Inductors are one of the primary components in electronic circuitry and have been since the earliest days of electricity. Inductors serve to store electrical energy in magnetic fields when electrical currents pass through them - the traditional design is simply a coil of wire surrounding a magnetic core.
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Given that they resist changes to the current that flows through them, and can provide voltages to circuits, they form an essential part of all kinds of electrical components.
Two inductors in close proximity are essentially a transformer: used to step-up and step-down voltages across the world’s electrical grids. Inductors are also used in virtually every signal-filtering system. Their design has not fundamentally changed since Faraday in the 19th century, with mostly tweaks to components rather than the original design.
This means that inductors are becoming a bottleneck in the miniaturization of electrical circuits. If the Internet of Things, which evangelizes about inserting chips and electrical circuits into every device, is to become a reality – we will need to get around this bottleneck. Now, new research led by Kaustav Banerjee, a professor in the Department of Electrical and Computer Engineering at UC Santa Barbara, has started the process of fundamentally redesigning this component.
"On-chip inductors based on magnetic inductance cannot be made smaller in the same way transistors or interconnects scale, because you need a certain amount of surface area to get a certain magnetic flux or inductance value," explained Jiahao Kang who recently completed his Ph.D. under Banerjee's supervision and was the lead author of the group’s paper in Nature Electronics.
All inductors have two components to their inductance – magnetic and kinetic inductance. Magnetic inductance is due to the resistance of the coil of wire to changes in current because of the electromagnetic forces in the wire. Kinetic inductance is that resistance due to the actual, inertial mass of the charge carriers.
As charge carriers in inductors are typically electrons, their mass and hence kinetic inductance is usually negligible – unless you’re alternating currents at very high frequencies (and consequently shifting the direction of travel of the electrons very rapidly) or else in superconductors. The kinetic inductance has therefore typically been minimal compared to the magnetic inductance in inductors and has mainly found applications in superconductors.
The key to kinetic inductance is that it’s independent of the surface area, which influences the amount of magnetic flux cut by the inductors. If materials with a high kinetic inductance could be constructed, the surface area would no longer be a restriction on the inductance of small coils.
The UCSB team set out to do this with a spiral inductor composed of multiple layers of graphene. A single layer of graphene has a large momentum relaxation time – typically hundreds or thousands of times larger than the traditional coils of copper used in inductors. This means that its kinetic inductance – the inertial resistance of the electron momentum to changes in current – can be high. But single-layer graphene has a resistance that’s too high for miniature electronic components.
The team sought to solve this with their multiple-layers; this reduces resistance, but couplings between the layers can decrease the kinetic inductance again. To fix this, they chemically inserted atoms of bromine between the layers which served to decouple them by separating them in space, increasing the inductance while maintaining a low resistance. This process of inserting the bromine atoms is called intercalcation.
"There is plenty of room to increase the inductance density further by increasing the efficiency of the intercalation process, which we are now exploring," said co-author Junkai Jiang.
"We essentially engineered a new nanomaterial to bring forward the previously 'hidden physics' of kinetic inductance at room temperature and in a range of operating frequencies targeted for next-generation wireless communications," Banerjee added.
This inductor works at 10-50GHz and offers 1.5x the inductance density of previous inductors, with a one-third reduction in surface area to achieve the same inductance. The team, which researched in collaboration with colleagues from Shibaura Institute of Technology in Japan and Shanghai Jiao Tong University in China, hopes that their new approach – exploiting kinetic inductance, rather than magnetic inductance – will see applications in nanocircuitry in the future.