We report a flexible carbon nanotube ͑CNT͒ thin-film transistor ͑TFT͒ fabricated solely by ink-jet printing technology. The TFT is top gate configured, consisting of source and drain electrodes, a carrier transport layer based on an ultrapure, high-density ͑Ͼ1000 CNTs/ m 2 ͒ CNT thin film, an ion-gel gate dielectric layer, and a poly͑3,4-ethylenedioxythiophene͒ top gate electrode. All the TFT elements are ink-jet printed at room temperature on a polyimide substrate without involving any photolithography patterning or surface pretreatment steps. This CNT-TFT exhibits a high operating frequency of over 5 GHz and an on-off ratio of over 100. Such an all-ink-jet-printed process eliminates the need for lithography, vacuum processing, and metallization procedures and thus provides a promising technology for low-cost, high-throughput fabrication of large-area high-speed flexible electronic circuits on virtually any desired flexible substrate.Printing thin-film transistors ͑TFTs͒ on flexible substrates at room temperature offers a cost-effective way to achieve mass production of large-area electronic circuits without using special lithography equipment. It is expected to provide an enabling technology for many emerging applications such as flexible displays, radio frequency identification ͑RFID͒ tags, electronic papers, and smart skins, just to name a few. Printed flexible electronics have been reported by using various organic semiconducting polymers. 1-3 However, the carrier mobility of organic semiconducting polymers is still less than 1.5 cm 2 / V s, 1-3 which limits the device operation speed to only a few kilohertz. Carbon nanotube ͑CNT͒, a material with exceptional aspect ratio and great mechanical flexibility, has shown great promises as an active carrier transport material in making high-speed flexible field-effect transistors ͑FETs͒. 4-12 Extraordinary field-effect mobility as high as 79 000 cm 2 / V s was reported in the FETs based on individual CNTs. 5 Due to the ultrahigh field-effect mobility, CNT-based flexible FETs are capable of achieving high-speed ͑gigahertz͒ operation. [13][14][15] However, most of the reported FETs were based on CNTs grown using chemical vapor deposition ͑CVD͒, 16,17 which generally requires an extremely high temperature, typically Ͼ900°C. 5,16,17 This represents a major obstacle to fabricating electronic devices on flexible substrates because most flexible substrates are unable to survive such a high CVD growth temperature. FETs based on solution-processable CNT thin films 6-12 can be fabricated at room temperature and are thus especially suitable for printed electronics on flexible substrates. However, the sidewalls of as-produced nanotubes are covered by amorphous carbon ͑␣-C͒, which is a very common carbonaceous impurity. 18 Such impurities would tremendously restrict the transport of carriers in the formed CNT thin films and seriously limit the field-effect mobility of the CNT-TFTs. 18,19 High field-effect mobility CNT-TFTs can be achieved by using ultrapure electronics-grade CN...
