1. A method for making a thin film transistor, the method comprising of: (a) providing a plurality of carbon nanotubes and an insulating substrate; (b) flocculating the carbon nanotubes to acquire a carbon nanotube structure; (c) applying the carbon nanotube structure on the insulating substrate; (d) forming a source electrode, a drain electrode, and a gate electrode; and (e) covering the carbon nanotube structure with an insulating layer, wherein the source electrode and the drain electrode are connected to the carbon nanotube structure; and the gate electrode is electrically insulated from the carbon nanotube structure by the insulating layer.