Metal capping (MC) layer for Si-Zn-Sn-O thin film of the back channel layer of thin film transistors is proposed to protect from ambient effect and to improve electrical properties. Field effect mobility is improved from 34.46 cm(2)/V s to 147.59 cm(2)/V s and excellent stability of V-th similar to 0.6 V is obtained. The floating MC-layer forms a strongly compact-shape current-path as a result of the effective control of the surface potential by the low-resistance of MC-layer. In addition, the proposed device structure effectively prevents the adsorption/desorption reaction of ambient oxygen and water molecules on the surface. (C) 2015 Elsevier B.V. All rights reserved.