We investigated the effect of Mg addition on solution-processed Zn-Sn-O (ZTO) thin-film transistors (TFTs). Because Mg affects the optical bandgap and the metal-oxygen bond in ZTO films, the carrier concentration of Mg-Zn-Sn-O (MZTO) films was suppressed by Mg. As the molar ratio of Mg increased in the MZTO TFTs annealed at 500 degrees C, the on/off ratio increased, and the sub-threshold gate swing (S.S) decreased considerably. As a result, the MZTO TFT showed a saturation mobility of 1.00 cm(2)/V s, an S. S of 0.92 V/decade, a threshold voltage (V-th) of 6.60 V, and an on/off ratio of 3.15x10(6). (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.012203esl] All rights reserved.