A thin film transistor (TFT) includes a gate, a drain, a source, an insulating layer, a metal oxide layer, and an etch stopper layer. The metal oxide layer includes a source area, a drain area, and a channel area. The source is electrically coupled to the source area and the drain is electrically coupled to the drain area. Oxygen ions are implanted into the channel area via a surface treatment process to make an oxygen concentration of the channel area be greater than an oxygen concentration of each of the source area and the drain area.