Abstract: A thin film transistor (TFT) and an organic light emitting diode (OLED) display device. The TFT and the OLED display device include a substrate, a buffer layer disposed on the substrate, a semiconductor layer disposed on the buffer layer, a gate electrode insulated from the semiconductor layer, a gate insulating layer insulating the semiconductor layer from the gate electrode, and source and drain electrodes insulated from the gate electrode and partially connected to the semiconductor layer, wherein the semiconductor layer is formed from a polycrystalline silicon layer crystallized by a metal catalyst and the metal catalyst is removed by gettering using an etchant. In addition, the OLED display device includes an insulating layer disposed on the entire surface of the substrate, a first electrode disposed on the insulating layer and electrically connected to one of the source and drain electrodes, an organic layer, and a second electrode.