Many people think that the future of electronics is going to include graphene, a sheet of carbon just a single atom thick that allows electrons to move through as if they were massless. Some of the first electronic devices involving graphene have already been built in the lab, but graphene presents its own challenges. It's not naturally a semiconductor, and most devices based on it have had poor performance due to current leakage. Now, researchers have put together a simple graphene-based test device that can operate at over 1.2GHz, and they used it as the basis for an all-graphene frequency mixer. Although the performance still isn't good enough for real-world use, it's a major step in the right direction.

The device in question is called a ring oscillator. It has a few uses in actual devices, but the authors of the new paper say it is "the most important class of circuits used to evaluate the performance limits of any digital technology." The Wikipedia entry on the circuits calls it the electronic equivalent of a "Hello, world!" program.

Ring oscillators work by putting an odd number of NOT gates in series, with the last one feeding back to the first as input. Because of the odd number of gates, the output will always be the opposite of the input. But the output is always slightly delayed due to the intervening circuitry. As a result of these two properties, the output will oscillate between two values, with the frequency of the oscillation becoming a measure of the circuit's performance. Graphene ring oscillators have been built, but they've only managed to perform in the Megahertz range.

The new work creates a graphene ring oscillator (with gold wiring) by growing the graphene directly on the device via chemical vapor deposition. The features of the devices are large by today's standards—anywhere from one to three micrometers—but they were able to control them, allowing them to track how the devices' performance scaled with decreasing voltage size. Because the idea was to run the device as fast as possible, the fact that the graphene wasn't semiconducting was an advantage. The device could run as fast as it could shuffle its electrons around.

At 3µm features, they managed to get an oscillation at 350MHz. Two microns raised that to more than 600MHz, while one-micron features created a 1.2GHz device. That's nothing special for current silicon devices, but it's a major step for graphene-based hardware.

Previously, researchers built graphene-based frequency mixers (an analog processor), but these needed to be driven by an external oscillator. Now that they had a graphene oscillator, the team put together an all-graphene frequency mixer.

Overall, the device had one aspect that works better than traditional circuits: fluctuations in the input voltage didn't affect its performance that much. Fluctuations in the output were seven times smaller than what you'd get from a silicon circuit. Unfortunately, the actual speed of the device was limited by internal resistance and current leakage—exactly the sort of thing that graphene is expected to avoid. The authors blame this on contact resistance between the graphene and its substrate as well as charge impurities that influenced the electrons' travel.

If both of these things could be corrected and features could be scaled down further, the authors suggest that graphene-based circuitry could reach hundreds of GHz in some specialized, simplified circuits. These aren't going to replace your desktop processor since they'd be too simple and still dissipate too much energy, but these sorts of devices are currently used for microwave-frequency communications equipment. It could be that graphene finds a home here—if performance can reach that of the current material used in these devices, InP.

ACS NANO, 2013. DOI: 10.1021/nn401933v (About DOIs).