Bilayer Graphene Field Effect Transistors Research Articles

Monolayer and bilayer graphene field effect transistor using Verilog-A

Monolayer and bilayer graphene field effect transistor modeling is presented in this paper. The transport model incorporated, works well for both drift diffusive and ballistic conditions. The validity of the model was checked for various device dimensions and bias voltages. Performance parameters affecting operation of graphene field effect transistor in various region of operation are optimized. Model was developed to verify transfer characteristics for monolayer and bilayer graphene field effect transistor. Results obtained prove the ambipolar property in Graphene. MATLAB is used for numerical modeling for systematic performance evaluation of parameters in graphene. The tool used to simulate the characteristics is cadence Verilog-A which describe analog component structure.

Influence of interlayer stacking arrangements on carrier accumulation in bilayer graphene field effect transistors

The electronic structure of bilayer graphene under an external electric field is studied in terms of the interlayer stacking arrangements using the density functional theory combined with the effective screening medium method. The calculations showed that the accumulated carrier distribution strongly depended on the interlayer stacking arrangement. The carriers were highly concentrated on the topmost layer of twisted bilayer graphene, while carriers were also found in the second layer of bilayer graphene with the AA and AB interlayer arrangements. In addition to the carrier accumulation, ab initio molecular dynamics calculations also revealed that the interlayer Seebeck coefficient and thermal diffusivity in the bilayer graphene depended on the interlayer stacking arrangements.

High temperature RF performances of epitaxial bilayer graphene field-effect transistors on SiC substrate

This paper investigated the temperature effects on the RF performances of the epitaxial bilayer graphene field-effect transistors (EBG-FETs) on a SiC substrate over a temperature range of 25–200 °C under the atmospheric environment. The temperature dependence of the cutoff frequency fT and the maximum oscillation frequency fmax of the EBG-FETs with an identical gate-length and different gate-widths were measured by small-signal measurement up to 40 GHz. The results show that EBG-FETs are capable of operating at high temperature up to 200 °C with workable amounts of thermally induced performance degradation. To gain further insight into the underlying physics of the device parameters affected by temperature, the small signal device parameter variations with ambient temperature are discussed. This work reveals the great potential of graphene in high temperature device applications.

Bilayer graphene nanoribbon field-effect transistor with electrically embedded source-side gate

In present-day technology, a bilayer graphene field-effect transistor (BLGNRFET) is known as a suitable alternative for continuing the scaling trend of transistor dimensions. This paper proposes an electrically embedded source-side gate BLGNRFET (EESG-BLGNRFET) in which an appropriate gate engineering takes into account near the source extension. In the proposed structure, this induces a vertical electric field and a larger bandgap in the vicinity of the source for low gate biases reducing off current. The EESG-BLGNRFET benefits from several analog and digital metrics. Intrinsic gain and cut-off frequency 84% and 44% are respectively improved in the EESG-BLGNRFET. Furthermore, the delay and power-delay product (PDP) of the proposed structure are lower than those of the conventional counterpart. It is shown that drain-induced barrier lowing (DIBL) in the EESG-BLGNRFET is decreased that makes the device less sensitive to short-channel effects.

Boltzmann transport equation‐based semi‐classical drain current model for bilayer GFET including scattering effects

In this study, a drain current model of bilayer graphene field-effect transistor (GFET) has been developed. The Boltzmann transport approach is considered to develop this model including the scattering effects. The effects of optical phonon scattering, acoustic phonon scattering and carrier-carrier scattering have been included. The effective voltages at top and back gates induce net mobile sheet charge density in graphene channel. Mobile sheet charge density helps to determine the quantum capacitance. Channel potential is derived from equivalent circuit. The electronic transport phenomena of carriers in channel are explained for the contribution of both electron and hole charge density. Finally, the expression of drain current is determined using the Boltzmann transport equation under non-equilibrium condition. Contact resistances at source and drain regions are considered to determine the accurate drain current. The small signal parameters of GFET are determined from drain current and net charge density, and finally using these parameters the authors have calculated the cut-off frequency of the model. We validate the model with several contemporary experimental results. Utilising the model, we have also explored the impact of back-gate bias, top-gate bias and channel length on the device characteristics.

A Surface Potential-Based Model for Dual Gate Bilayer Graphene Field Effect Transistor Including the Capacitive Effects

In this work, a surface potential modeling approach has been proposed to model dual gate, bilayer graphene field effect transistor. The equivalent capacitive network of GFET has been improved considering the quantum capacitance effect for each layer and interlayer capacitances. Surface potentials of both layers are determined analytically from equivalent capacitive network. The explicit expression of drain to source current is established from drift-diffusion transport mechanism using the surface potentials of the layers. The drain current characteristics and transfer characteristics of the developed model shows good agreement with the experimental results in literatures. The small signal parameters of intrinsic graphene transistor i.e., output conductance ([Formula: see text]), transconductance ([Formula: see text]), gate to drain capacitance ([Formula: see text]) and gate to source capacitance ([Formula: see text]) have been derived and finally, the cut-off frequency is determined for the developed model. The model is compared with reported experimental data using Normalized Root Mean Square Error (NRMSE) metric and it shows less than [Formula: see text] NRMSE. A Verilog-A code has been developed for this model and a single ended frequency doubler has been designed in Cadence Design environment using this Verilog-A model.

Optimization of 10nm Bi-GFET Device for higher ION/IOFF ratio using Taguchi Method

The simulation and statistical modeling are conducted using Silvaco TCAD tools and L9 orthogonal array (OA) of Taguchi method respectively to design a proposed layout of 10 nm gate length (Lg) Bilayer Graphene Field Effect Transistor (Bi-GFET). The investigated process parameters are halo implant dose. halo implant energy, source/drain (S/D) implant dose and source/drain (S/D) implant energy, while the noise factors are halo implant tilt angle and source/drain (S/D) implant tilt angle. The process parameters and the noise factors are optimized using the L9 orthogonal array (OA) of Taguchi method to achieve the highest possible ION/IOFF ratio. Utilizing both signal-to-noise ratio (SNR) and analysis of variance (ANOVA), the most dominant process parameters upon ION/IOFF ratio are identified as S/D implant energy and S/D implant dose with 56% and 37% factor effects on SNR respectively. The largest factor effects on SNR of S/D implant energy shows that it has dominantly affected the ION/IOFF ratio. The final results indicate that the 1.99 × 1013 atom/cm3 of halo implant dose. 174 keV of halo implant energy, 1.63 × 1014 atom/cm3 of S/D implant dose, 17 keV of S/D implant energy, 24° of halo implant tilt angle and 9° of S/D implant tilt angle are the best parameter setting in obtaining the highest Ion/Ioff ratio of the device which is measured at 4.811 × 105.

Electrically Inert h-BN/Bilayer Graphene Interface in All-Two-Dimensional Heterostructure Field Effect Transistors.

Bilayer graphene field effect transistors (BLG-FETs), unlike conventional semiconductors, are greatly sensitive to potential fluctuations because of the charged impurities in high- k gate stacks because the potential difference between two layers induced by the external perpendicular electrical filed is the physical origin behind the band gap opening. The assembly of BLG with layered h-BN insulators into a van der Waals heterostructure has been widely recognized to achieve the superior electrical transport properties. However, the carrier response properties at the h-BN/BLG heterointerface, which control the device performance, have not yet been revealed because of the inevitably large parasitic capacitance. In this study, the significant reduction of potential fluctuations to ∼1 meV is achieved in an all-two-dimensional heterostructure BLG-FET on a quartz substrate, which results in the suppression of the off-current to the measurement limit at a small band gap of ∼90 meV at 20 K. By capacitance measurement, we demonstrate that the electron trap/detrap response at such heterointerface is suppressed to undetectable level in the measurement frequency range. The electrically inert van der Waals heterointerface paves the way for the realization of future BLG electronics applications.

Bilayer graphene phonovoltaic-FET:In situphonon recycling

A new heat harvester, the phonovoltaic (pV) cell, was recently proposed. The device converts optical phonons into power before they become heat. Due to the low entropy of a typical hot optical phonon population, the phonovoltaic can operate at high fractions of the Carnot limit and harvest heat more efficiently than conventional heat harvesting technologies such as the thermoelectric generator. Previously, the optical phonon source was presumed to produce optical phonons with a single polarization and momentum. Here, we examine a realistic optical phonon source in a potential pV application and the effects this has on pV operation. Supplementing this work is our investigation of bilayer graphene as a new pV material. Our ab initio calculations show that bilayer graphene has a figure of merit exceeding 0.9, well above previously investigated materials. This allows a room-temperature pV to recycle 65% of a highly nonequilibrium, minimum entropy population of phonons. However, full-band Monte Carlo simulations of the electron and phonon dynamics in a bilayer graphene field-effect transistor (FET) show that the optical phonons emitted by field-accelerated electrons can only be recycled in situ with an efficiency of 50%, and this efficiency falls as the field strength grows. Still, an appropriately designed FET-pV can recycle the phonons produced therein in situ with a much higher efficiency than a thermoelectric generator can harvest heat produced by a FET ex situ.

Enhanced Intrinsic Voltage Gain in Artificially Stacked Bilayer CVD Graphene Field Effect Transistors

AbstractWe report on electronic transport in dual‐gate, artificially stacked bilayer graphene field effect transistors (BiGFETs) fabricated from large‐area chemical vapor deposited (CVD) graphene. The devices show enhanced tendency to current saturation, which leads to reduced minimum output conductance values. This results in improved intrinsic voltage gain of the devices when compared to monolayer graphene FETs. We employ a physics based compact model originally developed for Bernal stacked bilayer graphene FETs (BSBGFETs) to explore the observed phenomenon. The improvement in current saturation may be attributed to increased charge carrier density in the channel and thus reduced saturation velocity due to carrier‐carrier scattering.

Numerical simulation of vertical tunneling transistor with bilayer graphene as source and drain regions

In this paper, the electrical characteristics of vertical tunneling bilayer graphene field effect transistor (VTBGFET) are theoretically investigated. We evaluate the device behavior using nonequilibrium Green's function (NEGF) formalism combined with an atomistic tight binding model. By using this method, we extract the most significant figures of merit such as ON/OFF current ratio, subthreshold swing, and intrinsic gate‐delay time.The results indicate that using a bilayer graphene instead of a monolayer graphene as the base material for the source and drain regions leads to a larger ON/OFF current ratio due to the presence of an energy bandgap in biased bilayer graphene. Also, the subthreshold swing of VTBGFET can be much lower than that of vertical tunneling monolayer graphene field‐effect transistor (VTMGFET). We find that the increase of the number of hBN layers enhances the ON/OFF current ratio but degrades the intrinsic gate‐delay time.

Heterodyne detection at 216, 432, and 648 GHz based on bilayer graphene field-effect transistor with quasi-optical coupling

Graphene field-effect transistors (G-FETs) are being developed with ever increasing sensitivity as terahertz direct detectors at room temperature. The noise-equivalent power is being reduced towards the order of 1pW/Hz with frequency about 650 GHz. G-FET-based frequency multipliers and subharmonic mixers are becoming new active components for both millimeter and terahertz wave technologies. We report a 650-GHz G-FET coupled with terahertz antennas and a quasi-optical silicon lens for heterodyne/subharmonic detection. Heterodyne detection at 216 and 648 GHz, second-subharmonic detection at 432 GHz, and third-subharmonic detection at 648 GHz were demonstrated and characterized. Self-mixing/homodyne detection was used as a calibration tool for the power levels of the local pump source and the source to be detected. The conversion losses of heterodyne mixing at 216 GHz (off-resonant) and 648 GHz (resonant) were 38.4 dB and 57.9 dB with 12.8 dBm and −14 dBm of local pump power, respectively. The conversion loss of the second-harmonic mixing at 432 GHz is about 8 dB higher than that of the heterodyne mixing at 216 GHz. Calculations based on our quasi-static mixing model agree well with the measured conversion losses. To improve the conversion loss, more details on the G-FET noise, the optimal pump power, and the antenna should be examined.

Room-temperature, low-impedance and high-sensitivity terahertz direct detector based on bilayer graphene field-effect transistor

We report a room-temperature, low-impedance and high-sensitivity terahertz direct detector based on bilayer graphene field-effect transistor (GFET). Epitaxially grown on silicon carbide, the bilayer graphene had an as-grown carrier mobility of 3000 cm2/Vs. The source/drain contacts were formed on the freshly cleaned graphene sheet to minimize the contact resistance and served also as terahertz antennas. The gate and the dielectric layer underneath with a length of 145 nm and a gap of 203 nm to the source/drain antennas were formed in a self-alignment process. Although the carrier mobility in the GFET was reduced to about 405 cm2/Vs, the high-quality ohmic contacts and the short graphene channel delivered an overall source-drain resistance less than 203 Ω. A voltage responsivity of 30 V/W and a noise-equivalent power of 51 pW/Hz1/2 were estimated in direct detection at 0.33 THz. By using the GFET detector as a two-terminal detector, i.e., with the gate floating, a transmission-type terahertz imaging was demonstrated. Such a low-impedance GFET detector is ready to be matched to a commercial 50-Ω low-noise radio/microwave frequency amplifier allowing for high-speed homodyne detection and heterodyne detection in a quasi-optical configuration.

A new approach to study the effect of generation rate on drain-source current of bilayer graphene transistors

This paper presents a new approach to study the effect of impact ionization on the current of bilayer graphene field effect transistors. Analytical models for surface potential and current together with a Monte Carlo approach which include the edge effect scattering are used to calculate the current and generation rate in bilayer graphene transistors due to ionization. FlexPDE simulation is also employed for verification of surface potential modeling. Using the approach, the profile of generation rate, surface potential and current are plotted with respect to several structural parameters. We have shown that ignoring this effect in the modeling will result in an error of up to 10 % for a typical 30 nm bilayer graphene field effect transistor. As a result, we conclude that any analytical study ignoring the ionization is incomplete for bilayer graphene field effect transistors. The model presented here can be applied in optimization of photo detectors based on graphene.

Large-signal model of the bilayer graphene field-effect transistor targeting radio-frequency applications: Theory versus experiment

Bilayer graphene is a promising material for radio-frequency transistors because its energy gap might result in a better current saturation than the monolayer graphene. Because the great deal of interest in this technology, especially for flexible radio-frequency applications, gaining control of it requires the formulation of appropriate models for the drain current, charge, and capacitance. In this work, we have developed them for a dual-gated bilayer graphene field-effect transistor. A drift-diffusion mechanism for the carrier transport has been considered coupled with an appropriate field-effect model taking into account the electronic properties of the bilayer graphene. Extrinsic resistances have been included considering the formation of a Schottky barrier at the metal-bilayer graphene interface. The proposed model has been benchmarked against experimental prototype transistors, discussing the main figures of merit targeting radio-frequency applications.

Gap state analysis in electric-field-induced band gap for bilayer graphene.

The origin of the low current on/off ratio at room temperature in dual-gated bilayer graphene field-effect transistors is considered to be the variable range hopping in gap states. However, the quantitative estimation of gap states has not been conducted. Here, we report the systematic estimation of the energy gap by both quantum capacitance and transport measurements and the density of states for gap states by the conductance method. An energy gap of ~250 meV is obtained at the maximum displacement field of ~3.1 V/nm, where the current on/off ratio of ~3 × 103 is demonstrated at 20 K. The density of states for the gap states are in the range from the latter half of 1012 to 1013 eV−1cm−2. Although the large amount of gap states at the interface of high-k oxide/bilayer graphene limits the current on/off ratio at present, our results suggest that the reduction of gap states below ~1011 eV−1cm−2 by continual improvement of the gate stack makes bilayer graphene a promising candidate for future nanoelectronic device applications.

Chemically Modulated Band Gap in Bilayer Graphene Memory Transistors with High On/Off Ratio.

We report a chemically conjugated bilayer graphene field effect transistor demonstrating a high on/off ratio without significant degradation of the on-current and mobility. This was realized by introducing environmentally stable benzyl viologen as an electron-donating group and atmospheric dopants as an electron-withdrawing group, which were used as dopants for the bottom and top of the bilayer graphene, respectively. A high mobility of ∼3100 cm(2) V(-1) s(-1) with a high on/off ratio of 76.1 was obtained at room temperature without significant degradation of the on-current. This is attributed to low charge scattering due to physisorbed dopants without provoking sp(3) structural disorders. By utilizing our band-gap-opened bilayer graphene, excellent nonvolatile memory switching behavior was demonstrated with a clear program/erase state by applying pulse gate bias. The initial program/erase current ratio of ∼34.5 was still retained at higher than 10 even after 10(4) s.

Progress of terahertz devices based on graphene

Graphene has unique electronic properties stemming from a linear gapless carrier energy spectrum, and has dominant advantages in the research of devices such as lasers, detectors and modulators in terahertz region due to its tunable energy gap and extremely high carrier mobility. In this review, we summarize its latest progress in applications of terahertz devices such as lasers, detectors and modulators. Terahertz lasers based on graphene can reach a gain as high as 104 cm-1, and terahertz detectors with different structures such as a bilayer graphene field-effect transistor with top gate and buried gate can achieve NEP (noise equivalent power) ～ m nW/Hz. Graphene terahertz modulators, which are equipped with transmission configuration and reflection configuration, can have a very high modulation depth. These results may be helpful for developing the high-efficiency graphene terahertz devices.

Assessment of H-intercalated graphene for microwave FETs through material characterization and electron transport studies

Epitaxial graphene is grown on semi-insulating (SI) 4H-SiC in a hot wall CVD reactor by graphitization and in-situ intercalation with (H)ydrogen. A holistic material characterization is performed in order to ascertain the number of layers, layer uniformity, and electron transport properties of the epi-layers via electronic test structures and Raman spectroscopy. Bilayer graphene field effect transistors (GFETs) are fabricated using a full electron beam lithography (EBL) process which is optimized for low contact resistances of rc<0.2Ωmm. Mobilities of order 2500cm2/Vs are achieved on bilayer samples after fabrication. The devices demonstrate high transconductance gm=400mS/mm and high current density Ids=1.8A/mm. The output conductance at the bias of maximum transconductance is gds=300mS/mm. The GFETs demonstrate an extrinsic ftext and fmaxext of 20 and 13GHz, respectively and show 6dB power gain at 1GHz in a 50Ω system, which is the highest reported to date.

Analytical study of subthreshold behaviour of double gate bilayer graphene field effect transistors

In this paper, several analytical models have been developed for 2-D potential distribution, subthreshold current, drain induced barrier lowering (DIBL), and subthreshold-slope (SS) to study the subthreshold behaviour of bilayer graphene filed effect transistors (BLG-FETs). The models are grounded on the basis of the exact solution of the two-dimensional Poisson’s equation while the quantum capacitance effect has been considered throughout the models. The accuracy of the potential distribution model is verified by its analytical results that agree well with those of the FlexPDE Poissonʼs equation solver program. In addition, the effects of the channel length, the oxide thickness, quantum capacitance, and gate biases on subthreshold parameters of BLG-FETs have been explored and the results are compared with those of the silicon FETs.