Effect of external magnetic field on the instability of THz plasma waves in nanoscale graphene field-effect transistors

The instability of plasma waves in the channel of field-effect transistors will cause the electromagnetic waves with THz frequency.Based on a self-consistent quantum hydrodynamic model,the instability of THz plasmas waves in the channel of graphene field-effect transistors has been investigated with external magnetic field and quantum effects.We analyzed the influence of weak magnetic fields,quantum effects,device size,and temperature on the instability of plasma waves under asymmetric boundary conditions numerically.The results show that the magnetic fields,quantum effects,and the thickness of the dielectric layer between the gate and the channel can increase the radiation frequency.Additionally,we observed that increase in temperature leads to a decrease in both oscillation frequency and instability increment.The numerical results and accompanying images obtained from our simulations provide support for the above conclusions.

Design and Analysis of Graphene Based Tunnel Field Effect Transistor with Various Ambipolar Reducing Techniques

The fundamental advantages of carbon-based graphene material,such as its high tunnelling probability,symmetric band structure(linear dependence of the energy band on the wave direction),low effective mass,and characteristics of its 2D atomic layers,are the main focus of this research work.The impact of channel thickness,gate under-lap,asymmetric source/drain doping method,workfunction of gate contact,and High-K material on Graphene-based Tunnel Field Effect Transistor(TFET)is analyzed with 20 nm technology.Physical modelling and electrical characteristic performance have been simulated using the Atlas device simulator of SILVACO TCAD with user-defined material syntax for the newly included graphene material in comparison to silicon carbide(SiC).The simulation results in significant suppression of ambipolar current to voltage characteristics of TFET and modelled device exhibits a significant improvement in subthreshold swing(0.0159 V/decade),the ratio of Ion/Ioff(1000),and threshold voltage(-0.2 V with highly doped p-type source and 0.2 V with highly doped n-type drain)with power supply of 0.5 V,which make it useful for low power digital applications.

Room-temperature terahertz detection based on CVD graphene transistor

We report the fabrication and characterization of a single-layer graphene field-effect terahertz detector, which is cou- pled with dipole-like antennas based on the self-mixing detector model. The graphene is grown by chemical vapor deposi- tion and then transferred onto an SiO2/Si substrate. We demonstrate room-temperature detection at 237 GHz. The detector could offer a voltage responsivity of 0.1 V/W and a noise equivalent power of 207 nW/Hz 1/2. Our modeling indicates that the observed photovoltage in the p-type gated channel can be well fit by the self-mixing theory. A different photoresponse other than self-mixing may apply for the n-type gated channel.

High-Performance Photo-Modulated Thin-Film Transistor Based on Quantum dots/Reduced Graphene Oxide Fragment-Decorated ZnO Nanowires

In this paper, a photo-modulated transistor based on the thin-film transistor structure was fabricated on the flexible substrate by spin-coating and magnetron sputtering. A novel hybrid material that composed of Cd Se quantum dots and reduced graphene oxide(RGO) fragment-decorated ZnO nanowires was synthesized to overcome the narrow optical sensitive waveband and enhance the photo-responsivity. Due to the enrichment of the interface and heterostructure by RGO fragments being utilized, the photo-responsivity of the transistor was improved to 2000 AW^(-1) and the photo-sensitive wavelength was extended from ultraviolet to visible. In addition, a positive back-gate voltage was employed to reduce the Schottky barrier width of RGO fragments and ZnO nanowires. As a result, the amount of carriers was increased by 10 folds via the modulation of back-gate voltage. With these inherent properties, such as integrated circuit capability and wide optical sensitive waveband, the transistor will manifest great potential in the future applications in photodetectors.

A review for compact model of graphene field-effect transistors

Graphene has attracted enormous interests due to its unique physical, mechanical, and electrical properties. Specially, graphene-based field-effect transistors （FETs） have evolved rapidly and are now considered as an option for conventional silicon devices. As a critical step in the design cycle of modem IC products, compact model refers to the development of models for integrated semiconductor devices for use in circuit simulations. The purpose of this review is to provide a theoretical description of current compact model of graphene field-effect transistors. Special attention is devoted to the charge sheet model, drift-diffusion model, Boltzmann equation, density of states （DOS）, and surface-potential-based compact model. Finally, an outlook of this field is briefly discussed.

Study of short-term synaptic plasticity in Ion-Gel gated graphene electric-double-layer synaptic transistors

Multi-terminal electric-double-layer transistors have recently attracted extensive interest in terms of mimicking synaptic and neural functions.In this work,an Ion-Gel gated graphene synaptic transistor was proposed to mimic the essential synaptic behaviors by exploiting the bipolar property of graphene and the ionic conductivity of Ion-Gel.The Ion-Gel dielectrics were deposited onto the graphene film by the spin coating process.We consider the top gate and graphene channel as a presynaptic and postsynaptic terminal,respectively.Basic synaptic functions were successfully mimicked,including the excitatory postsynaptic current(EPSC),the effect of spike amplitude and duration on EPSC,and paired-pulse facilitation(PPF).This work may facilitate the application of graphene synaptic transistors in flexible electronics.

Layer by Layer Self-assembly Fiber-based Flexible Electrochemical Transistor

Poly(3,4-ethylenedioxyethiophene)-polystyrene sulfonic acid(PEDOT:PSS)/polyallyl dimethyl ammonium chloride modified reduced graphene oxide(PDDA-rGO)was layer by layer self-assembled on the cotton fiber.The surface morphology and electric property was investigated.The results confirmed the dense membrane of PEDOT:PSS and the lamellar structure of PDDA-rGO on the fibers.It has excellent electrical conductivity and mechanical properties.The fiber based electrochemical transistor(FECTs)prepared by the composite conductive fiber has a maximum output current of 8.7 mA,a transconductance peak of 10 mS,an on time of 1.37 s,an off time of 1.6 s and excellent switching stability.Most importantly,the devices by layer by layer self-assembly technology opens a path for the true integration of organic electronics with traditional textile technologies and materials,laying the foundation for their later widespread application.

High temperature characteristics of bilayer epitaxial graphene field-effect transistors on SiC Substrate

In this paper,high temperature direct current（DC） performance of bilayer epitaxial graphene device on SiC substrate is studied in a temperature range from 25℃ to 200℃.At a gate voltage of-8 V（far from Dirac point）,the drainsource current decreases obviously with increasing temperature,but it has little change at a gate bias of ＋8 V（near Dirac point）.The competing interactions between scattering and thermal activation are responsible for the different reduction tendencies.Four different kinds of scatterings are taken into account to qualitatively analyze the carrier mobility under different temperatures.The devices exhibit almost unchanged DC performances after high temperature measurements at 200℃ for 5 hours in air ambience,demonstrating the high thermal stabilities of the bilayer epitaxial graphene devices.

Radio-frequency transistors from millimeter-scale graphene domains

Graphene is a new promising candidate for application in radio-frequency （RF） electronics due to its excellent elec- tronic properties such as ultrahigh carrier mobility, large threshold current density, and high saturation velocity. Recently, much progress has been made in the graphene-based RF field-effect transistors （RF-FETs）. Here we present for the first time the high-performance top-gated RF transistors using millimeter-scale single graphene domain on a SiO2/Si substrate through a conventional microfabrication process. A maximum cut-off frequency of 178 GHz and a peak maximum os- cillation frequency of 35 GHz are achieved in the graphene-domain-based FET with a gate length of 50 nm and 150 nm, respectively. This work shows that the millimeter-scale single graphene domain has great potential applications in RF devices and circuits.

Impact of Channel Length and Width for Charge Transportation of Graphene Field Effect Transistor

The effect of channel length and width on the large and small-signal parameters of the graphene field effect transistor have been explored using an analytical approach.In the case of faster saturation as well as extremely high transit frequency,the graphene field effect transistor shows outstanding performance.From the transfer curve,it is observed that there is a positive shift of Dirac point from the voltage of 0.15 V to 0.35 V because of reducing channel length from 440 nm to 20 nm and this curve depicts that graphene shows ambipolar behavior.Besides,it is found that because of widening channel the drain current increases and the maximum current is found approximately 2.4 mA and 6 mA for channel width 2μm and 5μm respectively.Furthermore,an approximate symmetrical capacitance-voltage(C-V)characteristic of the graphene field effect transistor is obtained and the capacitance reduces when the channel length decreases but the capacitance can be increased by raising the channel width.In addition,a high transconductance,that demands high-speed radio frequency(RF)applications,of 6.4 mS at channel length 20 nm and 4.45 mS at channel width 5μm along with a high transit frequency of 3.95 THz have been found that demands high-speed radio frequency applications.

Application of graphene vertical field effect to regulation of organic light-emitting transistors

The luminescence intensity regulation of organic light-emitting transistor(OLED)device can be achieved effectively by the combination of graphene vertical field effect transistor(GVFET)and OLED.In this paper,we fabricate and characterize the graphene vertical field-effect transistor with gate dielectric of ion-gel film,confirming that its current switching ratio reaches up to 102.Because of the property of high light transmittance in ion-gel film,the OLED device prepared with graphene/PEDOT:PSS as composite anode exhibits good optical properties.We also prepare the graphene vertical organic light-emitting field effect transistor(GVOLEFET)by the combination of GVFET and graphene OLED,analyzing its electrical and optical properties,and confirming that the luminescence intensity can be significantly changed by regulating the gate voltage.

Comparative Study of Monolayer and Bilayer Epitaxial Graphene Field-Effect Transistors on SiC Substrates

Monolayer and bilayer graphenes have generated tremendous excitement as the potentially useful electronic materials due to their unique features. We report on monolayer and bilayer epitaxial graphene field-effect transistors （GFETs） fabricated on SiC substrates. Compared with monolayer GFETs, the bilayer GFETs exhibit a significant improvement in dc characteristics, including increasing current density I DS, improved transconductance g m, reduced sheet resistance lion, and current saturation. The improved electrical properties and tunable bandgap in the bilayer graphene lead to the excellent dc performance of the bilayer GFETs. Furthermore, the improved dc characteristics enhance a better rf performance for bilayer graphene devices, demonstrating that the quasifree-standing bilayer graphene on SiC substrates has a great application potential for the future graphene-based electronics.

Effect of Residual Charge Carrier on the Performance of a Graphene Field Effect Transistor

The temperature-dependent effect of residual charge carrier （no）, at the Dirac point, on mobility is studied. We fabricate and characterize a graphene field effect transistor （GFET） using 7nm TiO2 as the top-gate dielectric. The temperature-dependent gate voltage-drain current and room temperature gate capacitance are measured to extract the carrier mobility and to estimate the quantum capacitance of the GFET. The device shows the mobility value of gOO cm^2 /V.s at room temperature and it decreases to 45 cm^2 /V.s for 20 K due to the increase of n0. These results indicate that the phonon scattering is not the dominant process for the unevenness dielectric layer while the coulomb scattering by charged impurities degrades the device characteristically at low temperature.

Comparison of tunneling currents in graphene nanoribbon tunnel field effect transistors calculated using Dirac-like equation and Schrodinger’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 Schrodinger’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 Schrodinger’s equation.

Complete coverage of reduced graphene oxide on silicon dioxide substrates

Reduced graphene oxide （RGO） has the advantage of an aqueous and industrial-scale production route. No other approaches can rival the RGO field effect transistor platform in terms of cost （〈US$1） and portability （millimetre scale）. However the large deviations in the electrical resistivity of this fabricated material prevent it from being used widely. After an ethanol chemical vapor deposition （CVD） post-treatment to graphene oxide with ethanol, carbon islets are deposited preferentially at the edges of existing flakes. With a 2-h treatment, the standard deviation in electrical resistance of the treated chips can be reduced by 99.95%. Thus this process could enable RGO to be used in practical electronic devices.

Depositing aluminum as sacrificial metal to reduce metal–graphene contact resistance

Reducing the contact resistance without degrading the mobility property is crucial to achieve high-performance graphene field effect transistors. Also, the idea of modifying the graphene surface by etching away the deposited metal provides a new angle to achieve this goal. We exploit this idea by providing a new process method which reduces the contact resistance from 597Ω ·μm to sub 200 Ω ·μm while no degradation of mobility is observed in the devices. This simple process method avoids the drawbacks of uncontrollability, ineffectiveness, and trade-off with mobility which often exist in the previously proposed methods.

Characterization of Nanometer-Spaced Few-Layer Graphene Electrodes

We study graphene electrodes that can be used for contacting single molecules. The nanometer-scale gap is made by feedback controlled electroburning in few-layer graphene sheets. We analyze the time stability, and the influence of the temperature and gate voltage on the current flowing through the empty gaps. The electrodes are stable at room temper- ature for long periods of time. We show statistics of the relation between the initial resistance of the few-layer graphe- ne flakes and the final size of the gaps. We find that thicker flakes are more suitable for the fabrication of the elec-trodes.

Ultrasensitive detection of methamphetamine by antibody-modified transistor assay

Effective detection of methamphetamine(Met)requires a fast,sensitive,and cheap testing assay.However,commercially available methods require expensive instruments and highly trained operators,which are time-consuming and labor-intensive.Herein,an antibody-modified graphene transistor assay is developed for sensitive and minute-level detection of Met in complex environments.The anti-Met probe captured charged targets within 120 s,leading to a p-doping effect near the graphene channel.The limit of detection reaches 50 aM(5.0×10^(-17)M)Met in solution.The graphene transistor would be a valuable tool for Met detection effective prevention of drug abuse.

Flexible Graphene Field‑Effect Transistors and Their Application in Flexible Biomedical Sensing

Flexible electronics are transforming our lives by making daily activities more convenient.Central to this innovation are field-effect transistors(FETs),valued for their efficient signal processing,nanoscale fabrication,low-power consumption,fast response times,and versatility.Graphene,known for its exceptional mechanical properties,high electron mobility,and biocompatibility,is an ideal material for FET channels and sensors.The combination of graphene and FETs has given rise to flexible graphene field-effect transistors(FGFETs),driving significant advances in flexible electronics and sparked a strong interest in flexible biomedical sensors.Here,we first provide a brief overview of the basic structure,operating mechanism,and evaluation parameters of FGFETs,and delve into their material selection and patterning techniques.The ability of FGFETs to sense strains and biomolecular charges opens up diverse application possibilities.We specifically analyze the latest strategies for integrating FGFETs into wearable and implantable flexible biomedical sensors,focusing on the key aspects of constructing high-quality flexible biomedical sensors.Finally,we discuss the current challenges and prospects of FGFETs and their applications in biomedical sensors.This review will provide valuable insights and inspiration for ongoing research to improve the quality of FGFETs and broaden their application prospects in flexible biomedical sensing.

石墨烯场效应晶体管的X射线总剂量效应

本文针对不同结构、尺寸的石墨烯场效应晶体管(graphene field effect transistors,GFET)开展了基于10 keV-X射线的总剂量效应研究.结果表明,随累积剂量的增大,不同结构GFET的狄拉克电压V_(Dirac)和载流子迁移率μ不断退化;相比于背栅型GFET,顶栅型GFET的辐射损伤更加严重;尺寸对GFET器件的总剂量效应决定于器件结构;200μm×200μm尺寸的顶栅型GFET损伤最严重,而背栅型GFET是50μm×50μm尺寸的器件损伤最严重.研究表明:对于顶栅型GFET,辐照过程中在栅氧层中形成的氧化物陷阱电荷的积累是V_(Dirac)和μ降低的主要原因.背栅型GFET不仅受到辐射在栅氧化层中产生的陷阱电荷的影响,还受到石墨烯表面的氧吸附的影响.在此基础上,结合TCAD仿真工具实现了顶栅器件氧化层中辐射产生的氧化物陷阱电荷对器件辐射响应规律的仿真.相关研究结果对于石墨烯器件的抗辐照加固研究具有重大意义.