A thin film transistor (TFT) and a flat panel display device including the same. The TFT includes a substrate, a gate electrode formed over the substrate, the gate electrode formed with silicon doped with impurities, a gate wiring connected to the gate electrode, an active layer formed over the gate electrode, and source and drain electrodes connected to the active layer. According to such a structure, since heat flow to the gate electrode during crystallization of the active layer may be prevented, stable crystallization of the active layer may be performed, and thus an error rate of a product may be decreased.