World's fastest operation speed demonstrated using printed CNT transistor - NEC & TASC aim to apply newly developed CNT transistor to sheet-type electronic devices -

NEC and the Technology Research Association for Single Wall Carbon Nanotubes (TASC) have succeeded in reducing noise and improving output current during high-speed operation of a printed carbon nanotube network's (CNT) thin film transistor (printed CNT-TFT). In addition, having realized an operation speed of 500kHz, this transistor-which enables low-cost production of large-area electronic circuits-can now be applied to control circuits in electronic devices.

Printed electronics, which use printing technology to form electronic circuits on virtually any object, are attracting widespread attention as a technology that not only simplifies and reduces the cost of production but also has the potential for use in the fabrication of electronic devices in the form of thin sheets or transformable electronic devices. To make electronic devices like these a reality, control circuits capable of performing arithmetic operations and switching are necessary, and the development of TFT capable of being produced using printing processes will be required.

The printed CNT-TFT being developed by NEC and TASC have come under the spotlight as components that take advantage of the conductivity and other features of CNT and are capable of low-power, high-speed operation. Although NEC had developed printed CNT-TFT in the past that boasted mobility (the ease with which charge flows) more than ten times greater than that of ordinary printed transistors not using CNT, the company found itself faced with the challenge of reducing noise and improving output current during operation at high speeds in order for these transistors to be applied to control circuits used in electronic devices.

In addition to suppressing noise emitted during high-speed operation to one-tenth or less than that of ordinary printed transistors, this newly developed printed CNT-TFT realizes an operating speed of 500kHz, which is 10 to 50 times that of ordinary printed transistors, thanks to an output current dozens of times higher than existing printed transistors. As a result, the level of performance necessary for control circuits can be attained, thereby making it possible to apply this printed CNT-TFT to new devices in the future, including flexible large displays and sheets mounted with multiple sensors.

By further enhancing this newly developed technology, printed CNT-TFT capable of operating at speeds exceeding several MHz are anticipated, and their application to ultra-slim wireless communications devices and paper-thin computers that take advantage of the features of printed electronics will be made possible.

The main features of this newly developed technology are as follows.

1. Reduces noise through the use of narrow electrodes in super ink jet (SIJ) technology
   Boasting superior static properties, expectations are high for the application of printed CNT-TFT to printed electronics. However, since the noise content of TFT up until now has had a tendency to be significantly higher than transistor output current due to overlapping between drain and gate electrodes, it has been difficult to achieve high-speed operation of several tens of kHz. NEC therefore set out to use SIJ technology, which enables ultra-fine printing, to reduce overlapping between electrodes to the maximum extent possible by forming ultrafine electrodes, thereby allowing the company to succeed in lowering noise content to one-tenth or less than that of ordinary printed transistors. This in turn made it possible to reduce the ratio of noise content to output current.



2. Makes transistors featuring high output current a reality through the use of highly purified CNT ink
   CNT purity when performing semiconductive-metallic separation, which entails removing only the semiconductive components of CNT made of semiconductors and metal, has been improved to 98% (conventional purity: 95%). A technique for removing surfactant contained in the solution used during separation has also been developed and used to reduce the amount of surfactant in CNT ink to one-fiftieth that of what it once was. Through the use of this highly pure, low-surfactant CNT ink, CNT networks demonstrating conductivity higher than that of conventional CNT networks can be realized and printed CNT-TFT exhibiting high output current can be fabricated.

These recently developed technologies are partially based on results from TASC development that began in 2011 on the "Innovative Carbon Nanotubes and Their Application Project for Low Carbon Society"project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Additional aspects of these new technologies result from joint research between NEC and the University of Tokyo.

Going forward, NEC aims to apply logic circuits in combination with multiple transistor components as well as to apply multiple logic circuits in combination with printed control circuits. Moreover, NEC will continue its research and development of printed CNT-TFT, while seeking to improve the usability of electronics through their adoption.

NEC presented results from these findings at the 2013 International Conference on Solid State Devices and Materials (SSDM 2013) from 24 - 27 September, 2013 at the Hilton Fukuoka Sea Hawk, Fukuoka, Japan (http://www.ssdm.jp/).