A thin film transistor substrate is provided including a first thin film transistor and a second thin film transistor. The first thin film transistor comprises a first active layer, a first gate insulating film, and a first gate electrode. The second thin film transistor comprises a second active layer formed, a second gate insulating film, and a second gate electrode. A thickness of the second gate insulating film is larger than a thickness of the first gate insulating film, the second active layer has at least two impurity doping regions which overlap the second gate electrode, the first active layer has at least two impurity doping regions formed in a self-aligning manner with respect to the first gate electrode, and the second gate electrode comprises a semiconductor layer.