Various methods of improving on/off ratio of FET based on CNTs and graphene. a, Thinning of MWCNTs and CNT bundles by applying bias. Reproduced with permission . Copyright 2001, American Association for the Advancement of Science. b, Schematic and SEM image of a region of the random network SWCNT channel. A striping technique was used to cut metallic CNT paths. Reproduced with permission . Copyright 2008, Nature Publishing Group. c, Separation of semiconducting CNTs and metallic CNTs by density-gradient method. Reproduced with permission . Copyright 2006, Nature Publishing Group. d, BLG transistor with top and bottom gate to open band gap. Applying perpendicular field from bottom gate, band gap of the BLG can opened up to 250 meV. Reproduced with permission . Copyright 2009, Nature Publishing Group. e, Graphene nanoribbons with a width below 10 nm were obtained by upzipping CNTs. By narrowing the width of graphene to a few nanometers, a quantum confinement effect of carriers happens to open the band gap. Reproduced with permission . Copyright 2009, Nature Publishing Group.