A thin film transistor and a method of fabricating the same include: a semiconductor layer having a grain boundary disposed in a crystal growth direction and having a variation in height of a top surface of 15 nm or less formed by a thin beam directional crystallization method. Also, an organic light emitting diode (OLED) display device comprising the thin film transistor is provided and has excellent characteristics fabricated by a simple process. Also, a flat panel display device and a method of fabricating the same are provided and include: a polycrystalline silicon layer in a pixel region; and a polycrystalline silicon layer in a peripheral region formed by the thin beam directional crystallization method. Also, a semiconductor device and a method of fabricating the same include: an intrinsic region of a semiconductor layer in the photodiode region formed by the thin beam directional crystallization method.