Gated Graphene Nanoribbons Research Articles

Effectiveness of Graphene Nano-Ribbon Tunnel Field Effect Transistor for Bio-Molecular Identification

This paper examines the Double gate Graphene Nano-ribbon Tunnel Field-Effect Transistor (DG GNR TFET) for bio-sensing applications. The biomolecules are Streptavidin, APTES, Biotin, DNA and protein. Differentiation between biomolecules is possible based on dielectric permittivity and charge concentration. The protein switching ratio ([Formula: see text]/[Formula: see text] is 25.7 times that of Streptavidin and around 300 times that without any biomolecule in the cavity. The maximum current sensitivity among the biomolecules is achieved by protein biomolecule for VGS = 0.7[Formula: see text]V. The sub-threshold swing obtained for a biomolecule in DG GNR TFET is 40 mV/dec compared to that of 80[Formula: see text]mV/dec for an empty cavity. The device characteristics reveal that the drain current is maximum with a biomolecule in a cavity compared to an empty cavity. Results obtained emphasize that double gate Graphene Nano-Ribbon TFET is suitable as a biosensor. The simulations are executed using Silvaco Technology Computer Aided Design (TCAD).

Simulation insights of a new dual gate graphene nano-ribbon tunnel field-effect transistors for THz applications

This paper introduces a dual gate Graphene Nano-Ribbon Tunnel Field Effect Transistor (DG GNR TFET) structure and its performance characteristics are analysed. Technology Computer-Aided Design (TCAD) tool is used to simulate this novel non-silicon GNR TFET and to characterize various parameters of the device. A DG GNR TFET shows better device performance in terms of its surface potential, electric field and ON current (ION). The simulation results show that the proposed structure exhibits better ION/IOFF (109), average sub-threshold swing (43.16 mv/Dec) and threshold (0.3 V) when compared to a conventional Silicon TFET. Design parameters for the device such as dielectric thickness, body thickness, work function, source doping concentration, and channel length are analysed to deduce the fact that GNR can be used as an effective material in TFETs for low power electronics applications. Moreover, key parameters such as transconductance, transconductance generation efficiency and cut-off frequency (THz) are greatly improved in this proposed structure, which is highly desirable for analog/RF applications.

Tuning Anderson localization of edge-mode graphene plasmons in randomly gated nanoribbons.

Edge-mode graphene plasmons (EGPs) supported by graphene nanoribbons are highly confined, and they can show versatile tunability under electrostatic bias. In order to efficiently enhance and actively control the near-field intensity in integrated plasmonic devices, we theoretically study Anderson localization of EGPs in a graphene nanoribbon with an underlying electrode array in this work. By randomly arranging the electrodes in the array, positional disorder is introduced in the graphene nanoribbon system. Consequently, the Anderson localization of EGPs occurs with an exponentially decreased electric field, reduced propagation length, and rapid disappearance of the cross-correlation coefficient. Physically, inhomogeneous gating effectively creates a disordered distribution of Fermi levels in the graphene nanoribbon, which provides adequate fluctuation of the effective refractive index and results in strong localization of the EGPs at mid-infrared regime. By changing electrode array arrangements, the EGPs can be trapped at distinct locations in the nanoribbon. Further considering that the Fermi-level disorder can be introduced by randomly modulating the electrostatic bias, we apply different gate voltages at different electrodes in the array. Electrically tunable Anderson localization of EGPs are eventually realized in those randomly gated nanoribbons. Moreover, by combining both the positional and Fermi-level disorders in the system, the Anderson localization becomes more actively controlled in this electrically gated graphene nanoribbons. It is shown that the local field can be selectively trapped at single distinct location, or even several locations along the graphene nanoribbon. This investigation extends the Anderson localization to the EGPs in the mid-infrared range and enriches the graphene-based active plasmonic devices.

Inverse Stone Throwers Wales defect and enhancing ION/IOFF ratio and subthreshold swing of GNR transistors

In this paper, the impacts of the inverse Stone Thrower Wales (ISTW) defect as an ad-dimer defect on a double gate graphene nanoribbon field effect transistor (DGT) were studied. A DGT structure with a single ISTW defect is firstly analyzed for different positions of the ISTW defect across the width and along the length of the transistor channel. Then the impact of the random distributed ISTW (RDI) defect is investigated and the results indicate the defect density of 0.5% is more favorable due to its relatively better performance in off and on states. Considering how this ad-dimer defect can lead to enhance the transistor performance, a DGT structure including three ISTW defects in specific locations is also investigated which resulted in increasing the on-off current ratio up to 50 and decreasing the sub-threshold swing. The simulation results also show a decrease in ambipolar conduction and attenuation in kink effect. Our simulations has been done based on self-consistent solution of full 3D Poisson and Schrodinger equations within the non-equilibrium Green's function formalism. In the transistor channel, nanoribbons with non-functionalized edges are used.

Impact of carrier concentration and bandgap on the performance of double gate GNR-FET

Abstract This paper presents the impact of drain carrier density on the drain current saturation of 5 nm channel length double gate graphene nanoribbon field effect transistor (DGGNR-FET) near high drain voltage region. The impact of the substrate and energy band gap on the kink behavior of the device is analyzed by using Nonequilibrium Green's Function with mode space (NEGF_MS) method. We studied the effect of Si, SiC, and the oxide (SiO2) sub'strate on the device performance. The impact of the energy band gap is also examined on the increase in carrier density and current saturation. The necessity of constant carrier density in the drain side for better current saturation is emphasized. The carrier density in the drain side is increased due to the tunnelling from source to drain region (band to band tunnelling) and lateral carrier density modulation. We found that SiC is the better substrate over silicon and SiO2 for better current saturation of DGGNR-FET. Our simulation results provide detailed insight into analog and radio frequency application for the graphene device.

A computational study of a novel graphene nanoribbon field effect transistor

In this paper, using gate structure engineering and modification of channel dopant profile, we propose a new double gate graphene nanoribbon field effect transistor (DG-GNRFET) mainly to suppress the band-to-band tunneling (BTBT) of carriers. In the new device, the intrinsic part of the channel is replaced by an intrinsic-lightly doped-intrinsic [Formula: see text] configuration in a way that only the intrinsic parts are covered by the gate contact. Transport characteristics of the device are investigated theoretically using the nonequilibrium Green’s function (NEGF) formalism. Numerical simulations show that off-current, ambipolar behavior, on/off-current ratio and the switching characteristics such as intrinsic delay and power-delay product are improved. In addition, the new device demonstrates better sub-threshold swing and less drain-induced barrier lowering (DIBL).

Simulation of 50-nm Gate Graphene Nanoribbon Transistors

An approach to simulate the steady-state and small-signal behavior of GNR MOSFETs (graphene nanoribbon metal-semiconductor-oxide field-effect transistor) is presented. GNR material parameters and a method to account for the density of states of one-dimensional systems like GNRs are implemented in a commercial device simulator. This modified tool is used to calculate the current-voltage characteristics as well the cutoff frequency fT and the maximum frequency of oscillation fmax of GNR MOSFETs. Exemplarily, we consider 50-nm gate GNR MOSFETs with N = 7 armchair GNR channels and examine two transistor configurations. The first configuration is a simplified MOSFET structure with a single GNR channel as usually studied by other groups. Furthermore, and for the first time in the literature, we study in detail a transistor structure with multiple parallel GNR channels and interribbon gates. It is shown that the calculated fT of GNR MOSFETs is significantly lower than that of GFETs (FET with gapless large-area graphene channel) with comparable gate length due to the mobility degradation in GNRs. On the other hand, GNR MOSFETs show much higher fmax compared to experimental GFETs due the semiconducting nature of the GNR channels and the resulting better saturation of the drain current. Finally, it is shown that the gate control in FETs with multiple parallel GNR channels is improved while the cutoff frequency is degraded compared to single-channel GNR MOSFETs due to parasitic capacitances of the interribbon gates.

Double gate graphene nanoribbon field effect transistor with electrically induced junctions for source and drain regions

In this paper a novel graphene nanoribbon transistor with electrically induced junction for source and drain regions is proposed. An auxiliary junction is used to form electrically induced source and drain regions beside the main regions. Two parts of same metal are implemented at both sides of the main gate region. These metals which act as side gates are connected to each other to form auxiliary junction. A fixed voltage is applied on this junction during voltage variation on other junctions. Side metals have smaller workfunction than the middle one. Tight-binding Hamiltonian and nonequilibrium Green's function formalism are used to perform atomic scale electronic transport simulation. Due to the difference in metals workfunction, additional gates create two steps in potential profile. These steps increase horizontal distance between conduction and valance bands at gate to drain/source junction and consequently lower band to band tunneling probability. Current ratio and subthreshold swing improved at different channel lengths. Furthermore, device reliability is improved where electric field at drain side of the channel is reduced. This means improvement in leakage current, hot electron effect behavior and breakdown voltage. Application to multi-input logic gates shows higher speed and smaller power delay product in comparison with conventional platform.

Double gate graphene nanoribbon field effect transistor with single halo pocket in channel region

A new structure for graphene nanoribbon field-effect transistors (GNRFETs) is proposed and investigated using quantum simulation with a nonequilibrium Green's function (NEGF) method. Tunneling leakage current and ambipolar conduction are known effects for MOSFET-like GNRFETs. To minimize these issues a novel structure with a simple change of the GNRFETs by using single halo pocket in the intrinsic channel region, “Single Halo GNRFET (SH-GNRFET)”, is proposed. An appropriate halo pocket at source side of channel is used to modify potential distribution of the gate region and weaken band to band tunneling (BTBT). In devices with materials like Si in channel region, doping type of halo and source/drain regions are different. But, here, due to the smaller bandgap of graphene, the mentioned doping types should be the same to reduce BTBT. Simulations have shown that in comparison with conventional GNRFET (C-GNRFET), an SH-GNRFET with appropriately halo doping results in a larger ON current (Ion), smaller OFF current (Ioff), a larger ON–OFF current ratio (Ion/Ioff), superior ambipolar characteristics, a reduced power–delay product and lower delay time.

Charge Relaxation Resistances in Gated Graphene Nanoribbons

We investigate the charge relaxation resistances in a typical graphene nanoribbon FET (GNRFET). We show that the behavior of the charge relaxation resistances heavily depends on the exerted gate voltage and the structural details of the GNRFET. When there is 0 channel (blocked), 1 channel, and $N$ channels in the GNR as tuned by the gate voltage, the equilibrium charge relaxation resistance is roughly 1, $1/2$ , and $1/2N$ of Sharvin–Imry contact resistance ( $h/2e^{2}$ ), respectively, whereas the nonequilibrium charge relaxation resistance is much smaller. Our results indicate that the charge relaxation resistances characterizing the information of dissipation, $RC$ time and noise can be controlled by the gate voltage.

Theoretical analysis of a novel dual gate metal–graphene nanoribbon field effect transistor

A new double gate graphene nanoribbon field effect transistor with dual material for gate namely DMG-GNRFET is proposed. DMG-GNRFET includes a gate which is divided to the left and right side materials with different work functions. The left side metal (the source side) has lower work function than the right side one. This difference creates a step in potential profile which decreases band to band tunneling. Due to this step, the proposed structure has lower subthreshold swing, higher saturation current, lower leakage current and consequently higher current ratio than conventional structure. These advantages make DMG-GNRFET more suitable for digital applications. Transport properties of the proposed structure are compared to conventional GNRFET. The devices have been simulated based on self-consistent solution of Poisson and Schrodinger equations within non-equilibrium Green׳s function formalism. The effects of third nearest neighbor, dangling bands, and edge roughness have been included in the procedure to obtain more reliable simulations.

Strain-induced modifications of transport in gated graphene nanoribbons

We investigate the effects of homogeneous and inhomogeneous deformations and edge disorder on the conductance of gated graphene nanoribbons. Under increasing homogeneous strain the conductance of such devices initially decreases before it acquires a resonance structure and, finally, becomes completely suppressed at higher strain. Edge disorder induces mode mixing in the contact regions, which can restore the conductance to its ballistic value. The valley-antisymmetric pseudomagnetic field induced by inhomogeneous deformations leads to the formation of additional resonance states, which originate either from the coupling into Fabry-P\'erot states that extend through the system or from the formation of states that are localized near the contacts, where the pseudomagnetic field is largest. In particular, the $n=0$ pseudo-Landau level manifests itself via two groups of conductance resonances close to the charge neutrality point.

Strong Coupling in the Far-Infrared between Graphene Plasmons and the Surface Optical Phonons of Silicon Dioxide

We study plasmonic resonances in electrostatically gated graphene nanoribbons on silicon dioxide substrates. Absorption spectra are measured in the mid-far infrared and reveal multiple peaks, with width-dependent resonant frequencies. We calculate the dielectric function within the random phase approximation and show that the observed spectra can be explained by surface-plasmon-phonon-polariton modes, which arise from coupling of the graphene plasmon to three surface optical phonon modes in the silicon dioxide.

Electrically-activated source extension graphene nanoribbon field effect transistor: Novel attributes and design considerations for suppressing short channel effects

In this paper a double gate graphene nanoribbon field effect transistor with electrically-activated source extension is proposed. Source region of the proposed structure includes two sections, an electrically-activated extension and a doped section. The electrically extension, which is located between doped source section and gate region, is biased independent of the gate to form a virtual extension for source. The electrically-activated extension creates a step in potential profile which increases the horizontal distance between conduction and valance bands at channel to source junction. This step reduces the probability of band to band tunneling, lowers the leakage current and improves drain induced barrier lowering. The devices have been simulated based on self consistent solution of Poisson and Schrodinger equations within non-equilibrium Green’s function formalism. In addition, the effects of the edge and third nearest neighbor are included for more accurate outcomes. Simulations show that the proposed structure is a more reliable device because of its higher ON/Off current ratio, shorter delay time, and smaller power delay product beside lower subthreshold swing than conventional graphene nanoribbon field effect transistor.

Plasmonic extinction of gated graphene nanoribbon array analyzed by a scaled uniform Fermi level

A uniform Fermi level profile is typically assumed in the analysis of a gated graphene nanoribbon, whose Fermi level is actually nonuniform in the experimental measurements. Here, we show that the uniform Fermi level has to be downshifted when it is used to analyze a backgated graphene nanoribbon array (GNRA). The plasmonic extinction behaviors of the GNRAs are perfectly preserved by assuming properly scaled uniform Fermi levels. The scaling factor is independent of the average value of the actual Fermi level profile, but it is a function of the ratio of the nanoribbon width to the distance of the nanoribbons from the backgate. This study facilitates the data postprocessing in the experiments, and may be helpful for analyzing the electron behaviors in GNRAs.

Charge density and conductivity of disordered Berry-Mondragon graphene nanoribbons

We consider gated graphene nanoribbons subject to Berry-Mondragon boundary conditions in the presence of weak impurities. Using field-theoretical methods, we calculate the density of charge carriers (and, thus, the quantum capacitance) as well as the optical and DC conductivities at zero temperature. We discuss in detail their dependence on the gate (chemical) potential, and reveal a non-linear behaviour induced by the quantization of the transversal momentum.

A computational study of gate and channel engineering for GNRFETs performance enhancement

In this article, the effects of single-material-gate (SMG), halo doping (HALO), triple-material-gate (TMG) and triple-material halo doping (TMG-H) structures on electrical characteristics have been investigated for double gate graphene nanoribbon field-effect transistors (GNRFETs) using a full quantum transport simulation. The simulations are based on non-equilibrium Green’s functions (NEGF) solved self-consistently using 3D-Poisson’s equations. The results reveal that a larger difference between the work functions of gate metals of TMG structure will result in increasing the on–off current ratio in GNRFETs. HALO structure reduces the leakage current and alleviates the short channel effects (SCEs) like drain-induced barrier-lowering. Because of the perceivable steps in surface potential profile, which leads to additional lateral electric field peak along the channel, TMG and TMG-H structures have a larger average lateral electric field along the channel, which significantly promotes the transport efficiency and thus improves the subthreshold characteristics of devices.

Electron-electron interactions and topology in the electronic properties of gated graphene nanoribbon rings in Möbius and cylindrical configurations

We present a theory of the electronic properties of gated graphene nanoribbon rings with zigzag edges in M\obius and cylindrical configurations. The finite width opens a gap and nontrivial topology of the M\obius ring leads to a single edge with edge states with an induced, effective gauge field, in analogy to topological insulators. The single zigzag edge leads to a shell of degenerate states at the Fermi level and a ferromagnetic (FM) ground state at half-filling, i.e., at charge neutrality, due to electron-electron interactions. For fractional fillings, both the magnetic moment and the energy gap are found to oscillate as a function of the shell filling. In cylindrical rings, the two edges lead to AF ground state at half-filling but FM ground state at fractional fillings.

Modelling and simulation of saturation region in double gate graphene nanoribbon transistors

Novel analytical models for surface field distribution and saturation region length for double gate graphene nanoribbon transistors are proposed. The solutions for surface potential and electric field are derived based on Poisson equation. Using the proposed models, the effects of several parameters such as drain-source voltage, oxide thickness and channel length on the length of saturation region and electric field near the drain are studied.

Interacting electrons in graphene nanoribbons in the lowest Landau level

We study the effect of electron-electron interaction and spin on electronic and transport properties of gated graphene nanoribbons (GNRs) in a perpendicular magnetic field in the regime of the lowest Landau level (LL). The electron-electron interaction is taken into account using the Hartree and Hubbard approximations, and the conductance of GNRs is calculated on the basis of the recursive Greens function technique within the Landauer formalism. We demonstrate that, in comparison to the one-electron picture, electron-electron interaction leads to the drastic changes in the dispersion relation and structure of propagating states in the regime of the lowest LL showing a formation of the compressible strip and opening of additional conductive channels in the middle of the ribbon. We show that the latter are very sensitive to disorder and get scattered even if the concentration of disorder is moderate. In contrast, the edge states transport is very robust and can not be suppressed even in the presence of a strong spin-flipping.