1. A thin film transistor comprising: a source electrode; a drain electrode spaced from the source electrode; a semiconductor layer; a channel comprising a plurality of carbon nanotube wires, one end of each of the plurality of carbon nanotube wires is connected to the source electrode, and an opposite end of each of the plurality of carbon nanotube wires is connected to the drain electrode, each of the plurality of carbon nanotube wires comprises a plurality of successively oriented carbon nanotube segments joined end-to-end by van der Waals attractive force, each of the plurality of the successively oriented carbon nanotube segments comprises a plurality of carbon nanotubes in contact with each other; and a gate electrode insulated from the source electrode, the drain electrode, and the semiconducting layer by an insulating layer.