A thin film transistor includes a gate electrode on a semiconductor layer, a first insulation layer between the semiconductor layer and the gate electrode, a second insulation layer on the gate electrode, and a source and drain electrode on the semiconductor layer. The gate electrode includes a first part and a second part adjacent to the first part. A width of the second part is greater than a width of the first part. The source electrode and the drain electrode are on the semiconductor layer and arranged such that the first part of the gate electrode is between the source electrode and the drain electrode. The source electrode and the drain electrode are electrically connected to the semiconductor layer through the first insulation layer and the second insulation layer, respectively. A space between the source electrode and the drain electrode is greater than the width of the first part.