Embodiments of a semiconductor device capable of increasing an aperture ratio of an organic electroluminescence display device by decreasing the surface area of a capacitor in the organic electroluminescence display device and a method of manufacturing the semiconductor device are disclosed. By forming a gate insulating film of a gate electrode with a thickness different from that of a dielectric film of a capacitor, the surface area of the capacitor can be decreased without variation in capacitance, thereby increasing the aperture ratio of an organic electroluminescence display device.