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Posted: Feb 9th, 2012
Experimental carbon nanotube transistor breaks the 10-nanometer level
(Nanowerk Spotlight) For years it has been known that scaling bulk silicon transistors would be extremely challenging, if not impossible, when lengths close in on 15 nm. Already, attention has turned to 3D transistor design and silicon-on-insulator (SOI) devices to improve the scalability of silicon technology. Carbon nanotube (CNT) transistors have been touted as a possible replacement for silicon devices but the crucial question so far has been if CNT transistors can offer performance advantages over silicon at sub-10 nm lengths? New experimental results from IBM Research are indicating that the answer is 'yes'.
There have been mixed opinions in the nanoelectronics community regarding whether or not CNT transistors would maintain their impressive performance at extremely scaled lengths. Some argued that the very small effective mass of the carriers would contribute to a tunneling phenomena that would cause the devices to breakdown around 15 nm – an opinion supported by the few theoretical studies that explored nanotube devices at such dimensions (see for instance this paper in Nanotechnology: "Properties of short channel ballistic carbon nanotube transistors with ohmic contacts").
Meanwhile, others remained convinced that the ultrathin body of single-walled carbon nanotubes – only 1 nm in diameter – would allow for excellent transistor behavior even down to the sub-10 nm range.
"Ultimately, if nanotubes cannot provide devices that outperform silicon at such lengths, then pursuing them as a potential silicon replacement would be a waste of time," Dr. Aaron D. Franklin, a Research Staff Member at IBM's T. J. Watson Research Center in Yorktown Heights, tells Nanowerk. "So, our motivation for the work was simply that we needed to know how much value nanotubes can provide if they are able to be highly integrated at some point."
Schematic of a sub-10 nm carbon nanotube transistor configuration. (Image: Aaron Scott, Southern Illinois University)
The findings by a team of IBM researchers and scientists from ETH Zurich's Integrated Systems Laboratory and Purdue University defied the theoretical projections and exhibited encouraging performance for a device with a 9 nm channel length. They published their findings in a recent edition of Nano Letters ("Sub-10 nm Carbon Nanotube Transistor").
In this paper, first-authored by Franklin, the scaling performance of CNTs is unveiled. The researchers fabricated several transistors on the same single-walled nanotube, showing switching behavior that is comparable to the best silicon devices.
To observe the scaling behavior of transistors to below 10 nm, the researchers fabricated several devices with different channel lengths (between 320 nm and 9 nm) on the same CNT. They emphasize that using the same nanotube for all channel lengths is critical because the energy band gap for a CNT is inversely proportional to the diameter. Changes in band gap will affect performance at small channel lengths by affecting achievable on-current and ambipolar conduction behavior.
Defying the predictions of previous theoretical work, the characteristics of the 9 nm channel length device reflect superb switching behavior in the off-state and clear current saturation at a low drain-source bias of approximately 250 mV in the on-state.
Furthermore, as Franklin points out, the sub-10 nm CNT transistor provides low voltage operation that is superior to any similarly scaled device to date, a result that shows promise for further optimization of CNT transistors for future applications.
"There are several potential answers to why previous theory was not accurate for these devices, and most of them are related to a lack of complete understanding regarding transport of carriers at the metal-nanotube contact interfaces," he says. "These contacts play a primary role in the operation of devices that have such small channel lengths."
"The superb low voltage performance of our scaled carbon nanotube transistor is a sign post showing that there is a demonstrable alternative for extremely scaled transistors" he continues. "In this way, these results advance the field by showing the vitality of such devices for many future technology generations."
It needs to be pointed out that these are only experimental results and at this point there remain numerous challenges related to integrating CNT transistors into industrial-scale chip manufacturing. As Franklin notes: "At present, we simply hope that these encouraging results will inspire more researchers to start working on nanotube electronics."
Research results like these show that there still is much room and opportunity for carbon nanotube transistors to improve. The researchers say that the biggest challenge to research in this area is funding. "Though, once again, we are hopeful that results such as these will inspire funding agencies to acknowledge the tremendous value that carbon nanotube transistors could hold for future applications and that they will subsequently renew their drive to fund exploratory research in the field" says Franklin.