Graphene Nanoribbon Field-effect Transistors Research Articles

Size-controlled ambipolar graphene nanoribbon transistors by an all-dry mask method

Conventional graphene nanoribbon (GNR) field-effect transistor (FET) fabrication involved the wet process with the separated nanoribbon formation and device fabrication. Here, we demonstrate one simple and novel non-solution method to integrate the GNR formation and the FET fabrication, where a gold film is used as mask for electrode deposition, following by using a SnO2 nanoribbon as mask for the formation of GNR. The channel length and width can be controlled by the widths of the gold film and the SnO2 nanoribbon, respectively. It is found that the GNR with the width up to 330nm presents the promising ambipolar field-effect properties in air ambient, the hole and electron mobilities are respectively as high as 904 and 703cm2V−1s−1, which benefits from the all dry process for both GNR fabrication and device fabrication.

On Device Modeling for Circuit Simulation With Application to Carbon-Nanotube and Graphene Nano-Ribbon Field-Effect Transistors

This paper presents a method for deriving circuit model stamp equations from the characteristic equations of multiterminal devices. The method is applied to the derivation of stamp equations of carbon nanotube and graphene nano-ribbon field-effect transistors (FETs) for use in general-purpose circuit simulators. We first review existing methods of modeling FETs for circuit simulation and point out some of the weaknesses in these models. We then explain how to derive model equation stamps directly from the device physical characteristic equations without the need of eliminating internal device variables and without having to construct equivalent circuits consisting of interconnections of two-terminal resistors, controlled sources, and two-terminal capacitors.

Electrical Characteristics of Field‐Effect Transistors based on Chemically Synthesized Graphene Nanoribbons

The electronic properties of chemically synthesized graphene nanoribbons (GNRs) are investigated in a field‐effect transistor (FET) configuration. The FETs are fabricated by dispersing GNRs into an aqueous dispersion, depo­siting the GNRs onto an insulating substrate, and patterning of metal contacts by electron‐beam lithography. At room temperature, the GNR FET shows a large drain current of 70 μA, very good charge injection from the contacts, saturation of the drain current at larger drain‐source voltages, and an on/off current ratio of 2. The small on/off current ratio can be explained by either the unfavorable transistor geometry or by the unintentional agglomeration of two or more GNRs in the channel. Furthermore, it is demonstrated that, by quantum‐chemical calculations, the bandgap of a GNR dimer can be as small as 30% of the bandgap of a GNR monomer.

Lateral plasma etching enhanced on/off ratio in graphene nanoribbon field-effect transistor

Opening the transport gap in graphene by minimizing its width is highly desirable to achieve outstanding switching performance, i.e., the high on/off ratio, in its field effect transistors (FETs). In this letter, we propose a simple method to open a comparable transport gap in graphene by narrowing down it into graphene nanoribbon (GNR) via the conventional nanofabrication procedure. In the process, GNR capped with a 50-nm-wide hydrogen-silsesquioxane mask is trimmed down from the edges by lateral plasma etching. The on/off ratio of the FET device is dramatically enhanced by two orders of magnitude as etching duration increases. The large on/off ratios of ∼47 and ∼105 are achieved at room temperature and 5.4 K, respectively. The electrical measurement reveals a transport gap opening of ∼145 meV in GNR, which corresponds to a resulting width of <10 nm.

Graphene nanoribbon field-effect transistors on wafer-scale epitaxial graphene on SiC substrates

We report the realization of top-gated graphene nanoribbon field effect transistors (GNRFETs) of ∼10 nm width on large-area epitaxial graphene exhibiting the opening of a band gap of ∼0.14 eV. Contrary to prior observations of disordered transport and severe edge-roughness effects of graphene nanoribbons (GNRs), the experimental results presented here clearly show that the transport mechanism in carefully fabricated GNRFETs is conventional band-transport at room temperature and inter-band tunneling at low temperature. The entire space of temperature, size, and geometry dependent transport properties and electrostatics of the GNRFETs are explained by a conventional thermionic emission and tunneling current model. Our combined experimental and modeling work proves that carefully fabricated narrow GNRs behave as conventional semiconductors and remain potential candidates for electronic switching devices.

Non-equilibrium Green function simulations of graphene, silicene, and germanene nanoribbon field-effect transistors

Non-equilibrium Green function simulations of graphene, silicene, and germanene nanoribbon field-effect transistors

Improving Ion/Ioff in dual-gate graphene nanoribbon field-effect transistors using local uniaxial tensile strain

In this work, we present the Dual-Gate Graphene Nano Ribbon Field Effect Transistor (DG-GNRFET) under local uniaxial strain in source and drain regions as a device suitable for switching applications. Our investigations are based on the 2D Poisson based on an atomistic mode-space approach and Schrodinger equations, within the Non-Equilibrium Green’s (NEGF). We show a high on-current and on–off ratio which can be obtained using the combination of techniques such as applying uniaxial strain to the portion of channel/source regions and the gate overlap. We followed an optimization process to find the best performance of the device. Finally, the proposed device shows a higher on-current and on–off ratio becomes about 100 times greater than of the unstrained device.

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.

Asymmetric Gate Schottky-Barrier Graphene Nanoribbon FETs for Low-Power Design

The ambipolar behavior limits the performance of Schottky-barrier-type graphene nanoribbon field-effect transistors (SB-GNRFETs). We propose an asymmetric gate (AG) design for SB-GNRFETs, and show that it can significantly reduce the IOFF. Simulation results indicate at least 40% and 5× improvement in the subthreshold swing and the I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> /I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</sub> ratio, respectively. We build an accurate semianalytical closed-form model for the current-voltage characteristics of SB-GNRFETs. The proposed Simulation Program with Integrated Circuit Emphasis (SPICE)-compatible model considering various design parameters and process variation effects, which enables efficient circuit-level simulations of SB-GNRFET-based circuits. Simulation results of benchmark circuits show that the average energy-delay product of the AG SB-GNRFETs is only ~22% of that of a symmetric gate for the ideal case and ~88% for devices with line edge roughness.

Extraction of the voltage-dependent quantum capacitance and kinetic inductance of GNRFETs: a first-principles study

Graphene nanoribbon field effect transistors (GNRFETs) are prominent candidates for the near future nanoelectronics technology. In this work, a sample GNRFET is simulated utilizing density functional theory with non-equilibrium Green's function formalism (NEGF) to obtain the current---voltage relationship, the variation of the channel charge and the electrostatic potential with respect to applied voltage. It is shown that a 5th order polynomial model, which can be utilized in circuit design tools, accurately models the current---voltage relationship of GNRFETs. More importantly, the variations of the kinetic inductance, quantum capacitance and the Fermi velocity dependent on both the source---drain $$(\hbox {V}_\mathrm{DS})$$(VDS) and source---gate voltages $$(\hbox {V}_\mathrm{GS})$$(VGS) are extracted. Numerical values of these parameters are found to be consistent with the theoretical and experimental average values existing in the literature. The paper is concluded with the discussion of the voltage-dependencies of the kinetic inductance and the quantum capacitance for the circuit design using GNRFETs.

Graphene nanoribbon field-effect transistor at high bias.

Combination of high-mean free path and scaling ability makes graphene nanoribbon (GNR) attractive for application of field-effect transistors and subject of intense research. Here, we study its behaviour at high bias near and after electrical breakdown. Theoretical modelling, Monte Carlo simulation, and experimental approaches are used to calculate net generation rate, ionization coefficient, current, and finally breakdown voltage (BV). It is seen that a typical GNR field-effect transistor's (GNRFET) breakdown voltage is in the range of 0.5 to 3 V for different channel lengths, and compared with silicon similar counterparts, it is less. Furthermore, the likely mechanism of breakdown is studied.

Quantum Transport Simulations of CNTFETs: Performance Assessment and Comparison Study with GNRFETs

In this paper, we explore the electrical properties and high-frequency performance of carbon nanotube field-effect transistors (CNTFETs), based on the non-equilibrium Green’s functions (NEGF) solved self - consistently with Poisson’s equations. The calculated results show that CNTFETs exhibit superior performance compared with graphene nanoribbon field-effect transistors (GNRFETs), such as better control ability of the gate on the channel, higher drive current with lower subthreshold leakage current, and lower subthreshold-swing (SS). Due to larger bandstructure-limited velocity in CNTFETs, ballistic CNTFETs present better high-frequency performance limit than that of Si MOSFETs. The parameter effects of CNTFETs are also investigated. In addition, to enhance the immunity against short - channel effects (SCE), hetero - material - gate CNTFETs (HMGCNTFETs) have been proposed, and we present a detailed numerical simulation to analyze the performances of scaling down, and conclude that HMG-CNTFETs can meet the ITRS’10 requirements better than CNTs.

A novel graphene nanoribbon field effect transistor with two different gate insulators

In this paper, a novel structure for a dual-gated graphene nanoribbon field-effect transistor (GNRFET) is offered, which combines the advantages of high and low dielectric constants. In the proposed Two Different Insulators GNRFET (TDI-GNRFET), the gate dielectric at the drain side is a material with low dielectric constant to form smaller capacitances, while in the source side, there is a material with high dielectric constant to improve On-current and reduce the leakage current. Simulations are performed based on self-consistent solutions of the Poisson equation coupled with Non-Equilibrium Green's Function (NEGF) formalism in the ballistic regime. We assume a tight-binding Hamiltonian in the mode space representation. The results demonstrate that TDI-GNRFET has lower Off-current, higher On-current and higher transconductance in comparison with conventional low-K GNRFET. Furthermore, using a top-of-the-barrier two-dimensional circuit model, some important circuit parameters are studied. It is found that TDI-GNRFET has smaller capacitances, lower intrinsic delay time and shorter power delay product (PDP) in comparison with high-K GNRFET. Moreover, mobile charge and average velocity are improved in comparison with low dielectric constant GNRFET. The results show that the TDI-GNRFET can provide Drain Induced Barrier Lowering (DIBL) and Subthreshold Swing near their theoretical limits.

Simulation of Drain Currents of Double Gated Armchair Graphene Nanoribbon Field-Effect Transistors by Solving Dirac "Like" Equation and Using Transfer Matrix Method

We have modelled quantum mechanically the drain current in double gated armchair graphene nanoribbon field-effect transistors (AGNR-FETs). The Dirac 'like' equation was used to determine the electron wave functions in the AGNR. The electron transmittance and the drain current were calculated numerically by using a transfer matrix method (TMM). The results showed that the drain current of the AGNR-FET devices increases with the drain and gate voltages. In addition, the threshold voltage of the devices was obtained to be constant at about 0.3 V. Moreover, the drain current increased with decreasing the thickness of insulator. The different temperatures gave the different characteristics of the drain current.

Graphene nanoribbon crossbar architecture for low power and dense circuit implementations

Crossbar array is a promising nanoscale architecture which can be used for logic circuit implementation. In this work, a graphene nanoribbon (GNR) based crossbar architecture is proposed. This design uses parallel GNRs as device channel and metal as gate, source and drain contacts. Schottky-barrier type graphene nanoribbon field-effect transistors (SB-GNRFETs) are formed at the cross points of the GNRs and the metallic gates. Benchmark circuits are implemented using the proposed crossbar, Si-CMOS and multi-gate Si-CMOS approaches to evaluate the performance of the crossbar architecture compared to the conventional CMOS logic design. The compact SPICE model of SB-GNRFET was used to simulate crossbar-based circuits. The CMOS circuits are also simulated using 16nm technology parameters. Simulation results of benchmark circuits using SIS synthesis tool indicate that the GNR-based crossbar circuits outperform conventional CMOS circuits in low power applications. Area optimized cell libraries are implemented based on the asymmetric crossbar architecture. The area of the circuit can be more reduced using this architecture at the expense of higher delay. The crossbar cells can be combined with CMOS cells to exhibit better performance in terms of EDP.

Numerical investigation of the effect of substrate surface roughness on the performance of zigzag graphene nanoribbon field effect transistors symmetrically doped with BN.

The performance of field effect transistors comprised of a zigzag graphene nanoribbon that is symmetrically doped with boron nitride (BN) as a channel material, is numerically studied for the first time. The device merit for digital applications is investigated in terms of the on-, the off- and the on/off-current ratio. Due to the strong effect of the substrate roughness on the performance of graphene devices, three common substrate materials (SiO2, BN and mica) are examined. Rough surfaces are generated by means of a Gaussian auto-correlation function. Electronic transport simulations are performed in the framework of tight-binding Hamiltonian and non-equilibrium Green's function (NEGF) formalisms. The results show that with an appropriate selection of the substrate material, the proposed devices can meet the on/off-current ratio required for future digital electronics.

Investigation of the novel attributes of a double-gate graphene nanoribbon FET with AlN high-κ dielectrics

The use of high-κ dielectrics is an inevitable requirement to continue the historical performance scaling trend. Recently, the electrical transport characteristics of graphene FET on an aluminum nitride (AlN) substrate in spite of other common dielectric materials indicate significant improvement of carrier mobility. This paper proposes a novel double-gate graphene nanoribbon field-effect transistor (GNRFET) on AlN dielectric substrates. More studies exploring device characteristics are included to assess the performance over the conventional SiO2-based one. It is found that the high-κ GNRFET affords larger on current, on–off ratio, transconductance and intrinsic gain. The high-κ structure also provides further immunity against short-channel effects including drain-induced barrier-lowering and subthreshold swing.

Quantum simulation study of double gate hetero gate dielectric and LDD doping graphene nanoribbon p–i–n tunneling FETs

We perform a theoretical study of the effects of the lightly doped drain (LDD) and high-k dielectric on the performances of double gate p–i–n tunneling graphene nanoribbon field effect transistors (TFETs). The models are based on non-equilibrium Green's functions (NEGF) solved self-consistently with 3D-Poisson's equations. For the first time, hetero gate dielectric and single LDD TFETs (SL-HTFETs) are proposed and investigated. Simulation results show SL-HTFETs can effectively decrease leakage current, sub-threshold swing, and increase on–off current ratio compared to conventional TFETs and Si-based devices; the SL-HTFETs from the 3p + 1 family have better switching characteristics than those from the 3p family due to smaller effective masses of the former. In addition, comparison of scaled performances between SL-HTFETs and conventional TFETs show that SL-HTFETs have better scaling properties than the conventional TFETs, and thus could be promising devices for logic and ultra-low power applications.

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.

Carrier Velocity in High-Field Transport of Trilayer Graphene Nanoribbon Field Effect Transistor

Carrier velocity is one of the most significant characteristics for analytical modeling of field effect transistor based devices. The aim of the present paper is to evaluate the scaling behaviours of carrier velocity in trilayer graphene nanoribbon as a function of electron density, normalized Fermi energy and electric field in the degenerate and non-degenerate regimes. To this end, we derive an analytical model of carrier velocity with numerical solution for trilayer graphene nanoribbon field effect transistor in which the temperature and carrier concentration characteristics dependence is highlighted. Moreover, to determine the trilayer graphene nanoribbon field effect transistor performance the carrier velocity model is adopted to derive the current-voltage characteristics of the device. The simulated results proffer remarkable insights into the importance of carrier velocity impact in high performance trilayer graphene nanoribbon field effect transistor. We demonstrate that although there is no experimental evidence reported in the literature for carrier velocity of trilayer graphene nanoribbon, the proposed model can assist in comprehending experiments involving nanoscale field effect transistors.