Graphene mixer can speed up future electronics

Researchers at Chalmers University of Technology (Sweden) have for the first time demonstrated a novel subharmonic graphene FET mixer at microwave frequencies. The mixer provides new opportunities in future electronics, as it enables compact circuit technology, potential to reach high frequencies and integration with silicon technology.

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​A mixer is a key building block in all electronic systems – a device that combines two or more electronic signals into one or two composite output signals. Future applications at THz frequencies such as radar systems for security and safety, radio astronomy, process monitoring and environmental monitoring will require large arrays of mixers for high-resolution imaging and high-speed data acquisition. Such mixer arrays or multi-pixel receivers need new type of devices that are not only sensitive but also power-efficient and compact.

The ability in graphene to switch between hole or electron carrier transport via the field effect enables a unique niche for graphene for RF IC applications. Thanks to this symmetrical electrical characteristic, the researchers at Chalmers have managed to build the G-FET subharmonic resistive mixer using only one transistor. Hence, no extra feeding circuits are required, which makes the mixer circuit more compact as opposed to conventional mixers. As a consequence, the new type of mixer requires less wafer area when constructed and can open up for advanced sensor arrays, for example for imaging at millimetre waves and even sub millimetre waves as G-FET technology progress.

Schematic picture of a subharmonic graphene-FET mixer. The LO and RF signals are fed to the gate and drain terminals, respectively, and the IF signal is extracted from the drain terminal.

"The performance of the mixer can be improved by further optimising the circuit, as well as fabricating a G-FET device with a higher on-off current ratio", says Jan Stake, professor of the research team. "Using a G‐FET in this new topology enables us to extend its operation to higher frequencies, thereby exploiting the exceptional properties of graphene. This paves the way for future technologies operating at extremely high frequencies."

In addition to enabling compact circuits, the G-FET provides potential to reach high frequencies thanks to the high velocity in graphene, and the fact that a subharmonic mixer only requires half the local oscillator (LO) frequency compared to a fundamental mixer. This property is attractive especially at high frequencies (THz) where there is a lack of sources providing sufficient LO-power.

Moreover, the G-FET can be integrated with silicon technology. For example, it is CMOS compatible (Complementary Metal Oxide Semiconductor) and among other things it can be used in CMOS electronics for backend processing on a single chip.

How Molecules Move Energy After Light Absorption

Chemists Solve an 84-Year-Old Theory On How Molecules Move Energy After Light Absorption

ScienceDaily (Dec. 22, 2011) — The same principle that causes figure skaters to spin faster as they draw their arms into their bodies has now been used by Michigan State University researchers to understand how molecules move energy around following the absorption of light.

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Conservation of angular momentum is a fundamental property of nature, one that astronomers use to detect the presence of satellites circling distant planets. In 1927, it was proposed that this principle should apply to chemical reactions, but a clear demonstration has never been achieved.

In the current issue of Science, MSU chemist Jim McCusker demonstrates for the first time the effect is real and also suggests how scientists could use it to control and predict chemical reaction pathways in general.

"The idea has floated around for decades and has been implicitly invoked in a variety of contexts, but no one had ever come up with a chemical system that could demonstrate whether or not the underlying concept was valid," McCusker said. "Our result not only validates the idea, but it really allows us to start thinking about chemical reactions from an entirely different perspective."

The experiment involved the preparation of two closely related molecules that were specifically designed to undergo a chemical reaction known as fluorescence resonance energy transfer, or FRET. Upon absorption of light, the system is predisposed to transfer that energy from one part of the molecule to another.

McCusker's team changed the identity of one of the atoms in the molecule from chromium to cobalt. This altered the molecule's properties and shut down the reaction. The absence of any detectable energy transfer in the cobalt-containing compound confirmed the hypothesis.

"What we have successfully conducted is a proof-of-principle experiment," McCusker said. "One can easily imagine employing these ideas to other chemical processes, and we're actually exploring some of these avenues in my group right now."

The researchers believe their results could impact a variety of fields including molecular electronics, biology and energy science through the development of new types of chemical reactions.

Dong Guo, a postdoctoral researcher, and Troy Knight, former graduate student and now research scientist at Dow Chemical, were part of McCusker's team. Funding was provided by the National Science Foundation.