An organic light-emitting transistor (OLET) is provided. The OLET includes: a substrate; at least one first electrode on the substrate; a first semiconductor layer having a first conductive type on the first electrode; a second semiconductor layer having a second conductive type on the first semiconductor layer; a gate electrode disposed on a side surface of the second semiconductor layer; a gate insulating layer disposed between the gate electrode and the second semiconductor layer; an organic emission layer on the second semiconductor layer.