The optical and carrier transport properties of amorphous transparent zinc indium tin oxide (ZITO)(a-ZITO) thin films and the characteristics of the thin-film transistors (TFTs) were examined as a function of chemical composition. The as-deposited films were very conductive and showed clear free carrier absorption (FCA). The analysis of the FCA gave the effective mass value of 0.53 m(e) and a momentum relaxation time of 3.9 fs for an a-ZITO film with Zn:In:Sn=0.35:0.35:0.3. TFTs with the as-deposited channels did not show current modulation due to the high carrier density in the channels. Thermal annealing at 300 degrees C decreased the carrier density and TFTs fabricated with the annealed channels operated with positive threshold voltages (V-T) when Zn contents were 25 atom % or larger. V-T shifted to larger negative values, and subthreshold voltage swing increased with decreasing the Zn content, while large on-off current ratios 10(7)-10(8) were kept for all the Zn contents. The field effect mobilities ranged from 12.4 to 3.4 cm(2) V-1 s(-1) for the TFTs with Zn contents varying from 5 to 48 atom %. The role of Zn content is also discussed in relation to the carrier transport properties and amorphous structures. (c) 2008 The Electrochemical Society.