The wet etch process for amorphous indium gallium zinc oxide (a-IGZO or a-InGaZnO) by using various etchants is reported. The etch rates of a-IGZO, compared to another indium-based oxides including indium gallium oxide (IGO), indium zinc oxide (IZO), and indium tin oxide (ITO), are measured by using acetic acid, citric acid, hydrochloric acid, perchloric acid, and aqua ammonia as etchants, respectively. In our experimental results, the etch rate of the transparent oxide semiconductor (TOS) films by using acid solutions ranked accordingly from high to low are IZO, IGZO, IGO and ITO. Comparatively, the etch rate of the TOS films by using alkaline ammonia solution ranked from high to low are IGZO, IZO, IGO and ITO, in that order. Using the proposed wet etching process with high etch selectivity, bottom-gate-type thin-film transistors (TFTs) based on a-IGZO channels and Y(2)O(3) gate-insulators were fabricated by radio-frequency sputtering on plastic substrates. The wet etch processed TFT with 30 mu m gate length and 120 mu m gate width exhibits a saturation mobility of 46.25 cm(2) V(-1) s(-1), a threshold voltage of 1.3 V, a drain current on-off ratio >10(6), and subthreshold gate voltage swing of 0.29 V decade(-1). The performance of the TFTs ensures the applicability of the wet etching process for IGZO to electronic devices on organic polymer substrates. (C) 2009 Elsevier B.V. All rights reserved.