We investigated the effect of adsorption of specific molecules on undoped zinc tin oxide (ZTO) and zirconium (Zr)-doped zinc tin oxide (ZZTO) thin-film transistors (TFTs) using a solution process. The specific chemisorbed species on the back channel region produced unstable ZTO TFT electrical properties under a bias stress test. However, the amount of specific chemisorbed ZTO TFT species could be controlled by Zr doping. As a result, the threshold voltage shift of the ZZTO TFT was dramatically improved comparing the ZTO TFT under positive bias stress. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.019202esl] All rights reserved.