[show abstract][hide abstract] ABSTRACT: We propose that lateral heterostructures of single-atomic-layer graphene and hexagonal boron-carbon-nitrogen (hBCN) domains, can represent a powerful platform for the fabrication and the technological exploration of real two-dimensional field-effect transistors. Indeed, hBCN domains have an energy bandgap between 1 and 5 eV, and are lattice-matched with graphene; therefore they can be used in the channel of a FET to effectively inhibit charge transport when the transistor needs to be switched off. We show through ab initio and atomistic simulations that a FET with a graphene-hBCN-graphene heterostructure in the channel can exceed the requirements of the International Technology Roadmap for Semiconductors for logic transistors at the 10 and 7 nm technology nodes. Considering the main figures of merit for digital electronics, a FET with gate length of 7 nm at a supply voltage of 0.6 V exhibits I(on)/I(off) ratio larger than 10(4), intrinsic delay time of about 0.1 ps, and a power-delay-product close to 0.1 nJ/m. More complex graphene-hBCN heterostructures can allow the realization of different multifunctional devices, translating on a truly two-dimensional structure some of the device principles proposed during the first wave of nanoelectronics based on III-V heterostructures, as for example the resonant tunneling FET.
[show abstract][hide abstract] ABSTRACT: We present numerical simulations of high field transport in both suspended
and deposited armchair graphene nanoribbon (A-GNR) on HfO2 substrate. Drift
velocity in suspended GNR does not saturate at high electric field (F), but
rather decreases, showing a maximum for F=10 kV/cm. Deposition on HfO2 strongly
degrades the drift velocity by up to a factor of 10 with respect to suspended
GNRs in the low-field regime, whereas at high fields drift velocity approaches
the intrinsic value expected in suspended GNRs. Even in the assumption of
perfect edges, the obtained mobility is far behind what expected in
two-dimensional graphene, and is further reduced by surface optical phonons.
[show abstract][hide abstract] ABSTRACT: We investigate the low-field phonon-limited mobility in armchair graphene
nanoribbons (GNRs) using full-band electron and phonon dispersion relations. We
show that lateral confinement suppresses the intrinsic mobility of GNRs to
values typical of common bulk semiconductors, and very far from the impressive
experiments on 2D graphene. Suspended GNRs with a width of 1 nm exhibit a
mobility close to 500 cm^2/Vs at room temperature, whereas if the same GNRs are
deposited on HfO2 mobility is further reduced to about 60 cm^2/Vs due to
surface phonons. We also show the occurrence of polaron formation, leading to
band gap renormalization of ~118 meV for 1 nm-wide armchair GNRs.
[show abstract][hide abstract] ABSTRACT: We have investigated the main scattering mechanisms affecting mobility in
graphene nanoribbons using detailed atomistic simulations. We have considered
carrier scattering due to acoustic and optical phonons, edge roughness, single
defects, and ionized impurities, and we have defined a methodology based on
simulations of statistically meaningful ensembles of nanoribbon segments. Edge
disorder heavily affects mobility at room temperature in narrower nanoribbons,
whereas charged impurities and phonons are hardly the limiting factors. Results
are favorably compared to the few experiments available in the literature.
IEEE Transactions on Electron Devices 03/2011; 58(9). · 2.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present a full band investigation of electron-phonon interaction in Graphene NanoRibbons (GNRs) by exploiting a tight-binding description within the deformation potential approximation. We show that a full band approach is required to obtain accurate results: mobility as high as 800 cm<sup>2</sup>/Vs at room temperature can be achieved for 1 nm-wide ribbons, more than one order of magnitude higher than that obtainable in silicon nanowires, but still not enough to ensure ballistic transport in GNR-based devices.
Electron Devices Meeting (IEDM), 2010 IEEE International; 01/2011
[show abstract][hide abstract] ABSTRACT: We present a multi-scale investigation of graphene-based transistors with a hexagonal boron-carbon-nitride (h-BCN) barrier in the channel. Our approach exploits ab-initio calculations for an accurate extraction of energy bands and tight-binding simulations in order to compute charge transport. We show that the h-BCN barrier inhibits the ambipolar behavior of graphene transistors, leading to a large Ion/Ioff ratio, within the ITRS roadmap specifications for future semiconductor technology nodes.
[show abstract][hide abstract] ABSTRACT: We discuss the shot noise properties of carbon-based transistors in which the channel is laterally confined, either in the form of graphene nanoribbons or of carbon nanotubes. We show with an simple compact model and with computationally- intensive statistical simulations that electron-electron interaction can lead to a significant suppression of shot noise, often overlooked when the device is described with the Landauer- Buttiker formalism. Finally, we show that interband tunneling can play a significant role in enhancing shot noise due to exchange of holes between drain and channel, that is a peculiar effect observable in the case of channel materials with very small energy gaps.
[show abstract][hide abstract] ABSTRACT: Performance analysis of Field Effect Transistors based on hydrogenated graphene is performed through a multi-scale approach based on calculations of the energy bands by means of GW approximation, a three-nearest neighbor (3NN) sp<sup>3</sup> tight-binding Hamiltonian, and a ballistic transport model. The considered device exhibits large I<sub>on</sub> and I<sub>on</sub>/I<sub>off</sub> ratios, due to the large bandgap, with reduced lithographic constrains as compared to one-dimensional channels.
Computational Electronics (IWCE), 2010 14th International Workshop on; 11/2010
[show abstract][hide abstract] ABSTRACT: We predict the possibility of shot noise enhancement in defect-free Carbon Nanotube Field Effect Transistors, through a numerical investigation based on Monte Carlo simulations of randomly injected electrons from the reservoirs and the self-consistent solution of the Poisson and Schrodinger equations within the non-equilibrium Green's functions formalism. Such enhancement can be explained by a positive correlation between holes trapped in quasi-bound states in the channel valence band and thermionic electrons injected from the source and can yield a remarkable Fano factor at room temperature equal to 1.22.
Computational Electronics (IWCE), 2010 14th International Workshop on; 11/2010
[show abstract][hide abstract] ABSTRACT: We discuss an intriguing set of transport and noise properties of graphene-based transistors that can be investigated in a direct way with atomistic modeling - Non-Equilibrium Green's Functions with a Tight-Binding Hamiltonian - and are not directly accessible with models based on a higher level of physical abstraction. We present an investigation of the achievable electron mobility in channels based on graphene nanoribbons with realistic imperfections. Then, we will discuss how the small gap and small density of states of bilayer graphene can be used to design tunnel FETs with extremely steep subthreshold slope. Then, as far as noise is concerned, we will show the impact of electron-electron interaction and of interband transitions in enhancing the channel noise of FETs based on small-gap carbon nanotubes.
Simulation of Semiconductor Processes and Devices (SISPAD), 2010 International Conference on; 10/2010
[show abstract][hide abstract] ABSTRACT: In this work, we present a performance analysis of Field Effect Transistors based on recently fabricated 100% hydrogenated graphene (the so-called graphane) and theoretically predicted semi-hydrogenated graphene (i.e. graphone). The approach is based on accurate calculations of the energy bands by means of GW approximation, subsequently fitted with a three-nearest neighbor (3NN) sp3 tight-binding Hamiltonian, and finally used to compute ballistic transport in transistors based on functionalized graphene. Due to the large energy gap, the proposed devices have many of the advantages provided by one-dimensional graphene nanoribbon FETs, such as large Ion and Ion/Ioff ratios, reduced band-to-band tunneling, without the corresponding disadvantages in terms of prohibitive lithography and patterning requirements for circuit integration.
[show abstract][hide abstract] ABSTRACT: We present an investigation of the main mechanisms which limit mobility in GNR-FETs, by means of atomistic simulations based on the NEGF formalism. In particular, we focus on i) line edge roughness (LER), ii) single defects; iii) ionized impurities, iv) acoustic and optical phonons. Results show that the effect of ionized impurities is negligible, while phonons, LER and defects largely limits carrier mobility, especially for narrower GNRs.
Electron Devices Meeting (IEDM), 2009 IEEE International; 01/2010
[show abstract][hide abstract] ABSTRACT: We present an expression for the shot noise power spectral density in quasi-one dimensional conductors electrostatically controlled by a gate electrode, which includes the effects of Coulomb interaction and of Pauli exclusion among charge carriers. In this sense, our expression extends the well known Landauer-Büttiker noise formula to include the effect of Coulomb interaction inducing fluctuations of the potential in the device region. Our approach is based on evaluating the statistical properties of the scattering matrix and on a second-quantization many-body description. From a quantitative point of view, statistical properties are obtained by means of Monte Carlo simulations on an ensemble of different configurations of injected states, requiring the solution of the Poisson-Schrödinger equation on a three-dimensional grid, with the nonequilibrium Green's functions formalism. In a series of examples, we show that failure to consider the effects of Coulomb interaction on noise leads to a gross overestimation of the noise spectrum of quasi-one-dimensional devices.
[show abstract][hide abstract] ABSTRACT: We predict shot noise enhancement in defect-free carbon nanotube field-effect transistors through a numerical investigation based on the self-consistent solution of the Poisson and Schrödinger equations within the nonequilibrium Green’s functions formalism, and on a Monte Carlo approach to reproduce injection statistics. Noise enhancement is due to the correlation between trapping of holes from the drain into quasibound states in the channel and thermionic injection of electrons from the source, and can lead to an appreciable Fano factor of 1.22 at room temperature.
[show abstract][hide abstract] ABSTRACT: In this paper we show how numerical and analytical modeling of graphene-based devices is used to consider possible approaches to engineer a gap in graphene and to evaluate the perspectives of different technological options towards graphene nanoelectronics.
Electron Devices Meeting (IEDM), 2009 IEEE International; 12/2009
[show abstract][hide abstract] ABSTRACT: We present a novel method for the evaluation of shot noise in quasi-1-D field-effect transistors, such as those based on carbon nanotubes and silicon nanowires. The method is derived by using a statistical approach within the second quantization formalism and allows the inclusion of both the effects of Pauli exclusion and Coulomb repulsion among charge carriers. This way, it extends the Landauer-Buttiker approach by explicitly including the effect of Coulomb repulsion on noise. We implement the method through the self-consistent solution of the 3-D Poisson and transport equations within the nonequilibrium Green's function framework and a Monte Carlo procedure for populating injected electron states. We show that the combined effect of Pauli and Coulomb interactions reduces shot noise in strong inversion down to 23% of the full shot noise for a gate overdrive of 0.4 V, and that neglecting the effect of Coulomb repulsion would lead to an overestimation of noise up to 180%.
IEEE Transactions on Electron Devices 10/2009; · 2.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present an expression for the shot noise power spectral density in quasi-one dimensional conductors electrostatically controlled by a gate electrode, that includes the effects of Coulomb interaction and of Pauli exclusion among charge carriers. In this sense, our expression extends the well known Landauer-Buttiker noise formula to include the effect of Coulomb interaction through induced fluctuations in the device potential. Our approach is based on the introduction of statistical properties of the scattering matrix and on a second-quantization many-body description. From a quantitative point of view, statistical properties are obtained by means of Monte Carlo simulations on a ensemble of different configurations of injected states, requiring the solution of the Poisson-Schrodinger equation on a three-dimensional grid, with the non-equilibrium Green functions formalism. In a series of example, we show that failure to consider the effects of Coulomb interaction on noise leads to a gross overestimation of the noise spectrum of quasi-one dimensional devices.
[show abstract][hide abstract] ABSTRACT: We present a novel method for the evaluation of shot noise in quasi one-dimensional field-effect transistors, derived by means of a statistical approach within the second quantization formalism, which manages to include both the effects of Pauli exclusion and Coulomb interactions. The method has been applied to Carbon Nanotubes and Silicon Nanowire Transistors. We show that noise can significantly differ from that obtained by means of the Landauer-Biittiker's formula and that the main noise source is represented by the partition noise.
[show abstract][hide abstract] ABSTRACT: We propose a novel and general method to investigate shot noise in nanoscale devices by means of Monte Carlo simulations within the self-consistent 3D Poisson-NEGF framework, focusing our attention on Carbon Nanotube and Silicon Nanowire Field effect transistors. We will show that Pauli exclusion principle and Coulomb interactions play an important role in device electrical behavior. In particular, their combined effect leads to a reduction of shot noise in strong inversion down to 23% of the full shot power spectral density for a gate overdrive of 0.4 V.
Electron Devices Meeting, 2008. IEDM 2008. IEEE International; 01/2009