Computer chips from carbon nanotubes, not silicon, mark a milestone | Science News for Students

Computer chips from carbon nanotubes, not silicon, mark a milestone

The prototype could give rise to a new generation of faster, more energy-efficient electronics
Sep 25, 2019 — 6:45 am EST
a carbon nanotbue computer chip

The sun may be setting on silicon. The first computer chips made from carbon nanotubes (one pictured) are here. 

G. Hills et al/Nature 2019

A new type of computing chip could be a game-changer. That’s because its transistors are not made of silicon. Transistors are tiny electronic switches that together perform calculations. A new prototype uses carbon nanotubes. It is not yet as speedy or as small as the silicon devices found in today’s computers, phones and more. But these new computer chips may one day give rise to electronics that are faster and use less energy.

Researchers describe their advance in the August 29 Nature.

This is “a very important milestone in the development of this technology,” observes Qing Cao. He’s a materials scientist at the University of Illinois at Urbana-Champaign. He was not involved in the work.

The heart of every transistor is a semiconductor component. It’s usually made of silicon. This element can act like an electrical conductor. It also can act like an insulator. This lets a transistor have an “on” and an “off” state. When on, current flows through the semiconductor; when off, it doesn’t. And this on/off state is what encodes the 1s and 0s of digital computer data.

Max Shulaker is an electrical engineer. He works at the Massachusetts Institute of Technology in Cambridge. “We used to get exponential gains in computing every single year,” he says. Computer engineers were able to do so by building smaller and faster silicon transistors. But now, he says, “performance gains have started to level off.” Silicon transistors can’t get much smaller and more efficient than they already are.

Carbon nanotubes, though, are almost as thin as an atom. And they ferry electricity well. As a result, they make better semiconductors than silicon. In principle, carbon nanotube processors could run three times faster than silicon ones. And they would consume about one-third as much energy as silicon processors, Shulaker says. But until now, carbon nanotubes have proved too finicky to use in complex computing systems.

Carbon computing

One issue comes when a network of carbon nanotubes is deposited onto a computer chip wafer. At that point, the tubes tend to bunch into lumps. This prevents the transistor from working. It’s “like trying to build a brick patio, with a giant boulder in the middle of it,” Shulaker says. His team solved that problem. They spread nanotubes on a chip. Then they used vibrations to gently shake unwanted bundles off the layer of nanotubes.

an array of computer chips on a wafer
A new kind of computer chip (array of chips on the wafer pictured above) contains thousands of transistors made from carbon nanotubes, not silicon. The prototypes can't yet compete with silicon chips for size or speed. But carbon-nanotube-based computing promises to usher in a new era of electronics that are faster and more energy efficient.  
G. Hills et al/Nature 2019

The team also faced another problem. Each batch of carbon nanotubes contains about 0.01 percent metallic nanotubes. Metallic nanotubes can’t properly flip between conductive and insulating. So these tubes can muddle a transistor’s readout.

Shulaker and colleagues searched for a workaround. To perform different kinds of operations on bits of data, transistors can be configured in various ways. The researchers looked at how metallic nanotubes affected different configurations. They found that defective nanotubes affected the function of some configurations more than others. This is similar to the way a missing letter can make some words illegible, but leave others mostly readable. So the researchers carefully designed the circuitry of their microprocessor. They avoided configurations that were most confused by metallic-nanotube glitches.

“One of the biggest things that impressed me about this paper was the cleverness of that circuit design,” says Michael Arnold. He’s a materials scientist at the University of Wisconsin–Madison. He was not involved in the work.

The resulting chip has more than 14,000 carbon-nanotube transistors. It executed a simple program to write the message, “Hello, world!” This is the first program that many newbie computer programmers learn to write.

The new chips are not yet ready to unseat silicon ones in modern electronics. Each carbon transistor is about a millionth of a meter across. Current silicon transistors are smaller. They are tens of billionths of a meter across. Each carbon-nanotube transistor in this prototype can flip on and off about a million times a second. Silicon transistors can flicker billions of times per second. That puts nanotube transistors on a par with silicon transistors of the 1980s.

Shrinking the nanotube transistors would help electricity zip through them with less resistance. That would allow the devices to switch on and off faster, Arnold says. They could also align the nanotubes in parallel, rather than using a randomly oriented mesh. This could increase the electric current through the transistors. That would further boost processing speeds.

Power Words

(more about Power Words)

align     (noun: alignment) To place or organize things in a patterned order, following an apparent line.

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

carbon     The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

carbon nanotube     A billionth-of-a-meter scale, tube-shaped material that is made from carbon. It conducts heat and electricity well.

circuit     A network that transmits electrical signals. In the body, nerve cells create circuits that relay electrical signals to the brain. In electronics, wires typically route those signals to activate some mechanical, computational or other function.

colleague     Someone who works with another; a co-worker or team member.

component     Something that is part of something else (such as pieces that go on an electronic circuit board or ingredients that go into a cookie recipe).

computer chip     (also integrated circuit) The computer component that processes and stores information.

conductive     Able to carry an electric current.

conductor     (in physics and engineering) A material through which an electrical current can flow.

current     (in electricity) The flow of electricity or the amount of charge moving through some material over a particular period of time.

data     For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

digital     (in computer science and engineering)  An adjective indicating that something has been developed numerically on a computer or on some other electronic device, based on a binary system (where all numbers are displayed using a series of only zeros and ones).

electrical engineer     An engineer who designs, builds or analyzes electrical equipment.

electric current     A flow of electric charge — electricity — usually from the movement of negatively charged particles, called electrons.

electricity     A flow of charge, usually from the movement of negatively charged particles, called electrons.

electronics     Devices that are powered by electricity but whose properties are controlled by the semiconductors or other circuitry that channel or gate the movement of electric charges.

element     A building block of some larger structure. (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

encode     (adj. encoded) To use some code to mask a message.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

exponential     An adjective that describes things that vary (usually increase) by a factor of at least 10.

insulator     A substance or device that does not readily conduct electricity.

materials science     The study of how the atomic and molecular structure of a material is related to its overall properties. Materials scientists can design new materials or analyze existing ones. Their analyses of a material’s overall properties (such as density, strength and melting point) can help engineers and other researchers select materials that are best suited to a new application. 

micrometer     (sometimes called a micron) One thousandth of a millimeter, or one millionth of a meter. It’s also equivalent to a few one-hundred-thousandths of an inch.

milestone     An important step on the road to stated goal or achievement. The term gets its name from the stone markers that communities used to erect along the side of the road to inform travelers how far they still had to go (in miles) before reaching a town.

nanometer     A billionth of a meter.

network     A group of interconnected people or things. (v.) The act of connecting with other people who work in a given area or do similar thing (such as artists, business leaders or medical-support groups), often by going to gatherings where such people would be expected, and then chatting them up. (n. networking)

parallel     An adjective that describes two things that are side by side and have the same distance between their parts. In the word “all,” the final two letters are parallel lines. Or two things, events or processes that have much in common if compared side by side.

processor     (in computing) Also called a central processing unit, or CPU, it’s a part of the computer that performs numerical calculations or other types of data manipulation. It can also be a type of software, or programming, that translates some other program into a form that can be understood by the computer running it.

prototype     A first or early model of some device, system or product that still needs to be perfected.

resistance      (in physics) Something that keeps a physical material (such as a block of wood, flow of water or air) from moving freely, usually because it provides friction to impede its motion.

semiconductor     A material that sometimes conducts electricity. Semiconductors are important parts of computer chips and certain new electronic technologies, such as light-emitting diodes.

silicon     A nonmetal, semiconducting element used in making electronic circuits. Pure silicon exists in a shiny, dark-gray crystalline form and as a shapeless powder.

technology     The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

transistor     A device that can act like a switch for electrical signals.

Citation

Journal:​ ​​ G. Hills et al. Modern microprocessor built from complementary carbon nanotube transistors. Nature. Vol. 572, August 29, 2019, p. 595. doi:10.1038/s41586-019-1493-8.