Operation characteristics of amorphous In-Ga-Zn-O thin-film transistors (TFTs) having very thin channels were studied. TFTs thinner than 10 nm exhibit large mobilities 6-10 cm(2) (Vs)(-1), but that of a 5 nm thick TFT became 1 order of magnitude smaller. Threshold voltage increases with decreasing the channel thickness at < 20 nm. It was confirmed that continuous films were formed at film thickness' >= 1.3 nm, excluding the possibility that discontinuous film structures caused the deterioration. It was also found that shallow traps decreased but doping efficiency became worse for thinner films. These are explained by surface deep traps. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3555070] All rights reserved.