We investigated the thickness dependence of a room-temperature grown MgO0.3BST0.7 composite gate dielectric and an InGaZnO4 active semiconductor on the electrical characteristics of thin-film transistors (TFTs) fabricated on a polyethylene terephthalate substrate. The optimum gate dielectric and active semiconductor thickness were 300 and 30 nm, respectively. The TFT showed a high field-effect mobility of 21.34 cm(2)/V s, an on/off ratio of 8.27 x 10(6), a threshold voltage of 2.2 V, and a subthreshold swing of 0.42 V/dec. (C) 2008 The Electrochemical Society. [DOI: 10.1149/1.2978961] All rights reserved.