A thin film transistor (TFT) includes a semiconductor active layer, a gate electrode, a source electrode, and a drain electrode. The semiconductor active layer includes a first doped region as a source region, a second doped region as a drain region, an undoped region between the first and second doped regions. A third doped region is disposed between the second doped region and the undoped region. The gate electrode is insulated from the semiconductor active layer and overlaps the third doped region and the undoped region. The source electrode and the drain electrode are connected to the first and second doped regions.