Graphene Field-effect Transistors Research Articles

Quantitative assessment of thermal resistance in graphene field-effect transistors based on a phonon hydrodynamics and van der Waals coupling approach

In the progressive miniaturization of graphene field-effect transistors (GrFETs), pronounced thermal boundary resistance (TBR) and increasing ballistic-diffusive thermal resistance pose formidable challenges to thermal management. Quantitative analysis of thermal resistance is crucial for advancing efficient thermal designs in GrFETs using state-of-the-art technology. The inherently low-dimensional structure nature of graphene means that thermal conduction in GrFETs, governed by phonon hydrodynamic transport and van der Waals (vdW) interactions, cannot be accurately described by traditional Fourier's law-based models or the single-mode relaxation time approximation. This study introduces an innovative thermal simulation framework that addresses these challenges by incorporating phonon hydrodynamics equations and considering weak vdW coupling. Our findings indicate that thermal conduction in GrFETs is predominantly facilitated by flexural (ZA) phonons, and their thermal transport efficiency is significantly influenced by substrate perturbations. Specifically, the TBR in GrFETs is closely associated with the weak vdW coupling between ZA phonons and substrate atoms. Furthermore, this study employs dimensionless thermal resistance ratios and Knudsen numbers to explore the impact of silicon's geometric dimensions, heat source size, and ballistic phenomena on the substrate's thermal resistance. These insights provide substantial theoretical support for the development of efficient thermal management strategies in GrFETs.

Ultrafast Opto-Electronic and Thermal Tuning of Third-Harmonic Generation in a Graphene Field Effect Transistor.

Graphene is a unique platform for tunable opto-electronic applications thanks to its linear band dispersion, which allows electrical control of resonant light-matter interactions. Tuning the nonlinear optical response of graphene is possible both electrically and in an all-optical fashion, but each approach involves a trade-off between speed and modulation depth. Here, lattice temperature, electron doping, and all-optical tuning of third-harmonic generation are combined in a hexagonal boron nitride-encapsulated graphene opto-electronic device and demonstrate up to 85% modulation depth along with gate-tunable ultrafast dynamics. These results arise from the dynamic changes in the transient electronic temperature combined with Pauli blocking induced by the out-of-equilibrium chemical potential. The work provides a detailed description of the transient nonlinear optical and electronic response of graphene, which is crucial for the design of nanoscale and ultrafast optical modulators, detectors, and frequency converters.

Sensitive detection of infectious disease utilizing carbon Sphere@Fe3O4 micromotor combined with graphene field-effect transistor

BackgroundRapid on-site detection of infectious diseases is considerably essential for preventing and controlling major epidemics and maintaining social and public safety. However, the complexity of the natural environment in which infectious disease pathogens exist severely disrupts the performance of on-site detection, and rapid detection can become meaningless because of the cumbersome sample pretreatment process. ResultHerein, a new detection platform based on a carbon sphere@Fe3O4 micromotor (CS@Fe3O4) in combination with a graphene field-effect transistor (GFET) was designed and used for the on-site detection of SARS-CoV-2 coronavirus pathogens. The CS@Fe3O4 micromotor, surface-modified with anti-SARS-CoV-2 coronavirus antibody, could move at a velocity of 79.4 μm/s in a solution containing hydrogen peroxide (H2O2) and exhibited capture rates of 67.9% and 36.2% for the SARS-CoV-2 pathogen in phosphate buffered saline (PBS) and soil solutions, respectively. After magnetic field separation, the captured micromotor was used for GFET detection, with detection limits of 4.6 and 15.6 ag/mL in PBS and soil solutions, respectively. Significance and noveltyThis detection platform can be employed to avoid complex sample pretreatment procedures and achieve rapid on-site detection of SARS-CoV-2 coronavirus pathogens in complex environments. This study introduces a novel approach for the on-site detection of infectious diseases.

Enhancing Radio Frequency Performance of Graphene Field-Effect Transistors through Machine-Learning-Based Physical Prediction and Optimization

Enhancing Radio Frequency Performance of Graphene Field-Effect Transistors through Machine-Learning-Based Physical Prediction and Optimization

Study of graphene p-n junctions formed by the electrostatic modification of the SiO2 substrate.

We study the transport properties of mm-scale CVD graphene p-n junctions, which are formed in a single gated graphene field effect transistor configuration. Here, an electrical-stressing-voltage technique served to modify the electrostatic potential in the SiO2/Si substrate and create the p-n junction. We examine the transport characteristics about the Dirac points that are localized in the perturbed and unperturbed regions in the graphene channel and note the quantitative differences in the Hall effect between the perturbed and unperturbed regions. The results also show that the longitudinal resistance is highly sensitive to the external magnetic field when the Hall bar device operates as a p-n junction.

A Quasi-Ballistic Model for Short Channel Monolayer Graphene Field Effect Transistor Including Scattering Effects

In this work, the Landauer approach-based short channel model of a graphene field effect transistor has been presented. The quasi-ballistic and ballistic transport mechanism is explained by this approach for nanoscale transistors. The concept of the virtual source is considered to obtain the carrier density in active regions. Here, we apply the Landauer approach for short-channel graphene field effect transistors relating to the parameter in the virtual source model. The mobility of the model has been improved including various scattering effects like impurity scattering, acoustic phonon scattering, and optical phonon scattering. The dynamic resistance model is considered due to asymmetry transport in the electron branch and hole branch. The proposed model has been verified with several experimental works for model validation. The comparisons of the model simulation with several experimental results that have been reported in the literature match well for different short channel lengths of graphene.

High mobility graphene field effect transistors on flexible EVA/PET foils

Monolayer graphene is a promising material for a wide range of applications, including sensors, optoelectronics, antennas, EMR shielding, flexible electronics, and conducting electrodes. Chemical vapor deposition (CVD) of carbon atoms on a metal catalyst is the most scalable and cost-efficient method for synthesizing high-quality, large-area monolayer graphene. The usual method of transferring the CVD graphene from the catalyst to a target substrate involves a polymer carrier which is dissolved after the transfer process is completed. Due to often unavoidable damage to graphene, as well as contamination and residues, carrier mobilities are typically 1000–3000 cm2(Vs)−1 , unless complex and elaborate measures are taken. Here, we report on a simple scalable fabrication method for flexible graphene field-effect transistors that eliminates the polymer interim carrier, by laminating the graphene directly onto office lamination foils, removing the catalyst, and depositing Parylene N as a gate dielectric and encapsulation layer. The fabricated transistors show field- and Hall-effect mobilities of 7000–10 000 cm2(Vs)−1 with a residual charge-carrier density of 2×1011 1 cm−2 at room temperature. We further validate the material quality by terahertz time-domain spectroscopy and observation of the quantum Hall effect at low temperatures in a moderate magnetic field of ∼5 T. The Parylene encapsulation provides long-term stability and protection against additional lithography steps, enabling vertical device integration in multilayer electronics on a flexible platform.

Ultrasensitive Graphene Field-Effect Biosensors Based on Ferroelectric Polarization of Lithium Niobate for Breast Cancer Marker Detection.

Herein, we present a novel ultrasensitive graphene field-effect transistor (GFET) biosensor based on lithium niobate (LiNbO3) ferroelectric substrate for the application of breast cancer marker detection. The electrical properties of graphene are varied under the electrostatic field, which is generated through the spontaneous polarization of the ferroelectric substrate. It is demonstrated that the properties of interface between graphene and solution are also altered due to the interaction between the electrostatic field and ions. Compared with the graphene field-effect biosensor based on the conventional Si/SiO2 gate structure, our biosensor achieves a higher sensitivity to 64.7 mV/decade and shows a limit of detection down to 1.7 fM (equivalent to 12 fg·mL-1) on the detection of microRNA21 (a breast cancer marker). This innovative design combining GFETs with ferroelectric substrates holds great promise for developing an ultrahigh-sensitivity biosensing platform based on graphene that enables rapid and early disease diagnosis.

Heat Flow Overshooting in Suspended Graphene

ABSTRACT In this work we found that when the hydrodynamic heat conduction in suspended graphene is in the under-damped free oscillation regime the disturbance of heat flux at the boundaries of graphene and the joint action of the initial heat flux and its time change rate can cause the heat flow overshooting which refers to the excess heat flux established in the heat conduction medium when two heat flow wave-fronts meet. The overshooting can make the maximum amplitude of the heat flux more than twice that of the initial heat flux, which poses a great danger to the next high-frequency and high-efficiency electronic devices based on graphene, such as the graphene field-effect transistors. Interestingly, results show that when the scale ratio of graphene with rectangular shape in the heat flow direction to its vertical direction is greater than a certain value, the overshooting caused by the disturbance of the heat flux at the boundaries no longer occurs. For the overshooting induced by the uneven distribution of the initial heat flux and its time change rate, it is shown that if the length in the region with the higher heat flux and time change rate in the direction of heat flow is less than a critical value the overshooting phenomenon can be avoided. These findings provide a new way for thermal management of electronic devices based on graphene.

Electrically Oriented Antibodies on Transistor for Monitoring Several Copies of Methylated DNA.

An antibody transistor is a promising biosensing platform for the diagnosis and monitoring of various diseases. Nevertheless, the low concentration and short half-life of biomarkers require biodetection at the trace-molecule level, which remains a challenge for existing antibody transistors. Herein, we demonstrate a graphene field-effect transistor (gFET) with electrically oriented antibody probes (EOA-gFET) for monitoring several copies of methylated DNA. The electric field confines the orientation of antibody probes on graphene and diminishes the distance between graphene and methylated DNAs captured by antibodies, generating more induced charges on graphene and amplifying the electric signal. EOA-gFET realizes a limit of detection (LoD) of ∼0.12 copy μL-1, reaching the lowest LoD reported before. EOA-gFET shows a distinguishable signal for liver cancer clinical serum samples within ∼6 min, which proves its potential as a powerful tool for disease screening and diagnosis.

Dynamic characteristics of terahertz hot-electron graphene FET bolometers: Effect of electron cooling in channel and at side contacts

We analyze the operation of the hot-electron FET bolometers with the graphene channels (GCs) and the gate barrier layers. Such bolometers use the thermionic emission of the hot electrons heated by incident-modulated THz radiation. The hot electrons transfer from the GC into the metal gate. As the THz detectors, these bolometers can operate at room temperature. We show that the response and ultimate modulation frequency of the GC-FET bolometers are determined by the efficiency of the hot-electron energy transfer to the lattice and the GC side contacts due to the 2DEG lateral thermal conductance. The dependences of these mechanisms on the band structure and geometrical parameters open the way for the GC-FET bolometers optimization, in particular, for the enhancement of the maximum modulation frequency.

Deformed graphene FET biosensor on textured glass coupled with dielectrophoretic trapping for ultrasensitive detection of GFAP

Numerous efforts have been undertaken to mitigate the Debye screening effect of FET biosensors for achieving higher sensitivity. There are few reports that show sub-femtomolar detection of biomolecules by FET mechanisms but they either suffer from significant background noise or lack robust control. In this aspect, deformed/crumpled graphene has been recently deployed by other researchers for various biomolecule detection like DNA, COVID-19 spike proteins and immunity markers like IL-6 at sub-femtomolar levels. However, the chemical vapor deposition (CVD) approach for graphene fabrication suffers from various surface contamination while the transfer process induces structural defects. In this paper, an alternative fabrication methodology has been proposed where glass substrate has been initially texturized by wet chemical etching through the sacrificial layer of synthesized silver nanoparticles, obtained by annealing of thin silver films leading to solid state dewetting. Graphene has been subsequently deposited by thermal reduction technique from graphene oxide solution. The resulting deformed graphene structure exhibits higher sensor response towards glial fibrillary acidic protein (GFAP) detection with respect to flat graphene owing to the combined effect of reduced Debye screening and higher surface area for receptor immobilization. Additionally, another interesting aspect of the reported work lies in the biomolecule capture by dielectrophoretic (DEP) transport on the crests of the convex surfaces of graphene in a coplanar gated topology structure which has resulted in 10 aM and 28 aM detection limits of GFAP in buffer and undiluted plasma respectively, within 15 min of application of analyte. The detection limit in buffer is almost four decades lower than that documented for GFAP using biosensors which is is expected to pave way for advancing graphene FET based sensors towards ultrasensitive point-of-care diagnosis of GFAP, a biomarker for traumatic brain injury.

Temperature Dependence of Hot-Electron Graphene Fet Bolometric Detectors Response to Modulated Terahertz Radiation

In this paper, we analyze the modulation characteristics and the ultimate modulation frequency of the terahertz (THz) hot-electron FET bolometers with the graphene channels (GCs), metal gate (MG), and gate barrier layers (BLs) in a wide temperature range. Our results predict that the responsivity of GC-FET bolometers decreases with decreasing operating temperature. This is attributed to a dramatic drop in the thermionic GC-MG current (characterized by the relatively large activation energy) and its modulated component when the temperature lowers. A further decrease in the temperature results in a fairly strong responsivity roll-off. In contrast, the responsivity of the GC-FET detectors with the temperature-adjusted load resistance rises with decreasing temperature because the temperature lowering leads to an increase in the electron energy relaxation time and promotes more effective heating by the impinging THz radiation. In this case, the weakening of the thermionic current is compensated by the commensurate increase in the load resistance.

Effect of fabrication process on contact resistance and channel in graphene field effect transistors

Contact resistance, as one of the main parameters that limits the performance of graphene-based transistors, is highly dependent on the metal-graphene contact fabrication processes. These processes are investigated and the corresponding resistances are measured based on the transfer length method (TLM). In fabrication processes, when annealing is done on chemical vapor deposition (CVD)-grown graphene samples that are transferred onto SiO2/Si substrates, the adhesion of graphene to the substrate is improved, and poly methyl methacrylate (PMMA) residues are also reduced. When the metal deposition layer is first applied to the graphene, and then, the photolithography process is performed to define the electrodes and graphene sheet, the graphene-metal contact resistance is better than that in other methods due to the removal of photoresist residues. In fact, by changing the sequence of the fabrication process steps, the direct contact between photoresist and graphene surface can be prevented. Thus, the contact resistance is reduced and conductivity increases, and in this way, the performance of graphene transistor improves. The results show that the fabrication process has a noticeable effect on the transistor properties such as contact resistance, channel sheet resistance, and conductivity.‌ Here, by using the annealing process and changing the order of photolithography processes, a contact resistance of 470 Ω μm is obtained for Ni-graphene contact, which is relatively favorable.

A recurrence model capturing interface traps for non-zero bandgap GFETs towards dynamic mimicking of synaptic plasticity

This paper primarily focusses on developing an analytical model with a non-zero bandgap of boron (B)/nitrogen (N) substitution doped graphene field-effect transistors (GFETs) to mimic the synaptic behaviour. The trap charges at the channel and gate-insulator interface are utilized to induce the hysteresis conduction mechanism, which is further exploited to accomplish synaptic plasticity. The proposed recurrence, that is the time-dependent trap drain current model, accurately captures the physical insights of trap charges using an equivalent metal–insulator–graphene model. An interesting feature of the proposed model is that it is compatible with both the doped (B/N) and the undoped GFETs. The model is also investigated to generate the hysteresis characteristics of the GFET that are further utilized to simulate the synaptic behaviour. Another fact that must be noticed is the existence of complete OFF regions for doped B/N GFETs, unlike the undoped case, which manifest undesirable ambipolar behaviour. The synapse made up of B/N-doped GFETs predicts an optimistic learning and memory mechanism, termed as spike time-dependent plasticity (STDP). The STDP characteristics of B/N doped synaptic GFETs have been enhanced by more than 18 × compared to artificial synapses made of undoped GFETs. Hence, the hysteresis behaviour along with the non-zero bandgap of B/N substitution doped GFETs makes them highly favourable for the dynamic mimicking of synaptic plasticity to be efficiently biologically plausible.

Laser-induced graphene van der Waals contact-enabled high-performance 2D-materials-based field-effect transistor

Establishing robust electrical contact between electrodes and two-dimensional (2D) semiconductors has become an increasingly critical challenge in realizing high-performance 2D material-based devices. This study presents an easy-to-fabricate high-quality van der Waals (vdW) contact strategy, utilizing transferred laser-induced graphene (LIG), for monolayer graphene field-effect transistors (LIG-GFETs). LIG-graphene vdW contacts were formed via a simple transfer and alignment step without any high-temperature or vacuum processes. Excellent electrical properties of LIG-GFETs were achieved, with up to sevenfold enhanced carrier mobilities compared to conventional gold electrode-based GFETs. Experimental investigations and density functional theory simulations elucidated the mechanisms underlying this performance enhancement. Specifically, the superior performance of LIG-GFETs can be attributed to the formation of vdW contacts between the transferred LIG and the monolayer graphene, alongside the doping of the monolayer graphene induced by the O functional groups in the LIG. We also demonstrated that the high performance monolayer MoS2 FETs can be achieved with more than fivefold enhanced carrier mobilities by using LIG vdW contact compared to conventional gold electrode-based MoS2 FETs. This newly developed vdW contact strategy provides new insights into high-performance 2D material-based devices.

A scalable top-gate graphene field effect transistor with a polydimethylsiloxane dielectric

The limitations of modern CMOS technology have created a call to action for novel devices with great scalability potential. Graphene has been recognized as a suitable material for an enhanced transistor channel based on its incredibly large conductivity while also being easily scaled. Previous research has noted the importance of a top gate device structure, which is difficult to accomplish for graphene transistors due to graphene's incompatibility with oxide growth processes. A novel process flow for graphene field effect transistors with scalability is presented. The emphasis is on the growth of multilayer graphene using chemical vapor deposition and the implementation of polydimethylsiloxane as a gate dielectric. Polydimethylsiloxane gate insulator thickness of 815 nm and 570 nm were successfully developed on 4in large-scale wafers. Two devices of similar channel dimensions and different dielectric were compared and mobilities of 14.57cm2V−1s−1 and 0.44 cm2V−1s−1 were measured. Gate voltage sweeps from -20 V to 20 V also demonstrated channel current modulation with a charge neutrality point between 5 V and 8 V, indicating achievement of expected device operation.

Sonification methods for enabling augmented data analysis applied to graphene optoelectronics

AbstractThis paper presents a simple method to transform two‐dimensional data sets into a format that can be easily processed into sound files. These files can be loaded into software wavetable synthesizers to create audible forms of data that can represent complex information. Some background about sonification will be discussed, and the simple method developed here will be applied to graphene optoelectronics. Some key illustrative examples will be used to demonstrate the method, including data sets from previous work on light emission from graphene field effect transistors. We use the sonification method to show how changes in observed phenomena (e.g., light emission intensity and spectral shape) result in changes of the resulting sound (such as the timbre). Demonstrations are included in video format to hear and illustrate the method and resulting effects.

On-site detection of infectious disease based on CaCO3-based magnetic micromotor integrated with graphene field effect transistor.

Anew detection platform based on CaCO3-based magnetic micromotor (CaCO3@Fe3O4) integrated with graphene field effect transistor (GFET) was construct and used for on-site SARS-CoV-2 coronavirus pathogen detection. The CaCO3@Fe3O4 micromotor, which was modified with anti-SARS-CoV-2 (labelled antibody, AntiE1), can self-moved in the solution containing hydrochloric acid (HCl) and effective to capture the SARS-CoV-2 coronavirus pathogens. After magnetic field separation, the capture micromotor was detected by GFET, exhibiting a good linear relationship within the range of 1 ag/mL to 100ng/mL and low detection limit (0.39ag/mL). Furthermore, the detection platform wasalso successfully applied to detection of SARS-CoV-2 coronavirus pathogens in soil solution, indicating the potential use in on-site application.

Transfer-free robust n-type graphene field-effect transistors for digital logic electronic devices

Graphene has been hailed as wonder materials for electronics due to its outstanding charge transport properties. However, its lack of an electronic bandgap has hindered its application to large-scale field-effect transistor (FET) circuits, thereby making bandgap opening a key priority. Further, the fabrication of graphene devices to date conventionally requires the transfer of graphene from its growth substrate to a target substrate, which introduces defects and deteriorates the device performance. To overcome these challenges, we demonstrate a transfer-free approach for the low-temperature growth (∼100 °C) onto TiO2–x (∼10 nm) buffer layer and in-situ nitrogen doping of monolayer graphene, which enables stretchable nitrogen-doped graphene-based FETs with cutting-edge performance and stability. Hybridizing a TiO2–x channel with the nitrogen-doped graphene, nitrogen-doped graphene-based FETs could be achieved with an on-off ratio of ∼2 × 108 and an electron mobility of ∼1,500 cm2V−1s−1 for mainstream digital logic devices. Such devices exhibit high reproducibility and wafer-scale uniformity, thermal, bias-stress, long-term stability, and robust flexibility and stretchability. The scalability and versatility of this transfer-free approach for the flexible and stretchable FETs pave the way for high-performance nitrogen-doped graphene-based digital logic circuits.