Carbon nanotube transistors outperform silicon for the first time

University of Wisconsin-Madison claims nanotube breakthrough

A transistor made from carbon nanotubes has, for the first time, outperformed silicon-based transistors.

The breakthrough was achieved at the University of Wisconsin-Madison by a team led by professors of materials science and engineering Michael Arnold and Padma Gopalan.

"This achievement has been a dream of nanotechnology for the last 20 years," said Arnold. "Making carbon nanotube transistors that are better than silicon transistors is a big milestone. This breakthrough in carbon nanotube transistor performance is a critical advance toward exploiting carbon nanotubes in logic, high-speed communications, and other semiconductor electronics technologies."

They claim that the carbon nanotube transistors that they have developed were able to achieve an electrical current 1.9 times higher than silicon transistors.

If their experiments can be replicated and an efficient manufacturing process devised, the new transistors would initially be promising for wireless communications technologies that require a lot of current flowing across a relatively small area.

Carbon nanotubes have long been recognised as a promising material for next-generation transistors due to their electrical conductivity.

Transistors made from carbon nanotubes ought to be able to perform five times faster or use five times less energy than silicon transistors, claim the researchers. Nanotube's ultra-small size makes it possible to rapidly change a current signal travelling across it, which could lead to substantial gains in the bandwidth of wireless communications devices. It could also enable even smaller, yet more powerful, devices to be developed.

However, researchers have struggled to isolate purely carbon nanotubes, which are crucial, because metallic nanotube impurities act like copper wires and disrupt their semiconducting properties.

The research team at the university used polymers to selectively sort out the semiconducting nanotubes, achieving a solution of ultra-high-purity semiconducting carbon nanotubes. "We've identified specific conditions in which you can get rid of nearly all metallic nanotubes, where we have less than 0.01 per cent metallic nanotubes," said Arnold.

He added that to make a "good" carbon nanotube transistor, the nanotubes need to be aligned in exactly the right order, with precise spacing, before being assembled on a wafer. The team claim to have achieved this in 2014 when they developed a technique that they called "floating evaporative self-assembly".

The nanotubes must make good electrical contacts with the metal electrodes of the transistor. Because the polymer the UW-Madison researchers use to isolate the semiconducting nanotubes also acts like an insulating layer between the nanotubes and the electrodes, the team "baked" the nanotube arrays in a vacuum oven to remove the insulating layer and to achieve what they claimed was an "excellent electrical contact to the nanotubes".

The researchers also developed a treatment that removes residues from the nanotubes after they're processed in solution.

The researchers benchmarked their carbon nanotube transistor against a silicon transistor of the same size, geometry and leakage current in order to make performance comparison.

"There has been a lot of hype about carbon nanotubes that hasn't been realised, and that has kind of soured many people's outlook," said Arnold. "But we think the hype is deserved. It has just taken decades of work for the materials science to catch up and allow us to effectively harness these materials."