A substrate having a thin film transistor includes a buffer layer on a substrate, source and drain electrodes on the buffer layer, a portion of the buffer layer exposed between the source and drain electrodes, a small organic semiconductor layer on the source electrode and the drain electrode, the organic semiconductor layer contacting the exposed portion of the buffer layer, a gate insulating layer on the organic semiconductor layer, the gate insulating layer having substantially the same size as the organic semiconductor layer, a gate electrode on the gate insulating layer, a passivation layer over the surface of the substrate including the gate electrode; and a pixel electrode on the passivation layer, the pixel electrode electrically connected to the drain electrode.