Professor Kaustav Banerjee, leading the team at UC Santa Barbara (UCSB) in this project, explained that in typical metal conductors the kinetic inductance is so small it goes unnoticed. "The theory of kinetic inductance has long been known in condensed-matter physics,” he said. “But nobody ever used it for inductors because in conventional metallic conductors, kinetic inductance is negligible.”
Unlike magnetic inductance, kinetic inductance does not rely on the surface area of the inductor. Instead, it resists current fluctuations that alter the velocity of the electrons and according to Newton's law of inertia, the electrons resist such a change.
Prof Banerjee explained that as the links between transistors and interconnects have advanced, the elements have become smaller, yet the inductor has remained an 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," lead author, Jiahao Kang, added.
Single-layer graphene exhibits a linear electronic band structure and a correspondingly large momentum relaxation time (MRT) compared to conventional metallic conductors. USBC explained that the issue is a single-layer graphene has too much resistance for application on an inductor.
To overcome this, the team developed a spiral inductor comprising multiple layers of graphene. It claimed this offered a partial solution due to the lower resistance it provided. By chemically inserted bromine atoms between the graphene layers, the team said they were able to resolve the insufficiently small MRT, caused by the interlayer couplings. They added that this also further reduced the resistance, while decoupling the layers.
UCSB has said its inductor works in the 10 to 50GHz range and offers one-and-a-half times the inductance density of a traditional inductor. According to the team, this leads to a one-third reduction in area, while also providing extremely high efficiency, which they describe as a ‘previously elusive combination’.
"There is plenty of room to increase the inductance density further by increasing the efficiency of the intercalation process, which we are now exploring," ventured 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 concluded.