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; and a gate electrode insulated from the source electrode, the drain electrode, and the semiconducting layer by an insulating layer, wherein the semiconducting layer comprises at least two stacked carbon nanotube films, each of the carbon nanotube films comprises a plurality of carbon nanotubes, at least a part of the carbon nanotubes of each of the carbon nanotue films are aligned along a direction from the source electrode to the drain electrode.