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New Progress in Graphene-Based RF Circuits

02.24.2014 // Murray Slovick // New Technology

While everyone loves a good mystery, keeping engineers guessing as to which breakthroughs will shape future electronic component design can only benefit the makers of antacids. And while it’s hard to predict that a given research program will pan out and become workable new parts, it is worthwhile to track industrial and academic laboratory efforts, much as a fan of a major league sports team follows up-and- coming players in lesser “minor” or developmental leagues.

For example, the discovery of graphene in 2004 created buzz among researchers because this material, just one atom thick, possesses exceptional strength and unusual electronic properties, including high electron mobility: electrons travel through graphene more than 100 times as easily as they do through silicon. Its electrical, optical, mechanical and thermal properties make it well-suited for wireless RF communications. Graphene-based circuits could allow mobile devices such as smartphones, tablets or wearable electronics to transmit data much faster. Integrating graphene RF devices into silicon circuits could also potentially enable pervasive wireless communications (the “Internet of Things”). Graphene may further permit RFID tags and so-called “smart” sensors to send data at much greater distances than currently possible.

We’ve kept an eye on this technology, last reporting on graphene happenings on these pages almost two years ago and recently there have been enough new developments to warrant another visit.

For starters, IBM researchers last month reported building the best graphene circuit yet, sending a radio signal containing the letters “I, B and M”, to a device which received the text, making it the first working radio chip to be made from graphene. The experimental circuit built for wireless receivers also consumed less than 20 mW power during operation while demonstrating the highest conversion gain of any graphene RF circuit at multiple GHz frequency – successfully receiving and restoring the digital text message carried on a 4.3 GHz signal without any distortion.

Up to now there was not much hope that wireless chips would be possible with graphene transistors, because it is a delicate material; in 2011, IBM researchers made a radio microchip with graphene transistors, but they found that placing such metal components as resistors and coils on top of the transistors physically damaged them and the research team was not able to receive a broadcast.

This time IBM researchers found that by reversing the flow of the manufacturing process, placing the metal components on the chip first and then adding the graphene transistors, they could avoid the structural stresses and damage that befell the 2011 chip. They found that the gain of the graphene transistors is now 10,000 times better than before. The current device included three graphene transistors, whereas the 2011 circuit used just one. The team built the chips on standard silicon wafers, suggesting that no major changes in production technology will be necessary down the road.

One of the major potential applications for graphene is transistors, which control the flow of electricity in circuits. Researchers should be able to pack far more atom-thick graphene transistors into a chip than the relatively bulkier silicon currently used.

Unfortunately, the property that makes graphene a good conductor, its zero band gap, is the one characteristic that makes it very difficult to create graphene transistors, which would be the basic component of logic and memory circuits. By way of review, in semiconductors electrons are confined to a number of bands of energy, and forbidden from other regions. The term "band gap" refers to the energy difference between electrons residing in the two most important states of a material − valence band states and conduction band states. The band gap of a material largely determines its electrical and optical properties.

Because graphene does not naturally have a band gap, some fancy technical maneuvering is needed to allow graphene field-effect transistors (FETs) to be turned off effectively, so you can create digital logic. As a consequence, graphene must be modified to produce a band gap. Among the methods of controlling the band gap of graphene, doping methods show the most promise in terms of industrial scale feasibility.

Toward that end scientists from Korea’s Ulsan National Institute of Science and Technology (UNIST) have announced a method for the mass production of boron/nitrogen co-doped graphene nano-platelets, which, they claim, led to the fabrication of graphene-based FETs with semiconducting properties. According to the researchers, although the performance of the FET is not yet in the range of commercial silicon-based semiconductors, their work should be the proof of a new concept and is being described as “a great leap forward for studying graphene with band-gap opening."

By taking advantage of the unique electronic properties of graphene, Georgia Institute of Technology researchers now believe they're on track to connect networks of nanomachines powered by small amounts of scavenged energy. With antennas made from conventional materials like copper, communication between low-power nanomachines would be virtually impossible.

Based on a honeycomb network of carbon atoms, graphene could generate a type of electronic surface wave that would allow antennas just one micron long and 10 to 100 nanometers wide to do the work of much larger antennas. This is important because with the growth of Internet of Things applications, development of higher performance devices that can transmit and receive information more efficiently will become critical. While operating graphene nano-antennas have yet to be demonstrated, the researchers say their modeling and simulations show that nano-networks using the new approach are feasible using the new material to make a very small antenna that can radiate at much lower frequencies than classical metallic antennas of the same size.

The communications challenge, the scientists report, is that at the micron scale metallic antennas would have to operate at frequencies of hundreds of terahertz. While those frequencies might offer advantages in communication speed, their range would be limited by propagation losses to just a few micrometers. And they'd require more power than a nanomachine is likely to have.

At the risk of over-simplicity let me explain: when electrons in graphene are excited by an incoming electromagnetic wave, they start moving back and forth. Because of the unique properties of graphene, this oscillation of electrical charge results in a confined electromagnetic wave on top of the graphene layer. This is known as a surface plasmon polariton (SPP) wave − infrared or visible-frequency electromagnetic waves which travel along a metal-dielectric or metal-air interface.

Materials such as gold, silver and other noble metals also can support the propagation of SPP waves but only at much higher frequencies than graphene. Conventional materials such as copper don't support the waves.

By allowing electromagnetic propagation at lower terahertz frequencies, the SPP waves require less power − putting them within range of what might be feasible for nano-machines operated by energy harvesting technology.

In addition to giving nano-machines the ability to communicate, hundreds or thousands of graphene antenna-transceiver sets might be combined to help full-size cellular phones and Internet-connected devices communicate faster. The Georgia Tech researchers noted that data rates in current cellular systems are up to one gigabit-per-second in LTE advanced networks and 10 gigabits-per-second in the so-called millimeter wave or 60 gigahertz systems. They anticipate that graphene based systems will achieve data rates on the order of terabits-per-second in the terahertz band.

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Statements of fact and or opinions expressed in MarketEYE by its contributors are the responsibility of the authors alone and do not imply an opinion of the officers or the representatives of TTI, Inc.

Featured Contributor:
Murray Slovick

Murray Slovick

Murray Slovick is Editorial Director of Intelligent TechContent, an editorial services company that produces technical articles, white papers and social media posts for clients in the semiconductor/electronic design industry. Trained as an engineer, he has more than 20 years of experience as chief editor of award-winning publications covering various aspects of consumer electronics and semiconductor technology. He previously was Editorial Director at Hearst Business Media where he was responsible for the online and print content of Electronic Products, among other properties in the U.S. and China. He has also served as Executive Editor at CMP’s eeProductCenter and spent a decade as editor-in-chief of the IEEE flagship publication Spectrum.

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