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June 23, 2014

Carbon Nanotubes Pitched as Post-Silicon Contender

Tiffany Trader
Viterbi hybrid CNT-IGZO circuits

With TOP500 list stagnation likely signaling the slow-down of an exponential known as Moore’s law, what better time to consider alternatives to silicon-based microelectronics. Researchers from the USC Viterbi School of Engineering point to carbon nanotubes as a promising replacement for silicon as the traditional transistor material. The advantage of carbon nanotubes is that they are more transparent, flexible, and can be processed at a lower cost, according to the research team. In the journal Nature Communications, the researchers describe an energy-efficient hybrid circuit that combines carbon nanotube (CNT) thin film transistors (TFT) with thin film transistors comprised of indium, gallium and zinc oxide (IGZO).

In order to develop the carbon nanotube based circuit, the team, which includes electrical engineering professor Dr. Chongwu Zhou and USC Viterbi graduate students Haitian Chen, Yu Cao, and Jialu Zhang, had to overcome a major limitation in carbon nanotube technology.

“I came up with this concept in January 2013,” said Dr. Chongwu Zhou, professor in USC Viterbi’s Ming Hsieh Department of Electrical Engineering, in an article on the USC website. “Before then, we were working hard to try to turn carbon nanotubes into n-type transistors and then one day, the idea came to me. Instead of working so hard to force nanotubes to do something that they are not good for, why don’t we just find another material which would be ideal for n-type transistors—in this case, IGZO—so we can achieve complementary circuits?”

Carbon nanotubes are essentially one-atom thick sheets of carbon that have been rolled in a tube. The team achieved the integrated form by combining circuits that can operate complimentarily, optimizing power loss and efficiency. The addition of IGZO thin film transistors was the necessary step to increasing battery life. Using only carbon nanotubes constrains power efficiency. “By combining the two materials, their strengths have been joined and their weaknesses hidden,” explains the USC article.

“It’s like a perfect marriage,” said Zhou. “We are very excited about this idea of hybrid integration and we believe there is a lot of potential for it.”

The integrated circuitry created by the team could show up in a range of applications, including Organic Light Emitting Diodes (OLEDs), digital circuits, radio frequency identification (RFID) tags, sensors, wearable electronics, and flash memory devices. Electrodes based on the new technology could also benefit medical patient care.

There’s also a real possibility that the team could further develop the technique to use in more complex circuits, suitable for computers.

“We believe this is a technological breakthrough, as no one has done this before,” said Haitian Chen, research assistant and electrical engineering PhD student at USC Viterbi. “This gives us further proof that we can make larger integrations so we can make more complicated circuits for computers and circuits.”

In fact, building more complicated circuits with additional functionality is the next step for the team.