Simulation of Graphene Nanoribbon Field Effect Transistors

We present an atomistic three-dimensional simulation of graphene nanoribbon field effect transistors (GNR-FETs), based on the self-consistent solution of the 3D Poisson and Schroedinger equation with open boundary conditions within the non-equilibrium Green's Function formalism and a tight-binding hamiltonian. With respect to carbon nanotube FETs, GNR-FETs exhibit comparable performance, reduced sensitivity on the variability of channel chirality, and similar leakage problems due to band-to-band tunneling. Acceptable transistor performance requires effective nanoribbon width of 1-2 nm, that could be obtained with periodic etching patterns or stress patterns

Extending ballistic graphene FET lumped element models to diffusive devices

In this work, a modified, lumped element graphene field effect device model is presented. The model is based on the "Top-of-the-barrier" approach which is usually valid only for ballistic graphene nanotransistors. Proper modifications are introduced to extend the model's validity so that it accurately describes both ballistic and diffusive graphene devices. The model is compared to data already presented in the literature. It is shown that a good agreement is obtained for both nano-sized and large area graphene based channels. Accurate prediction of drain current and transconductance for both cases is obtained

Monolithically Patterned Wide-Narrow-Wide All-Graphene Devices

We investigate theoretically the performance advantages of all-graphene nanoribbon field-effect transistors (GNRFETs) whose channel and source/drain (contact) regions are patterned monolithically from a two-dimensional single sheet of graphene. In our simulated devices, the source/drain and interconnect regions are composed of wide graphene nanoribbon (GNR) sections that are semimetallic, while the channel regions consist of narrow GNR sections that open semiconducting bandgaps. Our simulation employs a fully atomistic model of the device, contact and interfacial regions using tight-binding theory. The electronic structures are coupled with a self-consistent three-dimensional Poisson's equation to capture the nontrivial contact electrostatics, along with a quantum kinetic formulation of transport based on non-equilibrium Green's functions (NEGF). Although we only consider a specific device geometry, our results establish several general performance advantages of such monolithic devices (besides those related to fabrication and patterning), namely the improved electrostatics, suppressed short-channel effects, and Ohmic contacts at the narrow-to-wide interfaces.Comment: 9 pages, 11 figures, 2 table

Introduction to Graphene Electronics -- A New Era of Digital Transistors and Devices

The speed of silicon-based transistors has reached an impasse in the recent decade, primarily due to scaling techniques and the short-channel effect. Conversely, graphene (a revolutionary new material possessing an atomic thickness) has been shown to exhibit a promising value for electrical conductivity. Graphene would thus appear to alleviate some of the drawbacks associated with silicon-based transistors. It is for this reason why such a material is considered one of the most prominent candidates to replace silicon within nano-scale transistors. The major crux here, is that graphene is intrinsically gapless, and yet, transistors require a band-gap pertaining to a well-defined ON/OFF logical state. Therefore, exactly as to how one would create this band-gap in graphene allotropes is an intensive area of growing research. Existing methods include nano-ribbons, bilayer and multi-layer structures, carbon nanotubes, as well as the usage of the graphene substrates. Graphene transistors can generally be classified according to two working principles. The first is that a single graphene layer, nanoribbon or carbon nanotube can act as a transistor channel, with current being transported along the horizontal axis. The second mechanism is regarded as tunneling, whether this be band-to-band on a single graphene layer, or vertically between adjacent graphene layers. The high-frequency graphene amplifier is another talking point in recent research, since it does not require a clear ON/OFF state, as with logical electronics. This paper reviews both the physical properties and manufacturing methodologies of graphene, as well as graphene-based electronic devices, transistors, and high-frequency amplifiers from past to present studies. Finally, we provide possible perspectives with regards to future developments.Comment: This is an updated version of our review article, due to be published in Contemporary Physics (Sept 2013). Included are updated references, along with a few minor corrections. (45 pages, 19 figures

External Bias Dependent Direct To Indirect Bandgap Transition in Graphene Nanoribbon

In this work, using self-consistent tight-binding calculations, for the first time, we show that a direct to indirect bandgap transition is possible in an armchair graphene nanoribbon by the application of an external bias along the width of the ribbon, opening up the possibility of new device applications. With the help of Dirac equation, we qualitatively explain this bandgap transition using the asymmetry in the spatial distribution of the perturbation potential produced inside the nanoribbon by the external bias. This is followed by the verification of the bandgap trends with a numerical technique using Magnus expansion of matrix exponentials. Finally, we show that the carrier effective masses possess tunable sharp characters in the vicinity of the bandgap transition points.Comment: Accepted for publication in Nano Letter

Atomistic Boron-Doped Graphene Field Effect Transistors: A Route towards Unipolar Characteristics

We report fully quantum simulations of realistic models of boron-doped graphene-based field effect transistors, including atomistic details based on DFT calculations. We show that the self-consistent solution of the three-dimensional (3D) Poisson and Schr\"odinger equations with a representation in terms of a tight-binding Hamiltonian manages to accurately reproduce the DFT results for an isolated boron-doped graphene nanoribbon. Using a 3D Poisson/Schr\"odinger solver within the Non-Equilibrium Green's Functions (NEGF) formalism, self-consistent calculations of the gate-screened scattering potentials induced by the boron impurities have been performed, allowing the theoretical exploration of the tunability of transistor characteristics. The boron-doped graphene transistors are found to approach unipolar behavior as the boron concentration is increased, and by tuning the density of chemical dopants the electron-hole transport asymmetry can be finely adjusted. Correspondingly, the onset of a mobility gap in the device is observed. Although the computed asymmetries are not sufficient to warrant proper device operation, our results represent an initial step in the direction of improved transfer characteristics and, in particular, the developed simulation strategy is a powerful new tool for modeling doped graphene nanostructures.Comment: 7 pages, 5 figures, published in ACS Nan