Flexible Field-effect Transistors Research Articles

Polymer‐modified solution‐processed metal oxide dielectrics on aluminum foil substrate for flexible organic transistors

Field‐effect transistors (FETs) employing the solution‐processed metal oxides as dielectrics have shown excellent performance on rigid silicon substrate. However, there have been very limited reports on FETs with solution‐processed, thermal‐annealed metal oxide dielectrics on low‐cost flexible substrates. To date, to our knowledge there is almost no report on flexible FETs having the similar or better performance than the FETs on Si substrate with solution‐processed, thermal‐annealed metal oxides as dielectrics. Herein, flexible pentacene‐based organic FETs have been demonstrated with polystyrene (PS)‐modified, solution‐processed yttrium oxide (Y2O3), hafnium oxide (HfO2), or aluminum oxide (Al2O3) as dielectrics on commercially available aluminum (Al) foil substrates. The transistors exhibit a good carrier mobility (µ), a large on/off current ratio (Ion/off), a low threshold voltage (VT), and a small subthreshold swing (SS) at a low‐operation voltage (−4 V), which is comparable with the device performance on rigid Si substrate. Therefore, the excellent electrical performance of the FETs on commercially available Al foil substrate, together with the virtue of light‐weight, makes the devices possess the promising potential for various electronic system applications.

PH에 민감한 그래핀 전계효과 트랜지스터(FET)

최근 환경, 의료분야에서 실시가 검출 및 인체 삽입형 pH 센서에 대한 요구가 증가하고 있다. 이에 본 연구에서는 생체적합성이 우수한 그래핀을 이용하여 실시간 pH 검출이 가능한 센서를 개발하였다. Polyethylene terephthalate(PET) 기판에 전사된 그래핀 표면에 이온 용액속에서 동작하는 전계효과 트랜지스터(solution-gated field-effect transistors; SGFETs)를 제작하였으며 이를 이용하여 이온 용액의 pH를 검출하였다. 제작한 트랜지스터의 게이트 채널 길이는 500 ㎛, 게이트 채널 폭은 8 ㎜이다. 이온 용액속에서 트랜지스터 동작특성 및 pH 감도를 평가하기 위하여 드레인-소스 전압(VDS)에 따른 드레인-소스 전류(IDS) 및 게이트-소스 전압(VGS)에 따른 드레인-소스 전류(IDS)를 측정하였다. PET기판에 전사된 그래핀 위에 제작한 그래핀 SGFETs의 전류-전압 특성은 이온 용액내에서 매우 안정적으로 동작하였으며 그래핀 SGFETs의 Dirac point는 이온 용액의 pH값이 증가함에 따라 양의 방향으로 19.32 ㎷/pH씩 증가하였다.

Determination of scattering time and of valley occupation in transition-metal dichalcogenides doped by field effect

The transition-metal dichalcogenides have attracted a lot of attention as a possible stepping-stone toward atomically thin and flexible field-effect transistors. One key parameter to describe the charge transport is the time between two successive scattering events---the transport scattering time. In a recent report, we have shown that it is possible to use density functional theory to obtain the band structure of two-dimensional semiconductors in the presence of field effect doping. Here, we report a simple method to extract the scattering time from the experimental conductivity and from the knowledge of the band structure. We apply our approach to monolayers and multilayers of ${\mathrm{MoS}}_{2}, {\mathrm{MoSe}}_{2}, {\mathrm{MoTe}}_{2}, {\mathrm{WS}}_{2}$, and ${\mathrm{WSe}}_{2}$ in the presence of a gate. In ${\mathrm{WS}}_{2}$, for which accurate measurements of mobility have been published, we find that the scattering time is inversely proportional to the density of states at the Fermi level. Finally, we show that it is possible to identify the critical doping at which different valleys start to be occupied from the doping dependence of the conductivity.

Synthesis of atomically thin GaSe wrinkles for strain sensors

A wrinkle-based thin-film device can be used to develop optoelectronic devices, photovoltaics, and strain sensors. Here, we propose a stable and ultrasensitive strain sensor based on two-dimensional (2D) semiconducting gallium selenide (GaSe) for the first time. The response of the electrical resistance to strain was demonstrated to be very sensitive for the GaSe-based strain sensor, and it reached a gauge factor of–4.3, which is better than that of graphene-based strain sensors. The results show us that strain engineering on a nanoscale can be used not only in strain sensors but also for a wide range of applications, such as flexible field-effect transistors, stretchable electrodes, and flexible solar cells.

Biaxially extended thiophene–isoindigo donor–acceptor conjugated polymers for high-performance flexible field-effect transistors

Biaxially-extended thiophene–isoindigo donor–acceptor conjugated polymers were explored for high-performance flexible field-effect transistors. A charge carrier mobility of 1.0 cm2 V−1 s−1 was achieved under ambient atmosphere with stable electrical properties.

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.

Flexible FETs using ultrathin Si microwires embedded in solution processed dielectric and metal layers

This work presents a novel manufacturing route for obtaining high performance bendable field effect transistors (FET) by embedding silicon (Si) microwires (2.5 μm thick) in layers of solution-processed dielectric and metallic layers. The objective of this study is to explore heterogeneous integration of Si with polymers and to exploit the benefits of both microelectronics and printing technologies. Arrays of Si microwires are developed on silicon on insulator (SOI) wafers and transfer printed to polyimide (PI) substrate through a polydimethylsiloxane (PDMS) carrier stamp. Following the transfer printing of Si microwires, two different processing steps were developed to obtain top gate top contact and back gate top contact FETs. Electrical characterizations indicate devices having mobility as high as 117.5 cm2 V−1 s−1. The fabricated devices were also modeled using SILVACO Atlas. Simulation results show a trend in the electrical response similar to that of experimental results. In addition, a cyclic test was performed to demonstrate the reliability and mechanical robustness of the Si μ-wires on flexible substrates.

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.

Flexible MoS2 Field-Effect Transistors for Gate-Tunable Piezoresistive Strain Sensors.

Atomically thin molybdenum disulfide (MoS2) is a promising two-dimensional semiconductor for high-performance flexible electronics, sensors, transducers, and energy conversion. Here, piezoresistive strain sensing with flexible MoS2 field-effect transistors (FETs) made from highly uniform large-area films is demonstrated. The origin of the piezoresistivity in MoS2 is the strain-induced band gap change, which is confirmed by optical reflection spectroscopy. In addition, the sensitivity to strain can be tuned by more than 1 order of magnitude by adjusting the Fermi level via gate biasing.

High Mobility of Graphene-Based Flexible Transparent Field Effect Transistors Doped with TiO2 and Nitrogen-Doped TiO2.

Graphene with carbon atoms bonded in a honeycomb lattice can be tailored by doping various species to alter the electrical properties of the graphene for fabricating p-type or n-type field-effect transistors (FETs). In this study, large-area and single-layer graphene was grown on electropolished Cu foil using the thermal chemical vapor deposition method; the graphene was then transferred onto a poly(ethylene terephthalate) (PET) substrate to produce flexible, transparent FETs. TiO2 and nitrogen-doped TiO2 (N-TiO2) nanoparticles were doped on the graphene to alter its electrical properties, thereby enhancing the carrier mobility and enabling the transistors to sense UV and visible light optically. The results indicated that the electron mobility of the graphene was 1900 cm(2)/(V·s). Dopings of TiO2 and N-doped TiO2 (1.4 at. % N) lead to n-type doping effects demonstrating extremely high carrier mobilities of 53000 and 31000 cm(2)/(V·s), respectively. Through UV and visible light irradiation, TiO2 and N-TiO2 generated electrons and holes; the generated electrons transferred to graphene channels, causing the FETs to exhibit n-type electric behavior. In addition, the Dirac points of the graphene recovered to their original state within 5 min, confirming that the graphene-based FETs were photosensitive to UV and visible light. In a bending state with a radius of curvature greater than 2.0 cm, the carrier mobilities of the FETs did not substantially change, demonstrating the application possibility of the fabricated graphene-based FETs in photosensors.

Synthesis of ultrathin polymer insulating layers by initiated chemical vapour deposition for low-power soft electronics.

Insulating layers based on oxides and nitrides provide high capacitance, low leakage, high breakdown field and resistance to electrical stresses when used in electronic devices based on rigid substrates. However, their typically high process temperatures and brittleness make it difficult to achieve similar performance in flexible or organic electronics. Here, we show that poly(1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane) (pV3D3) prepared via a one-step, solvent-free technique called initiated chemical vapour deposition (iCVD) is a versatile polymeric insulating layer that meets a wide range of requirements for next-generation electronic devices. Highly uniform and pure ultrathin films of pV3D3 with excellent insulating properties, a large energy gap (>8 eV), tunnelling-limited leakage characteristics and resistance to a tensile strain of up to 4% are demonstrated. The low process temperature, surface-growth character, and solvent-free nature of the iCVD process enable pV3D3 to be grown conformally on plastic substrates to yield flexible field-effect transistors as well as on a variety of channel layers, including organics, oxides, and graphene.

Tuning the optical, magnetic, and electrical properties of ReSe2 by nanoscale strain engineering.

Creating materials with ultimate control over their physical properties is vital for a wide range of applications. From a traditional materials design perspective, this task often requires precise control over the atomic composition and structure. However, owing to their mechanical properties, low-dimensional layered materials can actually withstand a significant amount of strain and thus sustain elastic deformations before fracture. This, in return, presents a unique technique for tuning their physical properties by "strain engineering". Here, we find that local strain induced on ReSe2, a new member of the transition metal dichalcogenides family, greatly changes its magnetic, optical, and electrical properties. Local strain induced by generation of wrinkle (1) modulates the optical gap as evidenced by red-shifted photoluminescence peak, (2) enhances light emission, (3) induces magnetism, and (4) modulates the electrical properties. The results not only allow us to create materials with vastly different properties at the nanoscale, but also enable a wide range of applications based on 2D materials, including strain sensors, stretchable electrodes, flexible field-effect transistors, artificial-muscle actuators, solar cells, and other spintronic, electromechanical, piezoelectric, photonic devices.

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.

Fabrication of flexible, transparent, and skin-attachable field-effect transistor (FET) sensors based on graphene-silver nanowires

Fabrication of flexible, transparent, and skin-attachable field-effect transistor (FET) sensors based on graphene-silver nanowires

Fabrication of flexible, transparent, and skin-attachable field-effect transistor (FET) sensors based on graphene-silver nanowires

Fabrication of flexible, transparent, and skin-attachable field-effect transistor (FET) sensors based on graphene-silver nanowires

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> .

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.

Silver Nanoparticles in Comparison with Ionic Liquid and rGO as Gate Dopant for Paper–Pencil-Based Flexible Field-Effect Transistors

Nanoparticle-based flexible field-effect transistors (FETs) containing carbon nanotubes (CNTs) and silicon nanowires (SiNWs) have attracted tremendous attention, since their interesting device performance can be utilized for integrated nanoscale electronics. However, use of CNTs and SiNWs on polymer substrates poses serious limitations in terms of their fabrication procedure, repeatability, and biodegradability. In this article, we report for the first time the fabrication and characteristics of solution-processed FETs on a paper substrate doped with easily prepared silver nanoparticles (AgNPs). To compare the FET performance, we fabricated two other FETs on paper containing ionic liquid (IL, 1-butyl-3-methylimidazolium octyl sulfate) and reduced graphene oxide (rGO) as dopants. We observe that the AgNP-based dopant generated good FET characteristics in terms of linear transconductance variations and higher carrier concentration values, showing negligible changes after bending and aging. In comparison with the AgNP-FET, the rGO- and IL-based dopants yielded high carrier mobilities, but the rGO-based FET is more susceptible to aging and bending. The excellent linearity of the I DS–V G curve found for the AgNP-FET ensures its applicability for devices requiring linear transfer characteristics such as linear amplifiers.

Enhancing the electrical properties of a flexible transparent graphene-based field-effect transistor using electropolished copper foil for graphene growth.

Flexible transparent graphene-based field-effect transistors (Gr-FETs) were fabricated using large-area single-layer graphene synthesized through low-pressure chemical vapor deposition on a pretreated copper (Cu) foil, followed by transfer of the graphene from the Cu foil to a poly(ethylene terephthalate) (PET) substrate. The electropolishing method was adopted to smooth the surface of the Cu foil, which is a crucial factor because it affects the defect density of graphene films on the PET substrate after transfer and the electronic transport property of the graphene-based devices. The influence of the electropolishing process on the graphene properties was examined using a Raman spectroscope, a scanning electron microscope, and an optical microscope. When the electropolishing process was adopted to improve the graphene quality, the carrier mobility of the flexible transparent Gr-FETs was enhanced from 90 to 340 cm(2)/(V s). Furthermore, variation of the carrier mobility was lower than 10% when the bending radius of the flexible device was decreased from 6.0 to 1.0 cm.