Flexible Field-effect Transistors Research Articles

Cellulose nanocrystals thin films as gate dielectric for flexible organic field-effect transistors

We report on the fabrication of flexible field-effect transistor using poly[N-9”-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] as active channel and cellulose nanocrystals (CNC) as gate dielectric on indium tin-oxide coated polyethyleneterephthalate substrate. The use of CNC as gate dielectric material was found favorable for carrier transport at the semiconductor/dielectric interface with the device showing an on–off ratio of ≈104. These results indicate that CNC is a promising gate dielectric layer for flexible organic field effect transistors.

Flexible ISFET Biosensor Using IGZO Metal Oxide TFTs and an ITO Sensing Layer

This letter presents the fabrication details and measured performance of a prototype flexible extended-gate ion-sensitive field effect transistor (ISFET) biosensor, manufactured using a metal oxide indium-gallium-zinc oxide thin film transistor and an indium-tin oxide sensing layer on a 125- μm thick flexible plastic substrate. ISFET drain current was shown to respond correctly to the pH buffer concentration with repeatable pH sensitivity observed over multiple cycles. These results demonstrate the initial viability of directly extending flexible plastic substrate organic light emitting diode display technology to the production of low-cost, plastic ISFET biosensors.

Flexible and Transparent Nanocomposite of Reduced Graphene Oxide and P(VDF‐TrFE) Copolymer for High Thermal Responsivity in a Field‐Effect Transistor

A new class of temperature‐sensing materials is demonstrated along with their integration into transparent and flexible field‐effect transistor (FET) temperature sensors with high thermal responsivity, stability, and reproducibility. The novelty of this particular type of temperature sensor is the incorporation of an R‐GO/P(VDF‐TrFE) nanocomposite channel as a sensing layer that is highly responsive to temperature, and is optically transparent and mechanically flexible. Furthermore, the nanocomposite sensing layer is easily coated onto flexible substrates for the fabrication of transparent and flexible FETs using a simple spin‐coating method. The transparent and flexible nanocomposite FETs are capable of detecting an extremely small temperature change as small as 0.1 °C and are highly responsive to human body temperature. Temperature responsivity and optical transmittance of transparent nanocomposite FETs were adjustable and tuneable by changing the thickness and R‐GO concentration of the nanocomposite.

Nanocrystalline cellulose applied simultaneously as the gate dielectric and the substrate in flexible field effect transistors

Cotton-based nanocrystalline cellulose (NCC), also known as nanopaper, one of the major sources of renewable materials, is a promising substrate and component for producing low cost fully recyclable flexible paper electronic devices and systems due to its properties (lightweight, stiffness, non-toxicity, transparency, low thermal expansion, gas impermeability and improved mechanical properties). Here, we have demonstrated for the first time a thin transparent nanopaper-based field effect transistor (FET) where NCC is simultaneously used as the substrate and as the gate dielectric layer in an ‘interstrate’ structure, since the device is built on both sides of the NCC films; while the active channel layer is based on oxide amorphous semiconductors, the gate electrode is based on a transparent conductive oxide. Such hybrid FETs present excellent operating characteristics such as high channel saturation mobility (>7 cm2 V −1 s−1), drain–source current on/off modulation ratio higher than 105, enhancement n-type operation and subthreshold gate voltage swing of 2.11 V/decade. The NCC film FET characteristics have been measured in air ambient conditions and present good stability, after two weeks of being processed, without any type of encapsulation or passivation layer. The results obtained are comparable to ones produced for conventional cellulose paper, marking this out as a promising approach for attaining high-performance disposable electronics such as paper displays, smart labels, smart packaging, RFID (radio-frequency identification) and point-of-care systems for self-analysis in bioscience applications, among others.

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.

Strain on field effect transistors with single–walled–carbon nanotube network on flexible substrate

We have systematically analyzed the effect of strain on the electrical properties of flexible field effect transistors with a single-walled carbon nanotube (SWCNT) network on a polyethersulfone substrate. The strain was applied and estimated at the microscopic scale (&amp;lt;1 μm) by using scanning electron microscope (SEM) equipped with indigenously designed special bending jig. Interestingly, the strain estimated at the microscopic scale was found to be significantly different from the strain calculated at the macroscopic scale (centimeter-scale), by a factor of up to 4. Further in-depth analysis using SEM indicated that the significant difference in strain, obtained from two different measurement scales (microscale and macroscale), could be attributed to the formation of cracks and tears in the SWCNT network, or at the junction of SWCNT network and electrode during the strain process. Due to this irreversible morphological change, the electrical properties, such as on current level and field effect mobility, lowered by 14.3% and 4.6%, respectively.

Quantum Confinement Effects in Transferrable Silicon Nanomembranes and Their Applications on Unusual Substrates

Two dimensional (2D) semiconductors have attracted attention for a range of electronic applications, such as transparent, flexible field effect transistors and sensors owing to their good optical transparency and mechanical flexibility. Efforts to exploit 2D semiconductors in electronics are hampered, however, by the lack of efficient methods for their synthesis at levels of quality, uniformity, and reliability needed for practical applications. Here, as an alternative 2D semiconductor, we study single crystal Si nanomembranes (NMs), formed in large area sheets with precisely defined thicknesses ranging from 1.4 to 10 nm. These Si NMs exhibit electronic properties of two-dimensional quantum wells and offer exceptionally high optical transparency and low flexural rigidity. Deterministic assembly techniques allow integration of these materials into unusual device architectures, including field effect transistors with total thicknesses of less than 12 nm, for potential use in transparent, flexible, and stretchable forms of electronics.

Immunologic and Tissue Biocompatibility of Flexible/Stretchable Electronics and Optoelectronics

Recent development of flexible/stretchable integrated electronic sensors and stimulation systems has the potential to establish an important paradigm for implantable electronic devices, where shapes and mechanical properties are matched to those of biological tissues and organs. Demonstrations of tissue and immune biocompatibility are fundamental requirements for application of such kinds of electronics for long-term use in the body. Here, a comprehensive set of experiments studies biocompatibility on four representative flexible/stretchable device platforms, selected on the basis of their versatility and relevance in clinical usage. The devices include flexible silicon field effect transistors (FETs) on polyimide and stretchable silicon FETs, InGaN light-emitting diodes (LEDs), and AlInGaPAs LEDs, each on low modulus silicone substrates. Direct cytotoxicity measured by exposure of a surrogate fibroblast line and leachable toxicity by minimum essential medium extraction testing reveal that all of these devices are non-cytotoxic. In vivo immunologic and tissue biocompatibility testing in mice indicate no local inflammation or systemic immunologic responses after four weeks of subcutaneous implantation. The results show that these new classes of flexible implantable devices are suitable for introduction into clinical studies as long-term implantable electronics.

Flexible air-stable three-dimensional polymer field-effect transistors with high output current density

Flexible air-stable short-channel polymer organic field-effect transistor (OFET) arrays with high saturated output current density are demonstrated by utilizing a novel solution-processed naphthobisthiadiazole (NTz) based donor–acceptor semiconducting polymer (PNTz4T) and designing a three-dimensional vertical channel structure with an extremely large ratio of channel width to channel length. The saturated mean field-effect mobility of 0.16cm2/Vs of the short-channel polymer devices remains over one month resulting in air-stable OFET arrays with high on/off ratio over 106 and powerful current–density exceeding 0.3A/cm2 under low operation voltage, both of which meet the requirements for such applications as driving organic light-emitting diodes in active-matrix displays.

Graphene-based Flexible Field Effect Transistor with Inkjet Printed Silver Electrodes

Graphene-based Flexible Field Effect Transistor with Inkjet Printed Silver Electrodes

Air-stability and bending properties of flexible organic field-effect transistors based on poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)

We have fabricated flexible field-effect transistors (FETs) using poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], PCDTBT, as an active channel, poly(methyl methacrylate) (PMMA) as gate dielectric and biaxially oriented polyethyleneterephthalate (BOPET) as supporting substrate. The output and transfer characteristics of the devices were measured as a function of channel length. It has been observed that various OFET parameters viz. on–off ratio (∼105), mobility (μ∼10−4cm2V−1s−1), threshold voltage (Vth∼−14V), switch-on voltage (Vso∼−6V), subthreshold slope (S∼7V/decade) and trap density (Nit∼1014cm−2V−1) are almost independent of the channel length, which suggested a very high uniformity of the PCDTBT active layer. These devices were highly stable under atmospheric conditions (temperature: 20–35°C and relative humidity: 70–85%), as no change in mobility was observed on a continuous exposure for 70days. The studies on the effect of strain on mobility revealed that devices are stable up to a compressive or tensile strain of 1.2%. These results indicate that PCDTBT is a very promising active layer for the air stable and flexible FETs.

Flexible field-effect transistor arrays with patterned solution-processed organic crystals

We demonstrate a fabrication process to develop field-effect transistor arrays based on patterned organic crystals as active semiconductor materials on flexible plastic substrates. Large plate-like organic crystals are produced by a direct spin-coating process on a substrate with patterned wettability. Resulting transistor arrays exhibit high device performance, indicating that the proposed method has great potential in flexible electronics.

Flexible Graphene Devices with an Embedded Back-Gate

We show the fabrication of flexible graphene devices with an embedded backgate. The resistance of these devices can be tuned by changing the strain through the bending of the substrate. These devices can be useful for applications requiring a flexible graphene-based field effect transistor in where the graphene channel is not covered (such as biological or chemical sensors and photo-detectors).

Multi-finger flexible graphene field effect transistors with high bendability

Highly bendable graphene field-effect transistors are fabricated on polyimide films. The device offers robust performance against various conditions including immersion in liquids, and dynamic loading tests, which are hazardous to conventional electronics. Bendability of the sample is tested with the bending radius of down to 1.3 mm; the devices remain fully functional with less than 8.7% reduction and no reduction in the electron and hole mobility after repeated bending tests, respectively. Multi-finger electrodes are implemented on flexible substrates to enhance its current drive. Silicon-nitride passivation offers efficient chemical protection over diverse liquids and robust mechanical protection against impacts.

Transistors: Flexible Non‐Volatile Ferroelectric Polymer Memory with Gate‐Controlled Multilevel Operation (Adv. Mater. 44/2012)

A flexible field effect transistor with a poly(3-hexylthiophene) (P3HT) active channel and a ferroelectric poly(vinlyidene fluoride-co-trifluoro ethylene) (PVDF-TrFE) insulator exhibits gate-voltage-controllable multilevel non-volatile memory characteristics with highly reliable data retention and endurance. The operating mechanism of the device is based on multileveled non-volatile current states of a polymeric semiconductor precisely controlled by various remnant polarization states of ferroelectric polymer domains. Further details can be found in the article by Cheolmin Park and co-workers on page 5910.

Ultrasensitive Flexible Graphene Based Field-Effect Transistor (FET)-Type Bioelectronic Nose

Rapid and precise discrimination of various odorants is vital to fabricating enhanced sensing devices in the fields of disease diagnostics, food safety, and environmental monitoring. Here, we demonstrate an ultrasensitive and flexible field-effect transistor (FET) olfactory system, namely, a bioelectronic nose (B-nose), based on plasma-treated bilayer graphene conjugated with an olfactory receptor. The stable p- and n-type behaviors from modified bilayer graphene (MBLG) took place after controlled oxygen and ammonia plasma treatments. It was integrated with human olfactory receptors 2AG1 (hOR2AG1: OR), leading to the formation of the liquid-ion gated FET-type platform. ORs bind to the particular odorant amyl butyrate (AB), and their interactions are specific and selective. The B-noses behave as flexible and transparent sensing devices and can recognize a target odorant with single-carbon-atom resolution. The B-noses are ultrasensitive and highly selective toward AB. The minimum detection limit (MDL) is as low as 0.04 fM (10(-15); signal-to-noise: 4.2), and the equilibrium constants of OR-oxygen plasma-treated graphene (OR-OG) and ammonia plasma-treated graphene (-NG) are ca. 3.44 × 10(14) and 1.47 × 10(14) M(-1), respectively. Additionally, the B-noses have long-term stability and excellent mechanical bending durability in flexible systems.

Flexible Non‐Volatile Ferroelectric Polymer Memory with Gate‐Controlled Multilevel Operation

A flexible field-effect transistor with a poly(3-hexylthiophene) (P3HT) active channel and a ferroelectric poly(vinlyidene fluoride-co-trifluoro ethylene) (PVDF-TrFE) insulator exhibits gate-voltage-controllable multilevel non-volatile memory characteristics with highly reliable data retention and endurance.

High Mobility Flexible Graphene Field-Effect Transistors with Self-Healing Gate Dielectrics

A high-mobility low-voltage graphene field-effect transistor (FET) array was fabricated on a flexible plastic substrate using high-capacitance natural aluminum oxide as a gate dielectric in a self-aligned device configuration. The high capacitance of the native aluminum oxide and the self-alignment, which minimizes access resistance, yield a high current on/off ratio and an operation voltage below 3 V, along with high electron and hole mobility of 230 and 300 cm(2)/V·s, respectively. Moreover, the native aluminum oxide is resistant to mechanical bending and exhibits self-healing upon electrical breakdown. These results indicate that self-aligned graphene FETs can provide remarkably improved device performance and stability for a range of applications in flexible electronics.

Direct Gravure Printing of Silicon Nanowires Using Entropic Attraction Forces

The development of a method for large-scale printing of nanowire (NW) arrays onto a desired substrate is crucial for fabricating high-performance NW-based electronics. Here, the alignment of highly ordered and dense silicon (Si) NW arrays at anisotropically etched micro-engraved structures is demonstrated using a simple evaporation process. During evaporation, entropic attraction combined with the internal flow of the NW solution induced the alignment of NWs at the corners of pre-defined structures, and the assembly characteristics of the NWs were highly dependent on the polarity of the NW solutions. After complete evaporation, the aligned NW arrays are subsequently transferred onto a flexible substrate with 95% selectivity using a direct gravure printing technique. As a proof-of-concept, flexible back-gated NW field-effect transistors (FETs) are fabricated. The fabricated FETs have an effective hole mobility of 17.1 cm(2) ·V(-1) ·s(-1) and an on/off ratio of ~2.6 × 10(5) .