We examine the performance limits of field-effect transistors (FETs) with chemically modified graphene as the channel materials. Graphene nanoroad (XNR) and graphene nanomesh (XNM) can be created through selective chemical modification by an X adsorbate (either H or F) on graphene, which generates a bandgap while conserving the continuous two-dimensional (2D) atomistic layer. We adopt a ballistic transistor model, where the band structures were calculated using ab initio simulations to assess the performance of graphene nanoroad and nanomesh transistors. It is shown that arrays of graphene nanoroads, defined by hydrogenation or fluorination of atomically narrow dimer lines in a 2D graphene, are most ideal for transistor channel materials in terms of delivering a large ON-current, and significantly outperform Si metal-oxide-semiconductor field-effect transistors (MOSFETs). Alternatively, comparable performance to silicon can be achieved by careful design of a graphene nanomesh through patterned hydrogenation or fluorination. Both hydrogenation and fluorination lead to similar transistor performance, with fluorination more preferred in terms of chemical energetics.

High frequency performance limits of graphene field-effect transistors (FETs) down to a channel length of 20 nm have been examined by using self-consistent quantum simulations. The results indicate that although Klein band-to-band tunneling is significant for sub-100 nm graphene FETs, it is possible to achieve a good transconductance and ballistic on–off ratio larger than 3 even at a channel length of 20 nm. At a channel length of 20 nm, the intrinsic cut-off frequency remains at a few THz for various gate insulator thickness values, but a thin gate insulator is necessary for a good transconductance and smaller degradation of cut-off frequency in the presence of parasitic capacitance. The intrinsic cut-off frequency is close to the LC characteristic frequency set by graphene kinetic inductance (L) and quantum capacitance (C), which is about 100 GHz·μm divided by the gate length.

The band structures of strained graphene nanoribbons (GNRs) are examined using a tight-binding Hamiltonian that is directly related to the type and magnitude of strain. Compared to a two-dimensional graphene whose band gap remains close to zero even if a large strain is applied, the band gap of a graphene nanoribbon (GNR) is sensitive to both uniaxial and shear strains. The effect of strain on the electronic structure of a GNR depends strongly on its edge shape and structural indices. For an armchair GNR, a weak uniaxial strain changes the band gap in a linear fashion, whereas a large strain results in periodic oscillation of the band gap. On the other hand, shear strain always tends to reduce the band gap. For a zigzag GNR, the effect of strain is to change the spin polarization at the edges of GNR, and thereby modulate the band gap. A simple analytical model, which agrees with the numerical results, is proposed to interpret the response of the band gap to strain in armchair GNRs.

The performance limits of a multilayer graphene nanoribbon (GNR) field-effect transistor (FET) are assessed and compared with those of a monolayer GNRFET and a carbon nanotube (CNT) FET. The results show that with a thin high dielectric constant (high-κ) gate insulator and reduced interlayer coupling, a multilayer GNRFET can significantly outperform its CNT counterpart with a similar gate and bandgap in terms of the ballistic on-current. In the presence of optical phonon scattering, which has a short mean free path in the graphene-derived nanostructures, the advantage of the multilayer GNRFET is even more significant. Simulation results indicate that multilayer GNRs with incommensurate non-AB stacking and weak interlayer coupling are the best candidates for high-performance GNRFETs.

We present a semi-analytical model incorporating the effects of edge bond relaxation, the third nearest neighbor interactions, and edge scattering in graphene nanoribbon field-effect transistors (GNRFETs) with armchair-edge GNR (AGNR) channels. Unlike carbon nanotubes (CNTs) which do not have edges, the existence of edges in the AGNRs has a significant effect on the quantum capacitance and ballistic I–V characteristics of GNRFETs. For an AGNR with an index of m=3p, the band gap decreases and the ON current increases whereas for an AGNR with an index of m=3p+1, the quantum capacitance increases and the ON current decreases. The effect of edge scattering, which reduces the ON current, is also included in the model.