Graphene Nanoribbon Field-effect Transistors Research Articles

Short Distance Channel Defect Intractions in Graphene Nanoribbon Field Effect Transistors

We have studied theoretically the channel defect interaction and for this we have used correlated nonradiative multiphonon theory in conjunction with the coulomb energy. We have presented the charge capture rate. To calculate the value of coulomb energy we have developed a quantum capacitive circuit considering the effect of the fringing electrical field distributed spatially in the dielectric. The calculated short distance charge capture rate for the channel subbands at room temperature revealed a significant correlation between the capture rate and singularity characteristics of the one dimensional density of states. Charge capture through the nonradiative multiphonon transition is responsible for the shortdistance interactions between the channel carriers and the interfacial dielectric defects that are typically located within a few nanometers of the dielectric channel interface.

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.

Erratum to: Exploiting Edge Effect to Control Generation Rate and Breakdown Voltage in Graphene Nanoribbon Field Effect Transistors

Erratum to: Exploiting Edge Effect to Control Generation Rate and Breakdown Voltage in Graphene Nanoribbon Field Effect Transistors

Understanding the impact of graphene sheet tailoring on the conductance of GNRFETs

The effect of tailoring the graphene sheets used as channel in a graphene nanoribbon field effect transistor (GNRFET) was investigated. The study was performed using self-consistent solution of Poisson’s and Schrodinger’s equation in combination with non-equilibrium Green’s function (NEGF) formalism. Graphene sheet channel was tailored into different shapes and found that with the introduction of edge roughness along the border of GNR sheet the bandgap of GNRFET channel increases. Tailoring the channel decreases mobility and transmission probability to a great extent and thus the performance of I–V characteristics of GNRFET degrades.

Scaling Effects on Static Metrics and Switching Attributes of Graphene Nanoribbon FET for Emerging Technology

In this paper, we have investigated the static metrics and switching attributes of graphene nanoribbon field-effect transistors (GNR FETs) for scaling the channel length from 15 nm down to 2.5 nm and GNR width by approaching the ultimate vertical scaling of oxide thickness. We have simulated the double-gate GNR FET by solving a numerical quantum transport model based on selfconsistent solution of the 3D Poisson equation and 1D Schrödinger equation within the non-equilibrium Green's function formulism. The narrow armchair GNR, e.g. (7,0), improved the device robustness to shortchannel effects, leading to better OFF-state performance considering OFF-current, 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, subthreshold swing, and drain-induced barrier-lowering. The wider armchair GNRs allow the scaling of channel length and supply voltage, resulting in better ON-state performance, such as the larger intrinsic cut-off frequency for the channel length below 7.5 nm at smaller gate voltage as well as smaller intrinsic gate-delay time with the constant slope for scaling the channel length and supply voltage. The wider armchair GNRs, e.g. (13,0), have smaller power-delay product for scaling the channel length and supply voltage, reaching to ~0.18 (fJ/μm).

Top-of-the-Barrier Ballistic Carbon Nanotubes and Graphene Nanoribbon Field-Effect Transistors Quantum Simulator

Top-of-the-Barrier Ballistic Carbon Nanotubes and Graphene Nanoribbon Field-Effect Transistors Quantum Simulator

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.

A SPICE-Compatible Model of MOS-Type Graphene Nano-Ribbon Field-Effect Transistors Enabling Gate- and Circuit-Level Delay and Power Analysis Under Process Variation

This paper presents the first parameterized SPICE-compatible compact model of a graphene nano-ribbon field-effect transistor (GNRFET) with doped reservoirs, also known as MOS-type GNRFET. The current and charge models closely match numerical TCAD simulations. In addition, process variation in transistor dimension, line edge roughness, and doping level in the reservoirs are accurately modeled. Our model provides a means to analyze delay and power of graphene-based circuits under process variation, and offers design and fabrication insights for graphene circuits in the future. We show that line edge roughness severely degrades the advantages of GNRFET circuits; however, GNRFET is still a good candidate for low-power applications.

High I on/I off current ratio graphene field effect transistor: the role of line defect.

The present paper casts light upon the performance of an armchair graphene nanoribbon (AGNR) field effect transistor in the presence of one-dimensional topological defects. The defects containing 5–8–5 sp2-hybridized carbon rings were placed in a perfect graphene sheet. The atomic scale behavior of the transistor was investigated in the non-equilibrium Green's function (NEGF) and tight-binding Hamiltonian frameworks. AGNRFET basic terms such as the on/off current, transconductance and subthreshold swing were investigated along with the extended line defect (ELD). The results indicated that the presence of ELDs had a significant effect on the parameters of the GNRFET. Compared to conventional transistors, the increase of the Ion/Ioff ratio in graphene transistors with ELDs enhances their applicability in digital devices.

Exploiting Edge Effect to Control Generation Rate and Breakdown Voltage in Graphene Nanoribbon Field Effect Transistors

Based on the influence of edge effect and channel shape, two new graphene nanoribbon field effect transistors are presented being useful in high-voltage and highly sensitive optical applications. We fabricated and examined our devices under different conditions. It is seen that by choosing the proper channel shape, ionization rate and breakdown voltage could be improved compared to a normal rectangular device. We report nearly 11 and 19 % improvements in breakdown voltage and ionization rate of graphene nanoribbon field effect transistors (GNRFET), respectively.

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.

Channel Length Modulation Effect on Monolayer Graphene Nanoribbon Field Effect Transistor

R Graphene Nanoribbon Field Effect Transistors (GNR FETs) is attracting a great deal of attention due to their better performance in comparison with conventional devices. In this paper, channel length Modulation (CLM) effect on the electrical characteristics of GNR FETs is analytically studied and modeled. To this end, the spatial distribution of the electric potential along the channel and current-voltage characteristic of the device are modeled. The obtained results of analytical model are compared against the experimental data of published works. As a result, it is observable that considering the effect of CLM, the current-voltage response of GNR FET is more realistic.

Improving [formula omitted] and sub-threshold swing in graphene nanoribbon field-effect transistors using single vacancy defects

Graphene nanoribbon field effect transistors are promising devices for beyond-CMOS nanoelectronics. Graphene is a semiconductor material with zero bandgap and its bandgap must be changed. One of the opening bandgap methods is using graphene nanoribbons. By applying a defect, there is more increase on band gap of monolayer armchair graphene nanoribbon field effect transistor. So, by applying more than one defect, we can reach to much more increase in bandgap of graphene nanoribbon field effect transistors (GNRFET). In this paper, double-gated monolayer armchair graphene nanoribbon field effect transistors (GNRFET) with one single vacancy (1SV) defect (so-called 1SVGNRFET)are simulated and after changing positions of defect in width and length of channel of GNRFET, a structure with three single vacancy (3SVs) defects(so-called 3SVsGNRFET) is offered that this structure has higher ION/IOFF ratio and lower sub-threshold swing than 1SVGNRFET and therefore has better performance. The energy band structure of nanoribbon is obtained by using nearest–neighbour interactions within an approximation tight binding model. Transfer characteristic of the transistor is simulated with solving Poisson-Schrodinger equation self-consistently by using Non- Equilibrium Green Function (NEGF) and in the real space approach.

Study of Nitrogen terminated doped zigzag GNR FET exhibiting negative differential resistance

This paper presents the study of Gallium and Aluminum doped Nitrogen terminated zigzag Graphene Nano Ribbon (GNR) FET with high-k dielectric. The GNR FET structure has been designed and simulated using Quantumwise Atomistix Toolkit software package. The presented GNR FET with n-type (Nitrogen doped) electrodes and p-type (Gallium or Aluminum doped) scattering region are simulated and analyzed using Density Functional Theory combined with NEGF formalism and Device Density of States (DDOS). The device shows a negative differential resistance phenomenon which can be controlled by the gate of the zigzag GNR FET. It is found that doping of Gallium and Aluminum in scattering region provides higher drain current, higher ION/IOFF and IP/IV ratios as compared to that of Boron doped zigzag GNR FET. The potential applications of the device are in logical, high frequency, and memory devices.

A seamless-pitched graphene nanoribbon field effect transistor

This paper proposes a graphene nanoribbon field effect transistor (GNRFET) consisting of pitched semiconducting GNRs as the channels that are connected to the metallic graphene source/drain in a seamless fashion. We obtained the diagrams for frequency bandwidths, step time responses, and Nyquist stability for the seamless pitched GNRFET (SP-GNRFET) with a channel having 100 pitched GNRs at 10nm pitch in the common source configuration with various dimensions of the GNRs. The aforementioned diagrams were also obtained for the pitched carbon nanotube field effect transistor (CNTFET) with a channel having 100 pitched CNTs at 10nm pitch in the common source configuration with various dimensions of the CNTs. In order to compare the SP-GNRFET and the pitched CNTFET, physical parameters of the GNRs/CNTs were assumed to be the same in both devices. The results show that when the dimensions of GNRs in the SP-GNRFET increase, the frequency bandwidth decreases, but relaxation time and Nyquist stability increase. Moreover, with an increase in the dimensions of CNTs, similar behavior is observed for the pitched CNTFET.The results also show that the frequency bandwidth of SP-GNRFET is in the range of 10THz and is more than that of the pitched CNTFET by two orders of magnitude. This is achieved by eliminating the Schottky barrier between the channels and source/drain contacts in the SP-GNRFET. Nevertheless, step time responses for the SP-GNRFET show multi-harmonic oscillations like those for the pitched CNTFET. This shows the importance of stability analysis as a challenge to the SP-GNRFET. Nyquist diagrams predict lower stability for SP-GNRFETs than for pitched CNTFETs. This is because elimination of the Schottky barrier results in a reduction in the overall impedance of the SP-GNRFET, which in turn leads to the frequency of the fluctuations in the SP-GNRFET being more than that in the pitched CNTFET.

A bilayer graphene nanoribbon field-effect transistor with a dual-material gate

This paper introduces dual-material gate (DMG) configuration on a bilayer graphene nanoribbon field-effect transistor (BLGNRFET). Its device characteristics based on nonequilibrium Green׳s function (NEGF) are explored and compared with a conventional single-material gate BLGNRFET. Results reveal that an on-off ratio of up to 10 is achievable as a consequence of both higher saturation and lower leakage currents. The advantages of our proposed DMG structure mainly lie in higher carrier transport efficiency by means of enhancing initial acceleration of incoming carriers in the channel region and the suppression of short channel effects. Drain-induced barrier lowering, subthreshold swing and hot electron effect as the key short channel parameters have been improved in the DMG-based BLGNRFET.

Analytical SPICE-Compatible Model of Schottky-Barrier-Type GNRFETs With Performance Analysis

This paper presents an accurate analytical compact model for Schottky-barrier-type graphene nanoribbon field-effect transistors (SB-GNRFETs). This is a physics-based analytical model for the current–voltage ( $I$ – $V$ ) characteristics of SB-GNRFETs. The proposed model considers various design parameters and process variation effects, including graphene-specific line-edge roughness, which allows thorough exploration and evaluation of SB-GNRFET circuits. We develop accurate approximations of SB tunneling, channel charge, and current, which provide accurate results while maintaining model compactness. We evaluate the effect of design parameters and process variations on the performance of SB-GNRFETs. We also compare circuit-level performance of SB-GNRFETs with multigate (MG) Si-CMOS (e.g., FinFETs). Our circuit simulations indicate that SB-GNRFET has an energy-delay product (EDP) advantage over Si-CMOS, although GNR-specific process variation, especially the line-edge roughness, would significantly downgrade such an advantage; the EDP of the ideal SB-GNRFET (assuming no process variation) is $\sim 2.5$ % of that of Si-CMOS, while the EDP of the nonideal case with process variation is $\sim 68$ % of that of Si-CMOS. Finally, we study technology scaling with SB-GNRFET and MG Si-CMOS. We show that the EDP of ideal (nonideal) SB-GNRFET is $\sim 0.88$ % (54%) EDP of that of Si-CMOS as the technology nodes scale down to 7 nm.

Investigation of the width-dependent static characteristics of graphene nanoribbon field effect transistors using non-parabolic quantum-based model

The performance of graphene nanoribbon field effect transistor (GNR FET) is investigated from a numerical model based on self-consistent non-equilibrium Green’s Function (NEGF) formulism in mode-space with position-dependent effective mass model and tight-binding model. The model accounts for the tunneling currents on the static performance of GNR FETs in two semiconducting families of armchair GNRs (3p,0) and (3p+1,0).We conclude that increasing the GNR width in both GNR families increases the leakage current and subthreshold swing, and decreases ION/IOFF ratio. In this scenario, GNR group (3p+1,0) leads to superior off-state performance such that GNR (7,0) has off-state current close to 2.5×10−16A, five orders of magnitude lower than GNR (6,0) as well as 67mV/decade subthreshold swing which is much smaller than that of 90mV/decade in GNR (6,0).

Improving performance of armchair graphene nanoribbon field effect transistors via boron nitride doping

Device performance of 10nm length armchair graphene nanoribbon field effect transistors with 1.5nm and 4nm width (13 and 33 atoms in width respectively) are compared in terms of Ion/Ioff, trans-conductance, and sub-threshold swing. While narrow devices suffer from edge roughness wider devices are subject to more substrate surface roughness and reduced bandgap. Boron Nitride doping is employed to compensate reduced bandgap in wider devices. Simultaneous effects of edge and substrate surface roughness are considered. Results show that in the presence of both the edge and substrate surface roughness the 4nm wide device with boron nitride doping shows improved performance with respect to the 1.5nm one (both of which incorporate the same bandgap AGNR as channel material). Electronic simulations are performed via NEGF method along with tight-binding Hamiltonian. Edge and surface roughness are created by means of one and two dimensional auto correlation functions respectively. Electronic characteristics are averaged over a large number of devices due to statistic nature of both the edge and surface roughness.

The vacancy defect in graphene nano-ribbon field-effect transistor in the presence of an external perpendicular magnetic field

In this paper, we investigate effect of vacancy defect on graphene nano-ribbon field effect transistor (GNR-FET) in the presence of an external perpendicular magnetic field. Our simulation is based on Non-Equilibrium Green's Function (NEGF). The nearest neighbor tight-binding model based on pz orbital construct the device Hamiltonian. Also we calculate magnetoresistance (MR) for different position vacancies in presence B-field. We find an external B-field causes the conductance of the GNR increases, giving rise to the MR effect. The simulation results indicate in the presence of a small B-field the current is degraded because the vacancy defect act as scattering and the conductance decreases due to defects compared with the perfect nanoribbons and the MR is reduced but in the hall quantum regime the probability of being scattered by the vacancy reduced due to the electrons are more localized at edge states and this phenomena to enhance the MR ratio of the device. The results indicate that the defect near drain and near edge has the largest MR ratio in the presence of a small B-Field and in the hall quantum the defect near source has the largest MR ratio.