1. A thin film transistor comprising: a source electrode; a drain electrode spaced from the source electrode; a semiconducting layer connected to the source electrode and the drain electrode; an insulating layer; and a gate electrode insulated from the source electrode, the drain electrode, and the semiconducting layer by the insulating layer; wherein the semiconducting layer comprises a carbon nanotube film, and the carbon nanotube film comprises a plurality of carbon nanotubes primarily oriented along a same direction and joined end to end by van der Waals attractive force, and the plurality of carbon nanotubes are oriented along a direction from the source electrode to the drain electrode.