Microprocessor Made Entirely Out Of Carbon Nanotubes (CNTs)

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As opposed to silicon, carbon nanotubes have higher processing speeds and are environment-friendly

Image courtesy MIT News

After overcoming numerous design challenges, MIT researchers have finally build a modern microprocessor from carbon nanotube transistors, which is as a faster, greener alternative to their silicon counterparts.

Silicon transistors have widely been used as microprocessor components in the computer industry for decades. As per Moore’s Law (which states that the processing power of computers doubles every two years), the industry has been able to shrink down and integrate more transistors onto chips every couple of years to carry out increasingly complex computations. But experts say the time will soon come when silicon transistors will stop shrinking and become inefficient.

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The microprocessor was built using traditional silicon-chip fabrication processes and is based on the RISC-V open-source chip architecture with an executable set of instructions. The microprocessor successfully executed the classic “Hello, World!” program, printing out, “Hello, World! I am RV16XNano, made from CNTs.” The researchers also demonstrated that this 16-bit microprocessor with more than 14,000 CNFETs can perform the same tasks as commercial microprocessors.

The invention will lead to a huge reduction in the number of defects while fabricating CNFETs (Carbon Nanotube Field Effect Transistors), using traditional silicon chip manufacturing processes.

Making CNFETs has become a major goal for developing next-generation computers. They are 10 times more energy efficient and have far greater speeds compared to silicon.

Max M. Shulaker, the Emanuel E Landsman Career Development Assistant Professor of Electrical Engineering and Computer Science (EECS) and a member of the Microsystems Technology Laboratories, said , “This is by far the most advanced chip made from any emerging nanotechnology that is promising for high-performance and energy-efficient computing.

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He added, “There are limits to silicon. If we want to continue to have gains in computing, carbon nanotubes represent one of the most promising ways to overcome those limits.”

The ‘DREAM’

For years, the defects intrinsic to carbon nanotubes have been its biggest problem. To be robust, they need to have a 99.999999 percent purity, which is virtually impossible to produce today.

To tackle this, researchers came up with a unique technique called DREAM (Designing Resiliency Against Metallic CNTs). The technique positions metallic CNFETs in such a way that they won’t disrupt computing. To achieve this, the stringent purity requirement was relaxed by around four orders of magnitude — or 10,000 times. This meant they only needed carbon nanotubes at about 99.99 percent purity, which is currently possible.

“The ‘DREAM’ pun is very much intended, because it’s the dream solution,” Shulaker says. “This allows us to buy carbon nanotubes off the shelf, drop them onto a wafer, and just build our circuit like normal, without doing anything else special.”

Binary computing requires two types of transistors: “N” types, which turn on with a 1 bit and turn off with a 0 bit, and “P” types, which do the opposite. Traditionally, making the two types out of carbon nanotubes has been challenging, often yielding transistors that vary in performance. A solution for this was developed called MIXED (Metal Interface Engineering Crossed with Electrostatic Doping).

In this technique, certain metals — platinum or titanium — were attached to each transistor which assigns that transistor as P or N. Then, the CNFETs were coated in an oxide compound through atomic-layer deposition, which gave transistors the characteristics for specific applications. Servers, for instance, often require very fast transistors but use up energy and power. Wearables and medical implants, on the other hand, may use slower, low-power transistors.

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With the support of Analog Devices, the National Science Foundation, U.S, and the Air Force Research Laboratory, U.S, the researchers aim to get the chips out into the real world. They have now started work on manufacturing techniques through a program by Defence Advanced Research Projects Agency, U.S.

Although no one can say when chips made entirely from carbon nanotubes will hit the shelves, Shulaker says it could be fewer than five years. “We think it’s no longer a question of if, but when.”

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