A thin film transistor includes a source electrode, a drain electrode, a semiconducting layer, and a gate electrode. The drain electrode is spaced from the source electrode. The semiconducting layer is connected to the source electrode and the drain electrode. The gate electrode is insulated from the source electrode, the drain electrode, and the semiconducting layer by an insulating layer. The semiconducting layer comprises at least two stacked carbon nanotube films, and each carbon nanotube film comprises a plurality of carbon nanotubes primarily oriented along a same direction, and the carbon nanotubes in at least two adjacent carbon nanotube films are aligned along different directions.