N2O plasma treatment suppressed the temperature-dependent sub-threshold leakage current of amorphous indium-gallium-zinc-oxide thin film transistors (a-IGZO TFTs). For untreated devices, the transfer curve exhibited abnormal electrical properties at high temperature. The abnormal electrical properties are explained by the energy band diagrams for both forward and reverse sweep. Above 400K, holes can be generated from trap-assisted transition, and drift to the source side which induces source barrier lowering. The source side barrier lowering enhances electron injection from the source to channel and causes an apparent sub-threshold leakage current. This phenomenon, which is experimentally verified, only appears in the device without N2O plasma treatment, but not in the device with N2O plasma treatment. The results suggested that the density of states for a-IGZO with N2O plasma treatment is much lower than that without plasma treatment.