Graphene Nanoribbon Tunnel Field Effect Transistor Research Articles

Computational study of bilayer armchair graphene nanoribbon tunneling field-effect transistors for digital circuit design

This study investigates the effects of a structural defect, specifically a single vacancy (SV), on the performance of bilayer armchair graphene nanoribbon tunnel field-effect transistors (BL-AGNR TFETs). Simulations are conducted using the Non-Equilibrium Green's Function (NEGF) formalism and the tight-binding Hamiltonian, along with a self-consistent solution of the Poisson and Schrödinger equations, to evaluate key performance parameters, including subthreshold swing (SS), ON-current (ION), OFF-current (IOFF), and the ON/OFF-current ratio (ION/IOFF). The results indicate that single vacancies have a negligible effect on ON-current (ION) but cause a significant increase in OFF-current (IOFF) and subthreshold swing (SS), leading to a degradation in the ION/IOFF ratio and overall switching performance. The subthreshold swing (SS) for a defect-free transistor is 65 mV/dec; however, in the presence of a single vacancy located near the source or in the middle of the channel, the SS increases to 97 mV/dec and 75 mV/dec, respectively. This study highlights the negative impact of structural defects on TFET performance and emphasizes the importance of precise device engineering to enhance performance in the presence of defects.

Electrostatically Doped Junctionless Graphene Nanoribbon Tunnel Field-Effect Transistor for High-Performance Gas Sensing Applications: Leveraging Doping Gates for Multi-Gas Detection.

In this paper, a new junctionless graphene nanoribbon tunnel field-effect transistor (JLGNR TFET) is proposed as a multi-gas nanosensor. The nanosensor has been computationally assessed using a quantum simulation based on the self-consistent solutions of the mode space non-equilibrium Green's function (NEGF) formalism coupled with the Poisson's equation considering ballistic transport conditions. The proposed multi-gas nanosensor is endowed with two top gates ensuring both reservoirs' doping and multi-gas sensing. The investigations have included the IDS-VGS transfer characteristics, the gas-induced electrostatic modulations, subthreshold swing, and sensitivity. The order of change in drain current has been considered as a sensitivity metric. The underlying physics of the proposed JLGNR TFET-based multi-gas nanosensor has also been studied through the analysis of the band diagrams behavior and the energy-position-resolved current spectrum. It has been found that the gas-induced work function modulation of the source (drain) gate affects the n-type (p-type) conduction branch by modulating the band-to-band tunneling (BTBT) while the p-type (n-type) conduction branch still unaffected forming a kind of high selectivity from operating regime point of view. The high sensitivity has been recorded in subthermionic subthreshold swing (SS < 60 mV/dec) regime considering small gas-induced gate work function modulation. In addition, advanced simulations have been performed for the detection of two different types of gases separately and simultaneously, where high-performance has been recorded in terms of sensitivity, selectivity, and electrical behavior. The proposed detection approach, which is viable, innovative, simple, and efficient, can be applied using other types of junctionless tunneling field-effect transistors with emerging channel nanomaterials such as the transition metal dichalcogenides materials. The proposed JLGNRTFET-based multi-gas nanosensor is not limited to two specific gases but can also detect other gases by employing appropriate gate materials in terms of selectivity.

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).

Graphene antidot nanoribbon tunnel field‐effect transistor

A graphene nanoribbon tunnel field-effect transistor (TFET) model is proposed, in which the antidot pattern is used to generate the heterojunction (HJ) band structure. The intrinsic strain at the HJ interface is absent naturally, greatly avoiding the tunnelling blocking effect of the interface states. The energy gap (Eg) of the graphene antidot nanoribbon (GANR) can be flexibly modulated through the antidot morphology, resulting in highly tunable device performance. Moreover, the stability of the TFET behaviours considering the patterning technology issue is studied. Simulation results indicate that the GANR is suitable for the TFET design in different application scenarios.

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.

Investigation of 6-armchair graphene nanoribbon tunnel FETs

A simulation-based study of an n-type six-dimer-line armchair graphene nanoribbon (6-AGNR) tunnel field-effect transistor with asymmetric reservoir doping density is carried out. Tunnel field-effect transistor (TFET) structures are proposed based on a detailed investigation of the device behavior for different applied voltages, channel lengths, temperatures, insulator thicknesses, dielectric constants, and source impurity molar fractions. By suppressing the tunneling transmission in the off-state, the channel length of the device using HfO2 can be scaled down to 5 nm without increasing the leakage current. When using a supply voltage of 0.4 V, the ION/IOFF ratio reaches a high value of 3.6 × 1010 for the device with a 5-nm channel. Besides, a subthreshold swing (SS) of 3.8 mV/dec is measured for the same GNR-TFET. The high-performance 10-nm-channel device, when supplied with 0.6 V, exhibits a boosted ION value of up to 4.3 × 103 µA/µm, with SS, gm, and Dini values of 28 mV/dec, 11 µS, and 11 fs, respectively. Nevertheless, conventional GNR-TFETs with various channel lengths exhibit rather outstanding characteristics. Such 6-AGNR TFETs display promising functionality for application in future digital and analog integrated circuits.

Performance enhancement of an ultra-scaled double-gate graphene nanoribbon tunnel field-effect transistor using channel doping engineering: Quantum simulation study

In this paper, new graphene nanoribbon tunnel field-effect transistors (GNRTFETs) endowed with specific doping profiles are proposed, assessed, and compared with the conventional design through a computational investigation. The used quantum simulation is based on solving self-consistently the Poisson and Schrödinger equations using the non-equilibrium Green's function formalism in the ballistic limit. The numerical study deals with GNRTFET designs with 5 nm gate length while comparing their I-V transfer proprieties, subthreshold swing, charge density, current ratio, intrinsic delay, and power-delay product. The proposed designs, which are endowed with new doping profiles, have been found efficient in mitigating the direct source-to-drain tunneling issue of ultrascaled GNR-based TFET and in improving the ambipolar behavior while boosting the performance of the GNRTFETs. As interesting results, highly improved minimum leakage current, subthreshold swing, and switching parameters have been recorded by means of the proposed channel doping engineering-based approach. The obtained substantial improvements in the performance of ultrascaled GNRTFETs make the proposed approach as a promising way for the continual shrinking of tunnel field-effect transistors (sub-5-nm) while maintaining the required high-performance.

A new ultra-scaled graphene nanoribbon junctionless tunneling field-effect transistor: proposal, quantum simulation, and analysis

In this paper, a new ultrascaled junctionless graphene nanoribbon tunnel field-effect transistor (JL GNRTFET) is proposed through a computational study. The quantum simulation approach is based on the resolution of the Schrodinger equation using the mode space non-equilibrium Green’s function formalism coupled self-consistently with a Poisson equation in the ballistic limit. The proposed nanodevice is endowed with ungated region between the auxiliary and control gates as well as with a laterally graded channel doping in order to improve the switching performance of the ultrascaled junctionless GNRTFET. The performance assessment has included the IDS–VGS transfer characteristics, subthreshold swing, current ratio, intrinsic delay, and power-delay product. It has been found that the proposed ultrascaled junctionless GNR tunneling FET can provide improved switching performance than its conventional counterpart. The proposed strategy can be applied to improve similar ultrascaled junctionless tunneling field-effect transistors for the future digital electronics, where the high-performance and the aggressive downscaling should be in agreement.

An analytical drain current model for graphene nanoribbon tunnel field-effect transistors

We develop a tunneling probability model based on a structure of p+-i-n+ of a graphene nanoribbon (GNR) tunnel field-effect transistors (TFETs), and present an analytical drain current model based on the tunneling probability model. Model results are compared with those in the literature, and good agreements are observed. With the drain current model, the output and transfer characteristics of currents in a GNR TFET can be obtained easily and quickly. Being in an explicit form, the drain current model can be embedded in the integrated circuit design simulation tools. We observe that low temperature favors both the GNR TFET’s drain current and subthreshold swing.

Comparison of tunneling currents in graphene nanoribbon tunnel field effect transistors calculated using Dirac-like equation and Schrödinger's equation

The tunneling current in a graphene nanoribbon tunnel field effect transistor (GNR-TFET) has been quantum mechanically modeled. The tunneling current in the GNR-TFET was compared based on calculations of the Dirac-like equation and Schrödinger's equation. To calculate the electron transmittance, a numerical approach-namely the transfer matrix method (TMM)-was employed and the Launder formula was used to compute the tunneling current. The results suggest that the tunneling currents that were calculated using both equations have similar characteristics for the same parameters, even though they have different values. The tunneling currents that were calculated by applying the Dirac-like equation were lower than those calculated using Schrödinger's equation.

The Understanding of SiNR and GNR TFETs for Analog and RF Application With Variation of Drain-Doping Molar Fraction

The 1-D nanoribbon (NR) of monolayer materials has gained immense interest due to their unique properties qualitatively distinct from their bulk properties and the demand for nanoscale applications. In this paper, the quantum transport properties of two most prominent 2-D materials, i.e., silicene NR (SiNR) and a graphene NR (GNR) tunnel field-effect transistor (TFET) with the effect of different dopant molar fractions in the drain region are studied numerically using nonequilibrium Green’s function formalism. In SiNR TFET, higher on-state current ( ${I}_{ \mathrm{\scriptscriptstyle ON}}$ ) is observed due to wider tunneling energy window and high transmission probability of carriers. In order to observe the effect of variation of doping density in the drain region, we have studied analog figures of merit such as the transconductance ( ${g}_{m}$ ), output resistance ( ${r}_{o}$ ), transconductance generation factor ( ${g}_{m}/{I}_{D}$ ), and the intrinsic gain ( ${g}_{m}{r}_{o}$ ) for different molar fractions. Similarly, we have evaluated the RF performance of the SiNR and GNR TFETs as a function of cutoff frequency ( ${f}_{T}$ ), gate capacitance ( ${C}_{G}$ ), and transport delay ( $\tau$ ).

Graphene Nanoribbon Tunnel Field-Effect Transistor via Segmented Edge Saturation

In this paper, segmented edge saturation (SES) is proposed as a novel method to design high-performance tunnel FETs (TFETs) using smooth graphene nanoribbon (GNR). When the edges of a smooth GNR are saturated by different elements in its different segments, the energy gaps ( ${E}_{g})$ of these segments can be tuned. Therefore, the tunneling and blocking effects are achieved simultaneously by reducing and enlarging the ${E}_{g}$ of GNR in its specific regions. Both high on-to-off current ( ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}})$ ratio and large ${I}_{ \mathrm{\scriptscriptstyle ON}}$ are obtained. Simulation results show that the $\mathrm {I}_{ \mathrm{\scriptscriptstyle OFF}}$ of the SES GNR TFET is below $1 \times 10^{-4} ~\mu \text{A}/\mu \text{m}$ and the ${I}_{{ \mathrm{\scriptscriptstyle ON}}}$ reaches 2000 $\mu \text{A}/\mu \text{m}$ at 0.4-V supply voltage. Detailed transport mechanisms of this TFET are investigated in comparisons with its uniform saturation counterparts. Dynamic performance and scaling ability analyzes are also carried out. The largest intrinsic delay and switching energy are only 0.17 ps and 0.09 fJ/ $\mu \text{m}$ . Meanwhile, it can maintain high ${I}_{{ \mathrm{\scriptscriptstyle ON}}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ratios when the channel length is scaled down to several nanometers. The discussions on GNR edge roughness suggest that the roughness should be avoided for better ${I}_{ \mathrm{\scriptscriptstyle ON}}$ behaviors. The good performance of the TFET highlights the capabilities of SES in the developments of GNR tunnel devices.

Performance Evaluation of Bilayer Graphene Nanoribbon Tunnel FETs for Digital and Analog Applications

The recent advancement in the fabrication of narrow graphene nanoribbon with smooth edges has catalyzed the growth of nanoribbon based transistors. Motivated by these advances, we suggest bilayer graphene nanoribbon tunnel field-effect transistors (BLGNR-TFETs) for low voltage digital and analog applications. The device performance is analyzed by quantum transport simulation, based on self-consistent solutions of two-dimensional Poisson's equation and nonequilibrium Green's function formalism. The results indicate that the use of BLGNR-TFET instead of monolayer graphene nanoribbon TFET with very narrow width, increases the ON current and reduces the intrinsic device delay without lowering the ON/OFF current ratio. The BLGNR-TFET for analog/RF applications has provided higher intrinsic gain at very narrow ribbon width; however, very narrow width reduces the cutoff frequency. Furthermore, RF figure of merits are investigated in the presence of external parasitics.

Analytical Current Transport Modeling of Graphene Nanoribbon Tunnel Field-Effect Transistors for Digital Circuit Design

A semi-classical and a semi-quantum current transport model for p-i-n n-type armchair graphene nanoribbon tunnel field effect transistor (TFET) are studied analytically. The results are compared with the numerical quantum transport simulation method using an atomistic Schrodinger–Poisson solver within the non-equilibrium Green function (NEGF) formalism. The channel length and width are 20 and 4.9 nm and a-GNR band gap is 0.289 eV. Current ratio ${\rm I_{ON}}/{\rm I_{OFF}}$ at 0.1 V supply voltage is calculated as follows: 122, 16.3 and 116 with a subthreshold slopes 26, 69 and 27.4 mV/decade from semi-classical, semi-quantum and NEGF simulation, respectively. Performance of a-GNR TFET is also studied analytically and numerically considering a-GNR width variation. Voltage transfer characteristics of a-GNR TFET inverter are computed for 0.1 V and 0.2 supply voltages using three current transport models which are in close agreement.

Simulation of Dirac Electron Tunneling Current in Armchair Graphene Nanoribbon Tunnel Field-Effect Transistors Using a Transfer Matrix Method

We simulate quantum mechanical tunneling current in armchair graphene nanoribbon tunnel field-effect transistors (AGNR-TFETs). The relativistic Dirac equation is used to determine electron wave functions in the AGNRs, while the potential profile is solved by the Poisson equation. We use a transfer matrix method (TMM) to calculate the electron transmittance and the Dirac electron tunneling current in the AGNR-TFETs. The results show that the Dirac electron tunneling current increases with increasing the drain and gate voltages. Moreover, the AGNR width and the thickness of insulator affect the characteristics of the Dirac electron tunneling currents.

TFET-Based Circuit Design Using the Transconductance Generation Efficiency &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$ {g}_{m}/ {I}_{d}$ &lt;/tex-math&gt;&lt;/inline-formula&gt; Method

Tunnel field effect transistors (TFETs) have emerged as one of the most promising post-CMOS transistor technologies. In this paper, we: 1) review the perspectives of such devices for low-power high-frequency analog integrated circuit applications (e.g., GHz operation with sub-0.1 mW power consumption); 2) discuss and employ a compact TFET device model in the context of the $g_{m}/I_{d}$ integrated analog circuit design methodology; and 3) compare several proposed TFET technologies for such applications. The advantages of TFETs arise since these devices can operate in the sub-threshold region with larger transconductance-to-current ratio than traditional FETs, which is due to the current turn-on mechanism being interband tunneling rather than thermionic emission. Starting from technology computer-aided design and/or analytical models for Si-FinFETs, graphene nano-ribbon (GNR) TFETs and InAs/GaSb TFETs at the 15-nm gate-length node, as well as InAs double-gate TFETs at the 20-nm gate-length node, we conclude that GNR TFETs might promise larger bandwidths at low-voltage drives due to their high current densities in the sub-threshold region. Based on this analysis and on theoretically predicted properties, GNR TFETs are identified as one of the most attractive field effect transistor technologies proposed-to-date for ultra-low power analog applications.

Graphene nanoribbon tunnel field effect transistor with lightly doped drain: Numerical simulations

By inserting a lightly doped region between the highly doped drain and the intrinsic channel of a graphene nanoribbon tunnel field effect transistor (GNR-TFET), we propose a new lightly doped drain (LDD)-GNR-TFET. Transport characteristics of the proposed transistor is numerically simulated, employing the third-nearest-neighbor tight-binding approximation in mode space non-equilibrium Green’s function formulism (NEGF), in ballistic regime. Simulations show, in comparison with a conventional GNR-TFET of the same dimensions, the proposed LDD-GNR-TFET exhibits a 102–103 times smaller OFF-current, an up to 105 times larger ON/OFF ratio, a shorter time delay, a smaller power-delay product (PDP) and a less drain induced barrier thinning (DIBT), besides preserving the subthreshold swing.

Optimum Bandgap and Supply Voltage in Tunnel FETs

A physics-based analytic model of the ON- and OFF-currents in a homojunction tunnel field-effect transistor (TFET) is used to understand the relationship between bandgap, gate length, ON-current, OFF-current, ON/OFF current ratio, and supply voltage to meet minimum energy requirements. The model, which applies to direct-bandgap semiconductors, is validated against numerical simulations to show that it captures the trends of more comprehensive simulations. The analytic model is then used to compare alternative channel materials for TFETs. Gate-all-around InAs nanowire and graphene nanoribbon TFETs are used as design examples at gate lengths of 10 and 15 nm and for an ON/OFF current specification of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> . The results suggest that TFETs based on 2-D materials can be more energy efficient than semiconductor nanowire TFETs and conventional metal-oxide-semiconductor field-effect transistors for low-power logic.

(Invited) Dielectrics for Graphene Transistors for Emerging Integrated Circuits

In this paper, we present the use of high-k dielectric material in design and physical modeling of following two types of transistors for emerging integrated circuits: graphene nano ribbon (GNR) tunnel field-effect transistors (TFETs) and graphene short channel FETs.Graphene nanoribbon field effect transistor (GNR FET) is one of the promising devices for emerging integrated circuits because of its exceptional electronic properties such as the large carrier mobility and planar structure. However, the problem is a high drain-source leakage current in standby state due to small band gap of graphene and short channel effect (SCE) enforced by the transistor feature size. We have proposed a new GNR FET structure which has two side gates with a lower work function than the main gate on top of the SiO2 and HfO2 insulating layers. The main gate and two side gates have work functions of Φ1=4.8eV and Φ2=4.5eV, respectively. The GNR FET width and length has been chosen 1.5nm and 15nm, where the source and drain regions are doped with a molar fraction of ionized donors equal to 5×10-3/cm3. The lengths of the main and side gates are 2.5nm and 1.25nm while those of insulator layers have been used as a variable. We have simulated the proposed GNR FET using the self-consistent solution of three-dimensional Poisson–Schrödinger equation, within the Non-equilibrium Green’s function (NEGF) formalism. In our new GNR FET structure, the two side gates induce the inversion layers next to drain and source regions, which increase the controllability of the main gate on the channel potential barrier by providing an effective screen for the main gate to prevent the change in drain current due to the change in drain voltage. We have found that the application of the HfO2 in combination of SiO2 in the proposed GNR FET not only shifts the drain source leakage current to the lower values by several orders of magnitude but also make the leakage current sensitive to the proportionality of thicknesses of SiO2 and HfO2. The proposed GNR FET introduce a minimum peak in the drain source leakage current much below that of a common GNR FET with the same insulator structure. The minimum leakage currents of the proposed structure for total insulator thickness of 10Å, 15 Å and 20Å are approximately 6×10-13A, 4×10-12A and 5×10-11A, respectively while those of a common GNR FET are 1.5×10-10A, 9×10-10A and 5×10-9A, respectively. The minimum peak in leakage current has been obtained for HfO2 thickness = 8Å and 14 Å corresponding to the total insulator thickness of 10 Å and 15 Å, respectively, while it shifts to purely HfO2 insulator layer for that of 20 Å. The proposed GNR FET can diminish the short channel effect and prevent an undesirable increase in the leakage current by decreasing the channel length. Effect of dielectric on performance of GNR TFETs is also studied through an analytical current transport model which we have developed for designing of integrated circuits. It is observed that choice of dielectric material and corresponding thickness along the lateral gate position severely modifies performance of GNR TFET. We have found that for constant oxide thickness of 0.76nm, threshold voltage reduces from 0.176V to 0.054V for dielectric permittivity of 2.5 to 47.5. We have also noticed that the performance of GNR TFET studied through threshold voltage and sub-threshold slope depends on selection of both dielectric material and their thickness. Specific results on influence of choice of dielectric in designing of GNR TFET will be presented included in the paper and presentation.

A Theoretical Model of Band-to-Band Tunneling Current in an Armchair Graphene Nanoribbon Tunnel Field-Effect Transistor

Tunneling current in an armchair graphene nanoribbon (AGNR) tunnel field-effect transistor (TFET) was modeled. A linear equation was employed in describing a potential distribution within the AGNR due to its simplicity. A parabolic dispersion and an electron effective mass obtained by approximating kx 0 to the parabolic dispersion were applied to AGNR. In order to obtain electron transmittance, electron wavefunctions in AGNR were based on Airy functions. The obtained transmittance was then applied to calculate the tunneling current by employing the Landauer formula. The calculated results showed that the tunneling current increases with the AGNR width. It was also shown that the tunneling current increases as temperature decreases. In addition, the gate voltage influences the saturation condition of tunneling current in AGNR TFETs.