A thin film transistor (TFT) may include a substrate, a gate electrode on the substrate, a gate insulating layer on the gate electrode, and a semiconductor layer on the gate insulating layer. The semiconductor layer may include a top surface, a channel area aligned in a vertical direction with the gate electrode, a plurality of doped areas proximate to the channel area, and a plurality of non-doped areas. Source and drain electrodes may be on the top surface of the semiconductor layer aligned above respective ones of the plurality of non-doped areas of the semiconductor layer. A planarization layer may be on the gate insulating layer, the source and drain electrodes and the semiconductor layer channel area, and may include a plurality of openings respectively exposing the plurality of doped areas of the semiconductor layer and a portion of the source electrode and the drain electrode.