high-performance computing ➤


electron current flow] field-effect transistor (FET), and a 20 nanometre CNT CMOS,’ said Lian-Mao Peng, the principal investigator of the study. ‘Our work pushed CNT CMOS transistors down to 10 nanometres, and a p-type FET down to five nanometers, which is likely the ultimate transistor size that the industry is likely to achieve.’ Peng’s team benchmarked their CNT FET


with a sub-10 nanometre silicon transistor of similar dimensions using the ‘energy delay product’ (EDP). Tis metric is the average energy consumed per switching event and measures the overall performance by compromising between power dissipation and performance. ‘Our experimental data indicated that CNT FETs can improve EDP by a factor of 10, than that of silicon CMOS FETs at the sub-10 nanometre technology node. Future computers based on CNT integrated chips should run much faster (10 times) than the silicon based integrated chips with similar power dissipation — or will require 1/10 power dissipation of silicon circuits with similar performance,’ said Peng. Tis demonstration is not only good news


for future fabrication scalability of transistors, based on carbon nanotubes, but has the potential for better mobile communication devices according to Pertti Hakonen, professor of physics at Aalto University in Espoo, Finland — who was not involved in the study. ‘Te concept introduced in the paper is


quite interesting,’ said Hakonen. ‘Te carbon nanotube is operated via graphene contacts, which act as the drain, and source electrodes that are reservoirs for charge carriers. Graphene contacts are modified by the gate electrode, but this is an integral part of the device. Consequently, the packing of devices can be made in a compact fashion. I believe this concept will be useful for many research groups working on miniaturisation of their circuits,’ said Hakonen. Although the results presented by the


Peking University researchers still needs to be reproduced in an industrial lab, researchers in the academic community commend the promising approach. ‘As far as I can judge, I do not see faults in their results,’ said Mikko Möttönen, leader of quantum computing and the devices lab at Aalto University, Finland, and professor in quantum computing at the University of Jyväskylä — who was also not involved in the study. ‘Tis is obviously a very important work for


the future industrial transistor development. Issues with low-cost, high-yield mass production and ageing should be addressed.’


12 SCIENTIFIC COMPUTING WORLD


and lower power advantage of the CNT FET is due to the carbon nanotubes properties such as higher carrier mobility and a thinner body. ‘Some of these problems have now been


solved and the remaining [issues] may be solved in the future. One of the other things is how to grow the CNTs exactly where one wants them to be,’ said Möttönen. Because carbon nanotubes are grown like


crystals, there is a certain randomness in how the final filament products end up. Even with the best automated design algorithms, impurities still end up in the material that results in electrical resistance, consuming more energy and reducing computation speeds. Some progress has been made in more efficient connections between carbon nanotubes and metal bonds by fusing the contacts directly. ‘One still needs to show that all parts of the


ACCORDING TO OUR WORK, AS WELL AS THOSE BY IBM, THERE IS A GOOD CHANCE THAT CNT FETS WOULD EXTEND MOORE’S LAW BEYOND 2020


Tis achievement supports one of the


main goals of the wider semiconductor industry’s partnership programme known as the International Technology Roadmap for Semiconductors (ITRS). One of the roadmap’s seven building blocks is to support ‘Beyond CMOS’ technologies. Tese are devices, focused on new physical states, which provide functional scaling. ‘Traditionally the problem in carbon


has been that it is difficult to make good transparent contacts to typical metal,’ said Möttönen. Tis is because electrons travelling within carbon nanotubes are considered to be in very different physical states compared to their CMOS transistor counterparts. Peng’s team, in their experiments, were


able to use only one electron per switching operation in their five-nanometre scale CNT FET. Te switching time of their CNT FET was about 42 femtoseconds. A femtosecond is one millionth of one billionth of a second; put another way, in Chemistry the time it takes for atoms in a molecule to perform one vibration is about 10-100 femtoseconds. In comparison the switching time in


a state-of-the-art silicon 14 nanometre transistor is 220 femtoseconds – the faster


transistor can be scaled down to the required size and that it can be done with very high yield. Also one needs to test that the transistors are very stable in time, i.e., none of them should break with years of operation time,’ said Möttönen. Unperturbed by these hurdles, Peng is not


shy in his ambitions for the semiconductor industry, as investors welcome the team’s breakthrough research. ‘According to our work, as well as those by


IBM, there is a good chance that CNT FETs would extend Moore’s Law beyond 2020. Our long-time goal for the development of CNT transistors and integrated circuits is to promote CNT chip technology to become the mainstream chip technology, and to provide more powerful chips with higher speed and lower power dissipation for China and the world,’ said Peng.


Building a carbon nanotube bridge to quantum computers Moreover, carbon nanotube-based transistors may open up the gateway to superconducting quantum-bits (qubits), which are perceived as a promising path for a scalable quantum system. Superconducting qubits are circuits made from superconducting components that offer zero-resistance to electrical currents at temperatures near absolute zero. Te components can comprise materials such as yttrium barium copper oxide to create wires, capacitors or non-linear inductors. Hakonen’s research goals are investigating


the possibilities of combining graphene and carbon nanotubes for quantum technology, such as in simple-charge-based qubits. Tese architectures have excellent electrical


@scwmagazine l www.scientific-computing.com ➤


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