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 includes a carbon nanotube film, a plurality of carbon nanotubes in the carbon nanotube film oriented along a direction from the source electrode to the drain electrode.