Research Proves Carbon Nanotube Transistors to be Better than Silicon Transistors

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. Credit: Stephanie Precourt 

Scientists have attempted to harness the unique properties of carbon nanotubes for decades, in order to develop high-performance electronics that are extremely fast or consume less power, prolonging the battery life and resulting in faster wireless communication and rapid processing speeds for devices such as laptops and smartphones.

A number of challenges have hampered the production of high-performance transistors made of carbon nanotubes, which are very small cylinders made of carbon just one atom thick. As a result, their performance is far behind semiconductors such as gallium arsenide and silicon used in personal electronics and computer chips.

Recently, materials engineers from the University of Wisconsin–Madison developed carbon nanotube transistors that perform better than state-of-the-art silicon transistors.

Michael Arnold and Padma Gopalan, UW–Madison professors of materials science and engineering, headed the team. The carbon nanotube transistors developed by the team obtained a current that is 1.9 times higher than silicon transistors.

The findings were reported by the researchers in a recent issue of the journal Science Advances.

This achievement has been a dream of nanotechnology for the last 20 years. 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.

Michael Arnold, Professor, UW-Madison

This achievement could enable carbon nanotube transistors to replace silicon transistors and consistently deliver the performance gains the computer industry depends on and that consumers demand. The new transistors are ideal for wireless communications technologies that need an increasing amount of current to pass through a relatively small area.

Carbon nanotubes, considered to be some of the best electrical conductors ever discovered, have long been recognized as a potential material for next-generation transistors.

Carbon nanotubes transistors should be capable of performing five times faster or use five times less energy compared to silicon transistors, based on extrapolations obtained from single nanotube measurements. The ultra-small dimension of the nanotube allows it to change a current signal passing through it in a rapid manner, leading to considerable gains in the bandwidth of wireless communication devices.

Isolation of purely carbon nanotubes has been a difficult task for researchers. These purely carbon nanotubes are vital because metallic nanotube impurities behave like copper wires and disturb their semiconducting features, like a short in an electronic device.

The UW–Madison researchers used polymers in order to selectively sort out the semiconducting nanotubes, thus obtaining 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 percent metallic nanotubes.

Michael Arnold, Professor, UW-Madison

Monitoring the placement and alignment of the nanotubes is also a complicated task.

A good transistor can be developed by aligning the nanotubes in the correct order with ideal spacing when arranged on a wafer. In 2014, the researchers overcame that challenging task when they developed a technique known as “floating evaporative self-assembly,” that provides them with this control.

Good electrical contacts are expected to be made by the nanotubes with the transistor’s metal electrodes. Nanotube arrays were backed in a vacuum oven in order to remove the insulating layer. This took place because the polymer used by the researchers to isolate the semiconducting nanotubes also behave as an insulating layer existing between the nanotubes and the electrodes. This resulted in exceptional electrical contacts to the nanotubes being achieved.

The team also came up with a treatment to remove residues from the nanotubes after being processed in solution.

In our research, 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.

Michael Arnold, Professor, UW-Madison

The carbon nanotube transistor developed by the team were benchmarked against a silicon transistor of similar size, leakage current, and geometry in order to carry out a comparative study.

The researchers are continuing to work towards adapting their device to match the geometry applied in silicon transistors, which becomes smaller with each new generation. Ongoing work also focuses on developing high-performance radio frequency amplifiers that may have the potential to enhance a cellphone signal.

The researchers have previously scaled their alignment and deposition process to 1x1" wafers, and are now aiming at commercializing the process.

Arnold states that it is exciting to finally arrive at the point where researchers can exploit the nanotubes to achieve performance gains in actual technologies.

There has been a lot of hype about carbon nanotubes that hasn’t been realized, and that has kind of soured many people’s outlook. 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.

Michael Arnold, Professor, UW-Madison

This new technology has been patented through the Wisconsin Alumni Research Foundation.

The researcher received financial aid from the Air Force, the Army Research Office and the National Science Foundation.

Additional authors on the paper include Harold Evensen, a University of Wisconsin-Platteville engineering physics professor, Gerald Brady, a UW–Madison materials science and engineering graduate student and lead author on the study, and graduate student Austin Way and postdoctoral researcher Nathaniel Safron.

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