High reliable bottom gate amorphous gallium indium zinc oxide (a-GIZO) thin-film transistors (TFTs) have been fabricated by using the double active layers. Top and bottom layers were CuGaInZnO (CGIZO) and GIZO. respectively. When the plasma-enhanced processes were introduced during fabrication of the TFTs, the TFT with a-GIZO single active layer did not exhibit electrically reliable performance due to forming the conducting surface layer front the plasma damages. The double-active-layer TFT with a CGIZO layer had the reliable performance (mu(FE) of 5.1 cm(2)/V s, V-th of 3.25 V, subthreshold gate swim, value of 0.68 V/decade. I-off of 3.8 X 10-(12) A) even under the same processes. This suggested that the Cu atom of CGIZO layer suppressed the carrier concentration during plasma-enhanced processes. The TFTs with double active layer showed excellent stability. which has the threshold voltage shift of < 0.6 V at 3 mu A drain current under 60 degrees C for 100 h. (c) 2009 The Electrochemical Society. [DOI: 10.1149/1.3060129] All rights reserved.