The present invention is directed toward field effect transistors (FETs) and thin film transistors (TFTs) comprising carbon nanotubes (CNTs) and to methods of making such devices using solution-based processing techniques, wherein the CNTs within such devices have been fractionated so as to be concentrated in semiconducting CNTs. Additionally, the relatively low-temperature solution-based processing achievable with the methods of the present invention permit the use of plastics in the fabricated devices.