a thin film transistor includes a gate electrode , a gate insulation layer on the gate electrode , source and drain electrodes formed on the gate insulation layer , a polysilicon channel layer overlapping the ohmic contact layers and on the gate insulation layer between the source and drain electrodes , ohmic contact regions over the source and drain electrodes for contacting the polysilicon channel to the source and drain electrodes , and doping layers over the source and drain electrodes .