Flexible Graphene Field Effect Transistors Research Articles

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.

Flexible field-effect transistors with high-quality and uniform single-layer graphene for high mobility

In this work, a fully flexible graphene field-effect transistor with high carrier mobility is reported. Patterned high-quality and uniform single-layer graphene films are successfully realized by combining the selective growth on a patterned copper foil and the direct transfer method to minimize degradation factors. The selectively grown single-layer graphene is directly transferred to the target substrate through the deposition of poly-para-xylylene (Parylene) C. The quality of the graphene films is confirmed by Raman spectroscopy. The analysis reveals that the use of Parylene C as the substrate, gate dielectric, and encapsulation layer has the advantage of reducing the scattering by the optical phonons and charge puddles. The estimated residual carrier density is 1.72 × 1011 cm−2, and the intrinsic hole and electron carrier mobilities are found to be as high as 10 260 and 10 010 cm2 V−1 s−1, respectively. This study can pave the way for the development and mass production of high-performance and fully flexible graphene electronics.

Advances in flexible graphene field-effect transistors for biomolecule sensing.

With the increasing demand for biomarker detection in wearable electronic devices, flexible biosensors have garnered significant attention. Additionally, graphene field-effect transistors (GFETs) have emerged as key components for constructing biosensors, owing to their high sensitivity, multifunctionality, rapid response, and low cost. Leveraging the advantages of flexible substrates, such as biocompatibility, adaptability to complex environments, and fabrication flexibility, flexible GFET sensors exhibit promising prospects in detecting various biomarkers. This review provides a concise summary of design strategies for flexible GFET biosensors, including non-encapsulated gate without dielectric layer coverage and external gate designs. Furthermore, notable advancements in sensing applications of biomolecules, such as proteins, glucose, and ions, are highlighted. Finally, we discuss the future challenges and prospects in this field, aiming to inspire researchers to address these issues in their further investigations.

Bending Stability of Ferroelectric Gated Graphene Field Effect Transistor for Flexible Electronics.

In this work, we explored the potential of the ferroelectric gate of (Pb0.92La0.08)(Zr0.30Ti0.70)O3 (PLZT(8/30/70)) for flexible graphene field effect transistor (GFET) devices. Based on the deep understanding of the VDirac of PLZT(8/30/70) gate GFET, which determines the application of the flexible GFET devices, the polarization mechanisms of PLZT(8/30/70) under bending deformation were analyzed. It was found that both flexoelectric polarization and piezoelectric polarization exist under bending deformation, and their polarization direction is opposite under the same bending deformation. Thus, a relatively stable of VDirac is obtained due to the combination of these two effects. In contrast to the relatively good linear movement of VDirac under bending deformation of relaxor ferroelectric (Pb0.92La0.08)(Zr0.52Ti0.48)O3 (PLZT(8/52/48)) gated GFET, these stable properties of the PLZT(8/30/70) gate GFETs make them have great potential for applications in flexible devices.

Electrolyte-Gated Graphene Field Effect Transistor-Based Ca2+ Detection Aided by Machine Learning.

Flexible electrolyte-gated graphene field effect transistors (Eg-GFETs) are widely developed as sensors because of fast response, versatility and low-cost. However, their sensitivities and responding ranges are often altered by different gate voltages. These bias-voltage-induced uncertainties are an obstacle in the development of Eg-GFETs. To shield from this risk, a machine-learning-algorithm-based LgGFETs' data analyzing method is studied in this work by using Ca2+ detection as a proof-of-concept. For the as-prepared Eg-GFET-Ca2+ sensors, their transfer and output features are first measured. Then, eight regression models are trained with the use of different machine learning algorithms, including linear regression, support vector machine, decision tree and random forest, etc. Then, the optimized model is obtained with the random-forest-method-treated transfer curves. Finally, the proposed method is applied to determine Ca2+ concentration in a calibration-free way, and it is found that the relation between the estimated and real Ca2+ concentrations is close-to y = x. Accordingly, we think the proposed method may not only provide an accurate result but also simplify the traditional calibration step in using Eg-GFET sensors.

Ultrasensitive Label-free MiRNA Sensing Based on a Flexible Graphene Field-Effect Transistor without Functionalization

A flexible graphene field-effect transistor (Gr-FET) biosensor for ultrasensitive and specific detection of miRNA without labeling and functionalization is reported. The flexible biosensor presents...

Flexible field-effect transistors with a high on/off current ratio based on large-area single-crystal graphene

Large-area single-crystal graphene (LSG) over half a millimeter were synthesized via chemical vapor deposition, and a flexible ion gel-gated graphene field-effect transistor (FET) was fabricated to explore the electrical properties of the graphene based on the mechanism of electronic double layers. A procedure of cyclical oxidation/hydrogen-annealing processes by chemical vapor deposition (CVD) was proposed and optimized as a new growth method for the large single-crystal graphene, to facilitate the reconstruction of copper foil surface. To form a flexible FET on the polyethylene terephthalate substrate, LSG was utilized as a channel layer, and a spin-coated ion gel film was used as a gate dielectric. The flexible device demonstrates excellent electrical properties and high degree of bendability. When the device was bent by 8.1%, the on/off current ratio exceeded 400 because the deformation of the graphene crystal lattice widened graphene’s bandgap. This work is a fundamental study on the growth mechanism of LSG and the two-dimensional-material-based ion gel-gated transistor for application in flexible electronic devices.

An integrated flexible and reusable graphene field effect transistor nanosensor for monitoring glucose

Diabetes is a chronic metabolic disease that has effect on blood sugar level and affects millions of people. We present an integrated flexible and reusable graphene-based field effect transistor (GFET) nanosensor for the detection of glucose using pyrene-1-boronic acid (PBA) as the receptor. The nanosensor fabricated on the polyimide performs GFET-based rapid transduction of the glucose-PBA binding, thereby potentially allowing the detection of glucose that are sampled reliably from human bodily fluids (e.g., sweat) in wearable sensing applications. Due to the reversible binding interaction between PBA and glucose, reusability of our nanosensor can be realized by exposing graphene surface to acidic solution. In characterizing the stability and reusability of the nanosensor for wearable applications, we investigated the effects of substrate bending, multiple reuse and long-time storage on the equilibrium dissociation constant between the PBA and glucose. Results show that bending, multiple reuse (over 10 times) and long-time storage has negligible effect on the sensing performance. The detection of glucose with a limit of detection (LOD) of 0.15 μM and a dynamic range of 0.05–100 mM, which covers the reference scope of physical examination or screening of diabetes. Hence, our flexible GFET nanosensor is promising for wearable and reusable biosensing applications.

Autonomous Self-Healing of Electrical Degradation in Dielectric Polymers Using In Situ Electroluminescence

Summary Dielectric polymers are ubiquitous as electrical insulation in electrical power systems and electronic devices. Electrical treeing is typically regarded as an irreversible degradation process of dielectric polymers subjected to high electric stresses, which significantly limits their lifespan and endangers the reliability of electronics and electrical power systems. Here, we report self-healing polymers consisting of microcapsules that are capable of completely restoring the dielectric properties as a direct response to electrical tree degradation. We design the microcapsules with a relatively higher dielectric constant than polymer matrix to guide the propagation of electrical trees. The electroluminescence generated in situ during electrical treeing of polymers triggers crosslinking of the healing agents released from the capsules to repair the electrically damaged areas under ambient conditions. Notably, the breakdown strength of the mended region is largely enhanced upon healing, which was initiated at different locations. The integration of autonomous healing capability into dielectric materials opens new avenues to smart polymers for electronics and energy applications.

Robust sandwiched fluorinated graphene for highly reliable flexible electronics

The high sensitivity of graphene to the surface condition of the gate dielectric layer and its poor van der Waals adhesion with a flexible substrate result in interfacial sliding and fracturing of graphene at low strains, making the successful utilization of pristine graphene (PG) in flexible electronics challenging. Here, we report a facile method for the fabrication of flexible graphene field effect transistors (F-GFETs) using sandwiched fluorinated graphene (FG). The “FG-PG-FG” sandwich structure shows a high optical transparency (>94%) with an average carrier mobility above 340 cm2/V·s, higher than that obtained when GO and Ion gel were used as gate dielectric materials on F-GFETs and a relatively low gate leakage current of ~160 pA. Furthermore, we observed a high mechanical stability, retaining >88% of the original current output against bending deformation of up to 6 mm and >77% after 200 bending cycles by applying a tensile strain of 1.56%, compared to the control sample. This improved performance is attributed to the fact that the sandwiched FG provides a good dielectric environment by tuning the C/F ratio, which tightly fixes the PG under strain. These findings provide a new route for the future development of graphene-based flexible electronics.

High‐Frequency Flexible Graphene Field‐Effect Transistors with Short Gate Length of 50 nm and Record Extrinsic Cut‐Off Frequency

Flexible graphene field effect transistors (GFETs) are fabricated on a polyimide (PI) substrate by an improved self‐aligned fabrication procedure. Short gate length of 50 nm is achieved. Ohmic contact resistance is depressed. The prepared GFET shows comparable intrinsic cut‐off frequency and maximum oscillation frequency of 116 and 110 GHz, respectively. The high frequency of flexible GFET demonstrated here paves the way for applications which require high flexibility and radio frequency operations.

Flexible graphene field effect transistor with ferroelectric polymer gate

A transparent, flexible graphene field effect transistor (GFET) based on ferroelectric gate is demonstrated. In this device, the single layer graphene was fabricated by chemical vapor deposition method, and transferred to the polyethylene terephthalate substrate. Then the poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) ferroelectric polymer layer film was used as the gate dielectric for the graphene FET. Based on the P(VDF-TrFE)/graphene heterojunction FET, Hall Bar structure was fabricated. The transport properties of the graphene channel at low temperature and retention characteristics at different temperature are investigated in detail. These special properties indicated that the GFET might be useful for many particular applications, such as a non-volatile memories, flexible electronic devices and phototransistors.

Towards the Realization of Graphene Based Flexible Radio Frequency Receiver

We report on our progress and development of high speed flexible graphene field effect transistors (GFETs) with high electron and hole mobilities (~3000 cm2/V·s), and intrinsic transit frequency in the microwave GHz regime. We also describe the design and fabrication of flexible graphene based radio frequency system. This RF communication system consists of graphite patch antenna at 2.4 GHz, graphene based frequency translation block (frequency doubler and AM demodulator) and graphene speaker. The communication blocks are utilized to demonstrate graphene based amplitude modulated (AM) radio receiver operating at 2.4 GHz.

High-performance and high-sensitivity applications of graphene transistors with self-assembled monolayers

Charge impurities and polar molecules on the surface of dielectric substrates has long been a critical obstacle to using graphene for its niche applications that involve graphene's high mobility and high sensitivity nature. Self-assembled monolayers (SAMs) have been found to effectively reduce the impact of long-range scatterings induced by the external charges. Yet, demonstrations of scalable device applications using the SAMs technique remains missing due to the difficulties in the device fabrication arising from the strong surface tension of the modified dielectric environment. Here, we use patterned SAM arrays to build graphene electronic devices with transport channels confined on the modified areas. For high-mobility applications, both rigid and flexible radio-frequency graphene field-effect transistors (G-FETs) were demonstrated, with extrinsic cutoff frequency and maximum oscillation frequency enhanced by a factor of ~2 on SiO2/Si substrates. For high sensitivity applications, G-FETs were functionalized by monoclonal antibodies specific to cancer biomarker chondroitin sulfate proteoglycan 4, enabling its detection at a concentration of 0.01fM, five orders of magnitude lower than that detectable by a conventional colorimetric assay. These devices can be very useful in the early diagnosis and monitoring of a malignant disease.

Flexible Graphene Field-Effect Transistors Encapsulated in Hexagonal Boron Nitride.

Flexible graphene field-effect transistors (GFETs) are fabricated with graphene channels fully encapsulated in hexagonal boron nitride (hBN) implementing a self-aligned fabrication scheme. Flexible GFETs fabricated with channel lengths of 2 μm demonstrate exceptional room-temperature carrier mobility (μFE = 10 000 cm(2) V(-1) s(-1)), strong current saturation characteristics (peak output resistance, r0 = 2000 Ω), and high mechanical flexibility (strain limits of 1%). These values of μFE and r0 are unprecedented in flexible GFETs. Flexible radio frequency FETs (RF-FETs) with channel lengths of 375 nm demonstrate μFE = 2200 cm(2) V(-1) s(-1) and r0 = 132.5 Ω. Unity-current gain frequencies, fT, and unity-power gain frequencies, fmax, reach 12.0 and 10.6 GHz, respectively. The corresponding ratio of cutoff frequencies approaches unity (fmax/fT = 0.9), a record value for flexible GFETs. Intrinsic fT and fmax are 29.7 and 15.7 GHz, respectively. The outstanding electronic characteristics are attributed to the improved dielectric environment provided by full hBN encapsulation of the graphene channel in conjunction with an optimized, self-aligned device structure. These results establish hBN as a mechanically robust dielectric that can yield enhanced electronic characteristics to a diverse array of graphene-based flexible electronics.

Graphene Field-Effect Transistors for Radio-Frequency Flexible Electronics

Flexible radio-frequency (RF) electronics require materials which possess both exceptional electronic properties and high-strain limits. While flexible graphene field-effect transistors (GFETs) have demonstrated significantly higher strain limits than FETs fabricated from thin films of Si and III-V semiconductors, to date RF performance has been comparatively worse, limited to the low GHz frequency range. However, flexible GFETs have only been fabricated with modestly scaled channel lengths. In this paper, we fabricate GFETs on flexible substrates with short channel lengths of 260 nm. These devices demonstrate extrinsic unity-power-gain frequencies, f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> , up to 7.6 GHz and strain limits of 2%, representing strain limits an order of magnitude higher than the flexible technology with next highest reported f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> .

High mobility flexible graphene field-effect transistors and ambipolar radio-frequency circuits

Field-effect transistors (GFETs) were fabricated on mechanically flexible substrates using chemical vapor deposition grown graphene. High current density (nearly 200 μA μm(-1)) with saturation, almost perfect ambipolar electron-hole behavior, high transconductance (120 μS μm(-1)) and good stability over 381 days were obtained. The average carrier mobility for holes (electrons) is 13,540 cm(2) V(-1) s(-1) (12,300 cm(2) V(-1) s(-1)) with the highest value over 24,000 cm(2) V(-1) s(-1) (20,000 cm(2) V(-1) s(-1)) obtained in flexible GFETs. Ambipolar radio-frequency circuits, frequency doubler, were constructed based on the high performed flexible GFET, which show record high output power spectra purity (∼97%) and high conversion gain of -13.6 dB. Bending measurements show the flexible GFETs are able to work under modest strain. These results show that flexible GFETs are a very promising option for future flexible radio-frequency electronics.

Flexible Solution-Gated Graphene Field Effect Transistor for Electrophysiological Recording

Graphene represents a high-performance material for field effect transistor (FET) manufacture. As a monolayer of carbon atoms, graphene offers flexibility, high carrier mobility, and transparency. Solution-gated graphene FETs based on flexible substrates have been developed for various applications. However, none of these FETs offer both flexibility and good insulation from the electrolyte simultaneously. This would restrict their applicability in certain areas, such as implantation. To solve this problem, we used commercially available graphene and polyimide to fabricate a graphene FET. Photosensitive polyimide was used as both substrate and insulator for our device. Two Au electrodes were used separately as source and drain, and the graphene was exposed to an electrolytic solution to form a solution-gated structure. In this paper, we have studied the resistance variation between source and drain during specific fabrication steps to optimize the fabrication process. By analyzing the FET performance after repeated bending tests, our device's performance reliability was also confirmed. When compared with similar published flexible devices, our FET showed higher transconductance and lower noise levels. Also, neural activities were recorded from cortex neurons cultured on graphene FET for the first time, which proved our device's biocompatibility and demonstrated its potential for electrophysiological applications.

Triangular-Pulse Measurement for Hysteresis of High-Performance and Flexible Graphene Field-Effect Transistors

We use a triangular-pulse measurement technique to obtain the hysteretic electrical characteristics of flexible graphene field-effect transistors (GFETs). To minimize charge trapping, the gate-voltage scanning rate was controlled (up to 2 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm V}/\mu{\rm s}$</tex></formula> ) by varying the triangular-pulse rise and fall times. This method makes it possible to measure the intrinsic-like transfer characteristics of chemical vapor deposition graphene devices. The maximum electron (hole) mobility measured by a dc measurement is <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{4800}$</tex></formula> (5200) <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\rm cm}^{2}/{\rm Vs}$</tex></formula> , whereas the maximum electron (hole) mobility measured by the triangular-pulse technique with a gate-voltage scanning rate of 0.4 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm V}/\mu{\rm s}$</tex></formula> is <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim{10600}$</tex></formula> (8500) <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm cm}^{2}/{\rm Vs}$</tex></formula> . For measurements with a triangular gate pulse, the shift of the Dirac voltage is less than that measured by the dc method. These results indicate that the triangular-gate-pulse measurement is a promising technique with which to determine the intrinsic properties and ambipolar transfer characteristics of GFETs.

Fabrication of free-standing Al2O3 nanosheets for high mobility flexible graphene field effect transistors

We propose a facile methodology for large-scale fabrication of graphene-based field effect transistors (GFETs) with two types of devices, top- and bottom-gated configurations. The top-gated GFETs containing a monolayer graphene channel, a free-standing Al2O3 nanosheet as the gate dielectrics and a top-gate electrode were assembled by a poly(methyl methacrylate)-assisted wet-transfer technique. As a result, the hole mobility of the top-gated GFETs was 2306.28 ± 295.53 cm2 V−1 s−1, which is much higher than that of top-gated GFETs with Al2O3 dielectrics prepared by atomic layer deposition (15.28 cm2 V−1 s−1). We also simultaneously synthesized graphene–graphite integrated electronic devices, in which we utilized graphene and graphite layers as an active channel layer and source/drain electrodes for bottom-gated all-graphene-based FETs, respectively. In addition, a layer-by-layer stacked multilayer graphene sheet and free-standing Al2O3 dielectrics were employed as a gate electrode and a dielectric layer, respectively. Significantly, all component layers were assembled by a poly(methyl methacrylate)-assisted wet-transfer technique, which is a progression towards high-performance, transparent, and flexible GFETs.