We generated solution-processed thin film transistor (TFTs) using gallium tin zinc oxide (GTZO, Ga-Sn-Zn-O) layers as the channel that exhibit improved bias-stress stability during device operation under ambient conditions. The cause of the bias-stress stability was investigated through comparisons with zinc tin oxide (ZTO, Zn-Sn-O)-based TFTs, which suffer red from bias stress instability. Based on in-depth analysis of the electrical characteristics and chemical structure of both GTZO and ZTO layers, it was discovered that the GTZO layers had a significantly lower oxygen vacancy concentration than did the ZTO layer, which influenced the electrical performance of the GTZO transistors as well as their bias-stress stability. When 5 mol% gallium was added, a bias stress-stable transistor was obtained, exhibiting typical semiconductor behavior with a field-effect mobility of 1.2 cm(2) V-1 s(-1) on/off ratio of 10(6), off-current of 1 x 10(-10) A, and threshold voltage of 19.6 V. Further doping of Ga deteriorated the device performance, which was found to be associated with decreased carrier concentration and segregation of an insulating secondary phase. (C) 2011 Elsevier B.V. All rights reserved.