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," they were able to achieve the alignment and assembly precision required to densely pack the transistors on a wafer. An insulating polymer layer coats each transistor until it is laid down and electrically bonded to the wafer contacts. Then the assembly is placed in an oven and the polymer layer is evaporated off. A final treatment step removes the residue.

Says Arnold, “we've shown that we can simultaneously overcome all of these challenges of working with nanotubes, and that has allowed us to create these groundbreaking carbon nanotube transistors that surpass silicon and gallium arsenide transistors."

The authors gave the reason why they achieved this exceptional performance, including electrical conductance that was seven times the previous state-of-the-art CNT FETs, as follows:

“The exceptional performance of the arrays achieved here is attributed to the combined outstanding alignment and spacing of the CNTs, the postdeposition treatment of the arrays to remove solvent residues and the insulating side chains of the polymers that wrap the CNTs, and the exceptional electronic-type purity of the semiconducting CNTs afforded by the use of polyfluorenes as CNT-differentiating agents.”

Arnold and his team are now working on producing geometries that match those commonly implemented in silicon. This includes high-performance RF amplifiers like those found in cellphones.

RP Siegel, PE, has a master's degree in mechanical engineering and worked for 20 years in R&D at Xerox Corp. An inventor with 50 patents, and now a full-time writer, RP finds his primary interest at the intersection of technology and society. His work has appeared in multiple consumer and industry outlets, and he also co-authored the eco-thriller Vapor Trails.

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