17.  Graphene-Based Nanoelectronics

The ultrahigh room-temperature carrier mobility in graphene makes it very useful for microwave and high-frequency devices. Additionally, high current-carrying capability >10⁸ A/cm² together with high thermal conductivity ~2000–4000 Wm⁻¹K⁻¹ for freely suspended graphene and ~600 Wm⁻¹K⁻¹ for SiO₂-supported graphene establish its superiority among nanoelectronic materials. Graphene flakes are easily isolated from graphite by mechanical exfoliation. Graphene can be grown on metal films and transferred to desired substrates. It can be grown epitaxially on silicon carbide. Graphene sheets can also be synthesized by a substrate-free process in the gas phase. Planarity of graphene makes widely practiced planar processes of semiconductor industry applicable to graphene. On the downside, the bandgap of graphene is zero. Hence, graphene transistors cannot be switched off effectively. However, single-layer graphene transistors show excellent performance in GHz analog circuits. By quantum confinement, a bandgap is opened in graphene when cut into nanoribbons. Bandgap is also created by applying a perpendicular electric field to bilayer graphene. However, carrier mobility in nanoribbons is lower than in large-area graphene. Present status of graphene nanoribbon and bilayer transistors is described. Although they display higher on-off current ratios than transistors fabricated on original graphene, intensive efforts are required to realize the full potentiality of graphene for nanoelectronics.