Inductors are everywhere, from your cell phone and laptop to your radio, television, and car. A basic building block of today’s technology, they are also a crucial component for realizing the potential of the Internet of Things (IoT), which promises to connect people to some 50 billion objects, by 2020, with an projected economic impact ranging from $2.7-$6.2 trillion annually by 2025.
Inductors today, however, are essentially the same as what English scientist Michael Faraday created in 1831, and because of that — and particularly the large size of inductors made according to Faraday’s design — they are a limiting factor in delivering miniaturized IoT devices.
Now, a team at the UC Santa Barbara College of Engineering, led by Kaustav Banerjee, professor in the Department of Electrical and Computer Engineering, has taken a materials-based approach to reinventing this fundamental component of modern electronics. The findings appear in a paper titled “On-chip intercalated-graphene inductors for next-generation radio frequency electronics,” published in Nature Electronics.
Banerjee and his UCSB team — lead author Jiahao Kang, Junkai Jiang, Xuejun Xie, Jae Hwan Chu, and Wei Liu, all members in Banerjee’s Nanoelectronics Research Lab — worked with colleagues from Shibaura Institute of Technology in Japan and Shanghai Jiao Tong University in China to exploit the phenomenon of kinetic inductance to demonstrate a fundamentally different kind of inductor.
Until now, all inductors have been made according to the same fundamental principle of magnetic inductance, which essentially opposes any fluctuations in the magnetic flux (total magnetic field flowing across the area of the inductor) caused by a change in the current through the inductor. Fluctuations in current can affect the performance of electronic components, but achieving sufficient magnetic inductance requires a large inductor area that encloses sufficient magnetic flux.
Historically, as the technology of transistors and interconnects that link them has advanced, the elements have become smaller, but the inductor, which, in its simplest form, is a metallic coil wound around a core material, has been the exception.
“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 first author Kang, who recently completed his PhD under Banerjee’s supervision.
In their paper, the researchers describe a new materials-based approach to miniaturizing high-performance inductors. The method exploits the physics of kinetic inductance, which, unlike magnetic inductance, does not depend on the surface area of the inductor. Rather, kinetic inductance resists current fluctuations that alter the velocity of the electrons, and the electrons resist such change according to Newton’s law of inertia.
All inductors generate both magnetic and kinetic inductance, but in typical metal conductors, the kinetic inductance is so small as to be unnoticeable. “The theory of kinetic inductance has long been known in condensed-matter physics, but nobody ever used it for inductors, because in conventional metallic conductors, kinetic inductance is negligible,” Banerjee explains.
That’s because metals have a low momentum relaxation time (MRT), which is the average time it takes for a charge carrier to lose its original momentum as a result of colliding with other particles. Kinetic inductance has a linear relationship to MRT, such that when MRT is small, the carriers collide more often with other particles and lose their kinetic energy quickly after colliding, thus reducing kinetic inductance.
Banerjee and his team took a materials approach to increasing kinetic inductance. They designed a new kind of “spiral inductor” that has no metal spool, but rather comprises multiple layers of graphene. Single-layer graphene exhibits a linear electronic band structure, and exhibits a correspondingly large MRT — a few picoseconds or even higher compared to that of conventional metallic conductors (like copper used in traditional on-chip inductors), which ranges from 1/1000 to 1/100 of a picosecond. But single-layer graphene has too much resistance for application on an inductor.
Multilayer graphene, however, offers a partial solution by providing lower resistance, but interlayer couplings cause its MRT to be insufficiently small. The researchers overcame that dilemma with a unique solution: they chemically inserted bromine atoms between the graphene layers (a process called intercalation). Doing so not only further reduced resistance, but also separated the graphene layers just enough to essentially decouple them, which extended the MRT, thereby increasing kinetic inductance.
The revolutionary inductor, which works in the 10-50 GHz range, offers one-and-a-half times the inductance density of a traditional inductor, leading to a one-third reduction in area, while also providing extremely high efficiency. Previously, high inductance and reduced size had been an elusive combination. Further, says co-author Jiang, “There is plenty of room to increase the inductance density further by increasing the efficiency of the intercalation process, which we are now exploring.”
“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,” added Banerjee.
Faraday invented the inductor while discovering the phenomenon of electromagnetic-induction nearly two hundred years ago, which subsequently proved crucial for practical large scale use of electricity for power and telegraph communication. Renewing the cycle, Banerjee and his colleagues are re-inventing it now to pave the way for next-generation communications, sensing, and energy storage for smarter living.