Research Breakthrough in Carbon Nanotube Transistors

The superiority of semiconducting single-walled carbon nanotubes (CNTs) as field effect transistors (FET) was demonstrated over a decade ago. Their suitability for this application comes about due to their exceptional charge transport characteristics, specifically, their current-carrying capacity, carrier velocity, and exceptional electrostatics due to their ultrathin bodies. There is little question that once commercially available, these super-lightweight high-performance circuits will soon replace silicon as the dominant semiconductor material.

However, difficulties in production have kept this material out of reach all this time. Among the challenges faced have been making them small enough, achieving their theoretical performance potential, and the challenge of massively packing them into integrated circuits without short circuiting.

Last year, IBM announced a breakthrough. The company had managed to successfully reduce the size of the contacts by attaching inside the ends of the tubes instead of on top as had been done previously. This was hailed by many, including Michael Arnold, a professor of materials science and engineering at the University of Wisconsin, who called it a “fantastic strategy” for the contact problem. This will allow extremely small CNT transistors to be made. Still, that leaves two remaining problems: removing embedded metals from the nanotubes, and finding a way of packing billions of these transistors on a tiny wafer.

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In September, Dr. Arnold had an announcement of his own. He and his colleague, Padma Gopalan, demonstrated CNT field-effect transistors (FET) that for the first time ever, actually outperformed silicon. The work has been published in the journal, Science Advances . The carbon transistors demonstrated a current capacity 1.9 times higher than their silicon counterparts. It is expected, based on extrapolation, that these transistors will be five times faster and five times less expensive. The fabrication approach Arnold and Gopalan used was “a combination of CNT purification, solution-based assembly, and CNT treatment.”

The following excerpt from the paper describes the performance:

“The saturated on-state current density is as high as 900 μA μm−1 and is similar to or exceeds that of Si FETs when compared at and equivalent gate oxide thickness and at the same off-state current density. The on-state current density exceeds that of GaAs FETs, as well. This breakthrough in CNT array performance is a critical advance toward the exploitation of CNTs in logic, high-speed communications, and other semiconductor electronics technologies.”

A big part of what had been holding back the development of carbon nanotube transistors was the presence of metallic impurities in the nanotubes. These impurities create short circuits.

The researchers made use of polymers to sort and purify the nanotubes, achieving a metallic content of less than 0.01%. Using a construction method that they call “floating evaporative self-assembly,"

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