The UW–Madison engineers use a solution process to deposit aligned arrays of carbon nanotubes onto 1 inch by 1 inch substrates. The researchers used their scalable and rapid deposition process to coat the entire surface of this substrate with aligned carbon nanotubes in less than 5 minutes. The team’s breakthrough could pave the way for carbon nanotube transistors to replace silicon transistors, and is particularly promising for wireless communications technologies. STEPHANIE PRECOURT
For decades, scientists have tried to harness the unique properties of carbon nanotubes to create high-performance electronics that are faster or consume less power – resulting in longer battery life, faster wireless communication and faster processing speeds for devices like smartphones and laptops. But a number of challenges have impeded the development of high-performance transistors made of carbon nanotubes, tiny cylinders made of carbon just one atom thick. Consequently, their performance has lagged far behind semiconductors such as silicon and gallium arsenide used in computer chips and personal electronics.
Now, for the first time, U-WM materials engineers have created carbon nanotube transistors that outperform state-of-the-art silicon transistors. Their carbon nanotube transistors achieved current 1.9X higher than silicon transistors. “This achievement has been a dream of nanotechnology for the last 20 years,” says Arnold. “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.”
This advance could pave the way for carbon nanotube transistors to replace silicon transistors and continue delivering the performance gains the computer industry relies on and that consumers demand. The new transistors are particularly promising for wireless communications technologies that require a lot of current flowing across a relatively small area.
Carbon nanotube transistors should be able to perform 5X faster or use 5X less energy than silicon transistors. The nanotube’s ultra-small dimension makes it possible to rapidly change a current signal traveling across it, which could lead to substantial gains in the bandwidth of wireless communications devices.
But 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 UW-Madison team 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% metallic nanotubes,” says Arnold.
To make a good transistor, the nanotubes need to be aligned in just the right order, with just the right spacing, when assembled on a wafer. In 2014, the UW-Madison researchers overcame that challenge when they announced a technique, called “floating evaporative self-assembly,” that gives them this control.
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. The result: excellent electrical contacts 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 an apples-to-apples comparison. They are continuing to work on adapting their device to match the geometry used in silicon transistors, which get smaller with each new generation. Work is also underway to develop high-performance radio frequency amplifiers that may be able to boost a cellphone signal. While the researchers have already scaled their alignment and deposition process to 1 inch by 1 inch wafers, they’re working on scaling the process up for commercial production.
Arnold says it’s exciting to finally reach the point where researchers can exploit the nanotubes to attain performance gains in actual technologies. The researchers have patented their technology through the Wisconsin Alumni Research Foundation. http://news.wisc.edu/for-first-time-carbon-nanotube-transistors-outperform-silicon/
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