1. A thin film transistor comprising: a source electrode; a drain electrode spaced from the source electrode; a semiconductor layer comprising a carbon nanotube structure, the carbon nanotube structure comprises of carbon nanotubes; and a gate electrode insulated from the source electrode, the drain electrode, and the semiconducting layer by an insulating layer, wherein the carbon nanotube structure is connected to both the source electrode and the drain electrode, an angle exist between each carbon nanotube of the carbon nanotube structure and a surface of the semiconductor layer, and the angle ranges from about 0 degrees to about 15 degrees.