The off current of amorphous silicon (a-Si: H) thin film transistors (TFTs) depends on fabrication after back-channel etching. Plasma treatment after back-channel etching affects off current in the lower subthreshold voltage region. Off current is high in O₂ plasma treatment and low in N₂ plasma treatment. Without plasma treatment, off current can not be controlled. N₂ plasma treatment affects the interface characteristics between a-Si: H layer and passivation layer, obstructing electron transport near the back-channel inerface.