Expanding applications for microelectronics in large-area sensor arrays, disposable sensor tapes, timeϪtemperature smart labels, radio frequency identification tags, and roll-up displays 1Ϫ4 motivate efforts to integrate electronics onto flexible plastic, paper, or metal substrates. A principal strategy for achieving flexible electronics is to employ graphic arts methods such as flexographic or ink-jet printing to pattern metallic, semiconducting, and insulating inks onto foils and paper. 5Ϫ10 Liquid phase printing offers the potential for high-throughput roll-toroll or sheet-to-sheet processing of electronics on large-area substrates, facilitating applications where large areas are necessary (e.g., displays) and also potentially translating into low production cost. Yet the challenge for printed electronics is to achieve high-performance circuits. The inherently low carrier mobilities of many printable organic or nanoparticle-based semiconductors lead to reduced transistor switching frequencies and high circuit supply voltages. Alternative strategies in which silicon chips are bonded to flexible substrates (by transfer printing or pick-andplace methods) are also attractive because they benefit from the superior electronic properties of silicon and the very advanced state of silicon microelectronics technology.11 In a competitive environment, the success of liquid phase printed electronics depends on substantial performance improvements, in particular, the development of faster, lower power printed circuits. Figure 1a displays a summary of reported signal delay times versus supply voltages for ring oscillator circuits based on organic semiconductors and carbon nanotube (CNT) arrays. It is evident that for nonprinted organic ring oscillators (open blue symbols) signal delays of 1Ϫ10 s have been achieved but only for supply voltages of 10Ϫ100 V, 12Ϫ24 while for supply voltages in the range of 4Ϫ10 V, the delay is above 10 s for the fastest circuits, with most displaying Ͼ1 ms switching times.25Ϫ28 Reports of printed ring oscillators are less common (solid green symbols in Figure 1a), and these circuits have generally required tens of volts to achieve switching times on the order of 1 ms.29Ϫ32 Such large voltages are not practical for many potential applications of flexible electronics where power will be supplied by thin-film batteries or radio frequency fields. Very recently, unipolar, p-type electrolyte-gated ring oscillator circuits have been demonstrated that indeed operate at very low

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...

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