In this work, the influence of copper on amorphous type Indium-Gallium-Zinc-Oxide (a-IGZO) thin-film transistor's (TFTs) transfer curve is studied. The I-D-V-G curves of a-IGZO TFTs with source/drain in the structures of Cu/Ti and Ti/Al/Ti are compared. The results show that the copper greatly deteriorates the performance of the TFTs. The presence of the copper in the channel region of the device is verified by SIMS analysis. A Cu-dipping experiment is conducted by dipping devices into the solution of CuSO4 to confirm the role of copper in the deterioration of the ID-VG curves. The hypothesis is also verified through ATLAS device simulator. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.025206jes]