Effects of plasma treatments on the back-channel of amorphous Ga2O3-In2O3-ZnO (GIZO) thin film transistors (TFTs) are compared for N-2 and N2O plasma. Acceptor-like states originating from the oxygen adsorbed on the back-channel of the GIZO TFTs suppress the back-channel current by capturing the electrons in the GIZO active layer and thus shift the threshold voltage to the positive direction. It is also shown that the oxygen in a silicon oxide passivation layer reduces the back-channel current. An enhancement-mode GIZO TFT has been successfully fabricated by combining the N2O plasma treatment and the silicon oxide passivation layer.