The change of electrical performance of amorphous SiZnSnO thin film transistors (a-SZTO TFTs) has been investigated depending on various metal capping layers on the channel layer by causing different contact property. It was confirmed that the change of electrical characteristics was sensitively dependent on the change of the capping layer materials on the same channel layer between the source/drain electrodes. This sensitive change in the electrical characteristics is mainly due to different work function of metal capping layer on the channel layer. The work function of each capping layer material has been analyzed and derived by using Kelvin probe force microscopy and compared with the energy bandgap of the SZTO layer. When the work function of the capping layer is larger than that of the channel layer, electrons are depleted from the channel layer to the capping layer. On the contrary, in the case of using a material having a work function smaller than that of the channel layer, the electrical characteristics were improved because electrons were injected into the channel layer. Based on depletion and injection mechanism caused by different contact barrier between metal capping layer and channel layer, NOT, NAND, and NOR logic circuits have been implemented simply by changing metal capping layer on the channel layer.