An thin film transistor includes an insulating substrate, an MgO layer, a semiconductor carbon nanotube layer, a functional dielectric layer, a source electrode, a drain electrode, and a gate electrode. The semiconductor carbon nanotube layer is sandwiched between the MgO layer and the functional dielectric layer. The source electrode and the drain electrode electrically connect the semiconductor carbon nanotube layer. The gate electrode is sandwiched between the insulating substrate and the MgO layer.