Graphene gate

Abstract

Researchers from Consorzio Nazionale Interuniversitario per le Telecomunicazion, the University of Palermo, University of Münster, Yonsei University, and the Karlsruhe Institute of Technology, have reported an in-depth statistical and parametrical investigation on the microwave performance of graphene field effect transistors (GFETs) on sapphire substrates. Their investigation led to the discovery of an optimal region in the gate parameters that maximises microwave performance. Microwave bench developed to perform GFETs characterizations. Sketch of the GFET device; (right) Micrograph of a fabricated sample. First fabricated in 2004, graphene represents the first truly two-dimensional atomic crystal available to the scientific community. As such, it is still one of the most investigated materials, showing exceptional electrical and thermal conductivity, high carrier mobility and saturation velocity, strength and elasticity, wavelength-independent optical absorption coefficient, and so on. These properties suggest that graphene could replace, in the next few years, more conventional materials in many fields, such as high-frequency electronics and photonics. In fact, graphene has already been successfully applied in energy conversion devices (such as solar cells and batteries) and in flexible electronics (such as touch screens). While graphene is certainly very promising, there have been problems in its implementation. Marco Angelo Giambra, one of the authors of the research, told us that “as a general rule, the development of new materials deeply relies on the progress of the production techniques, due to the need of finding the best compromise for each particular application between quality and cost of the final product. This is also true for graphene production, in which additional difficulties arise from its intrinsic 2D nature. For this reason, despite its appealing properties, graphene will be able to rebuild from scratch the well assessed industrial processes only when it will be competitive enough in terms of applications and production costs.” The properties listed above make graphene particularly suitable for high-frequency transistors. However, as Giambra explained, “in this field, graphene has to fight with more mature technologies, such as III-V materials. For the realization of high performance microwave FETs, the active material has to satisfy different peculiarities, such as wide bandgap, excellent carrier transport properties, high thermal conductivity, possibility of being “friendly”, produced on large-area substrates with processes compatible with Si CMOS technology, possibility of interfacing with good dielectric materials, and low contact resistance. All of these characteristics could be potentially satisfied and, in the near future, overtaken by graphene.” Geometry and layout play an important role in the performance of these microwave devices, and this is what the team investigated in their Letter: “In order to perform a meaningful comparison among different GFET geometries in terms of microwave parameters,” said Giambra, “particular attention has been paid to their layout design. In detail, to fulfil the primary task of this work, great care was taken to design drain, source, and gate launching structures, to reduce the parasitic elements, decrease their negative influence on the device performance and, in addition, simplify and make more accurate the identification of the intrinsic transistor in the subsequent measurements de-embedding phase. Moreover, since we are dealing with a 2D material for applications in high frequency electronics, the choice of the substrate plays a fundamental role. Graphene charge mobility, in fact, is strictly connected to the scattering induced by the substrate. An optimal substrate should be flat, insulating, and free of charge traps. Since sapphire possesses all these properties, it has been chosen as the substrate of our GFETs.” After fabrication, the team's measurements allowed them to infer that the cut-off frequency does not monotonically increase when scaling the device geometry; instead an optimal region in the gate-drain/source and gate length space exists which maximises the microwave performance. Given this research, and other successes in a number of applications, Giambra is confident in graphene's future. “There is no doubt that graphene will be integrated into industrial production processes in the near future, providing wide benefits,” he said. When the technology matures, graphene will be able, according to Giambra, “in principle to be completely integrated with more conventional materials in several fields, such as high-frequency electronics, photonics, and energy conversion with possible extensive usage in ultra-high speed transistors, solar cells, batteries, and touch screens and other flexible devices.”