnO powers. The combination of transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses elucidated that as the Zn ratio increased from 0 to 0.69, the microstructure changed from amorphous IGZO to nanocrystalline IGZO and then to columnar ZnO. The dynamic transitions of the microstructure, in turn, profoundly affected the electrical properties including the mobility and carrier concentration. The sample with a mixing ratio of 0.25:0.2:0.55 (In:Ga:Zn) exhibited the best performance with a mobility of nearly 30 cm(2)/V.s. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3615534] All rights reserved.