A thin film transistor includes a substrate, a crystallized semiconductor layer formed over the substrate having a channel region, low-density impurity regions and high-density impurity regions, a gate insulating layer formed on the crystallized semiconductor layer, a first gate electrode formed on the gate insulating layer having a width corresponding to the channel region, a second gate electrode formed on the first gate electrode and on the gate insulating layer such that the second gate electrode overlaps the low-density impurity regions and a source electrode and a drain electrode respectively contacting the high-density impurity regions.