Vertical Organic Field-effect Transistors Research Articles

A design strategy for high-performance vertical organic field-effect transistors based on reduced graphene oxide electrode

Achieving a high current density in vertical organic field-effect transistors (VOFETs) based on reduced graphene oxide (rGO) electrode is a big challenge. The unique gate-tunable operation mechanism makes it difficult...

Low-temperature vapor reduction of graphene oxide electrodes for vertical organic field-effect transistors

Vertical organic field-effect transistors based on a low-temperature-prepared reduced graphene oxide electrode.

Predicting the miniaturization limit of vertical organic field effect transistor (VOFET) with perforated graphene as a source electrode

Vertical organic field effect transistors (VOFETs) are of paramount importance due to their fast switching speed, low power consumption, and higher density on a chip compared to lateral OFETs. The low charge carrier mobility in organic semiconductors and longer channel lengths in lateral OFETs lead to higher operating voltages. The channel length in VOFETs can be less than 100 nm which reduces the size of the channel and hence the operating voltages. Another important factor in the operation of VOFETs is the thickness and width of the source electrode. The channel length, source electrode thickness and width sets the miniaturization limit of the VOFETs. The graphene monolayer can be exploited as a source electrode due to its thinness, high carrier mobility, and metallic behaviors. However, for better gate modulation, perforations in the source material are desired. Here, we simulate the VOFET having perforated graphene monolayer as a source electrode and n-type organic semiconductor N, N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) as an active channel material, while aluminum as a drain electrode to predict the best-miniaturized device. The miniaturization limit of such a VOFET has a limit to the gate opening/perforation in which the minimum source width is 10 nm, as in the sub 10 nm range graphene starts behaving like a semiconductor. The subthreshold swing, deduced from the drain current (J D) versus gate voltage (V G) graph, advocates the limit of the organic semiconductor height/channel length to 50 nm, while 50 nm for the gate.

Simulation study of various factors affecting the performance of vertical organic field-effect transistors

Abstract Vertical organic field-effect transistors (VOFETs) can offer a short channel architecture that can further enhance the performance at low operating voltages, which makes it more viable for organic electronics applications. VOFETs can be prepared using low-cost techniques that reduce the high processing costs and can operate at high current densities and relatively high fRequencies. To further improve the performance, high current density, and operating frequency, the physics of charge carrier transport should be understood well with the simulation. The main problem with VOFET is the high off-current which is inevitable due to conduction from the source to the drain contact. Many efforts have been made to reduce the off-state current by the addition of an insulating layer on top of the source electrode, which further increases the processing complexity and cost of fabrication. Simulations based on device geometry, contact barriers, and organic semiconductor parameters are carried out to study the charge carrier transport in VOFET. The simulation results show that the most important factor, to enhance the performance is the device geometry or architecture, which requires a specific fill factor, a ratio of the exposed gate dielectric width to the total width of the device with the source electrode. The simulation results also show a different type of working physics of the basic VOFET architecture where the On/Off ratio and subthreshold swing are largely dependent on the initial negative gate field instead of the accumulated charge carriers at positive gate fields. Optimized VOFET architecture is then simulated for variation in contact barrier and semiconductor properties, which show further enhancement in performance.

Artificial optoelectronic synaptic devices based on vertical organic field-effect transistors with low energy consumption

Artificial optoelectronic synaptic devices based on vertical organic field-effect transistors with low energy consumption

Pushing On-Chip Photosensitivity Forward Using Edge-Driven Vertical Organic Phototransistors

Developing high-performance photosensors using prototype device architectures is essential to pushing forward developing and advancing next-generation optoelectronic applications. This work reports an organic phototransistor (OPT) with an ultra-short conducting channel (tens of nanometers) and outstanding photoelectric conversion efficiency. The OPT is based on a vertical organic field-effect transistor (VOFET) architecture, which utilizes a rolled-up metallic nanomembrane (NM) as the drain electrode and a photolithographically patterned (rectangular-shaped) perforated source electrode. These features expand the concept of conventional VOFETs as the former enables the incorporation of ultra-thin active layers and allows reliable control over gate-induced modulation of channel current. Using the engineering as abovementioned strategies, we focused on obtaining an improved device performance, studying their fundamental operating principle, and further investigating their application as photosensors. The optimized devices exhibited low operating voltages (<5 V) and enhanced on/off current ratio (∼105). The VOFET photoresponse was characterized by measuring the electrical characteristics in the dark and under illumination using three different monochromatic light colors. Under blue light, our devices demonstrated impressive photosensitivity (Pmax ≈ 105) and fast photoelectric conversion (steep light-induced threshold voltage shift), demonstrating that the rolled-up NM OPT shows excellent potential as a highly sensitive photodetector with low power consumption.

Impact of Planar and Vertical Organic Field-Effect Transistors on Flexible Electronics.

The development of flexible and conformable devices, whose performance can be maintained while being continuously deformed, provides a significant step toward the realization of next-generation wearable and e-textile applications. Organic field-effect transistors (OFETs) are particularly interesting for flexible and lightweight products, because of their low-temperature solution processability, and the mechanical flexibility of organic materials that endows OFETs the natural compatibility with plastic and biodegradable substrates. Here, an in-depth review of two competing flexible OFET technologies, planar and vertical OFETs (POFETs and VOFETs, respectively) is provided. The electrical, mechanical, and physical properties of POFETs and VOFETs are critically discussed, with a focus on four pivotal applications (integrated logic circuits, light-emitting devices, memories, and sensors). It is pointed out that the flexible function of the relatively newer VOFET technology, along with its perspective on advancing the applicability of flexible POFETs, has not been reviewed so far, and the direct comparison regarding the performance of POFET- and VOFET-based flexible applications is most likely absent. With discussions spanning printed and wearable electronics, materials science, biotechnology, and environmental monitoring, this contribution is a clear stimulus to researchers working in these fields to engage toward the plentiful possibilities that POFETs and VOFETs offer to flexible electronics.

Self-powered high-sensitivity all-in-one vertical tribo-transistor device for multi-sensing-memory-computing

Devices with sensing-memory-computing capability for the detection, recognition and memorization of real time sensory information could simplify data conversion, transmission, storage, and operations between different blocks in conventional chips, which are invaluable and sought-after to offer critical benefits of accomplishing diverse functions, simple design, and efficient computing simultaneously in the internet of things (IOT) era. Here, we develop a self-powered vertical tribo-transistor (VTT) based on MXenes for multi-sensing-memory-computing function and multi-task emotion recognition, which integrates triboelectric nanogenerator (TENG) and transistor in a single device with the simple configuration of vertical organic field effect transistor (VOFET). The tribo-potential is found to be able to tune ionic migration in insulating layer and Schottky barrier height at the MXene/semiconductor interface, and thus modulate the conductive channel between MXene and drain electrode. Meanwhile, the sensing sensitivity can be significantly improved by 711 times over the single TENG device, and the VTT exhibits excellent multi-sensing-memory-computing function. Importantly, based on this function, the multi-sensing integration and multi-model emotion recognition are constructed, which improves the emotion recognition accuracy up to 94.05% with reliability. This simple structure and self-powered VTT device exhibits high sensitivity, high efficiency and high accuracy, which provides application prospects in future human-mechanical interaction, IOT and high-level intelligence.

Performance Improvement with an Ultrathin p-Type Interfacial Layer in n-Type Vertical Organic Field-Effect Transistors Based on Reduced Graphene Oxide Electrode.

Vertical organic field-effect transistors (VOFETs) with a large current on/off ratio and easy fabrication process are highly desirable for future organic electronics. In this paper, we proposed an ultrathin p-type copper (II) phthalocyanine (CuPc) interfacial layer in reduced graphene oxide (rGO)-based VOFETs. The CuPc interfacial layer was sandwiched between the rGO electrode and the N,N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) organic layer. The introduced CuPc interfacial layer not only decreased the off-current density of the device but also slightly enhanced the on-current density. The threshold voltage of the device was also effectively improved and stabilized at around 0 V. The obtained device exhibited a current on/off ratio exceeding 106, which is the largest value reported for rGO-based VOFETs. The vertical electron mobility of the PTCDI-C8 layer estimated by the space-charge-limited current technique was 1.14 × 10–3 cm2/(V s). However, it was not the main limiting factor for the current density in this device. We totally fabricated 48 devices, and more than 75% could work. Besides, the device was stable with little performance degradation after 1 month. The use of low-cost, solution-processable rGO as work-function-tunable electrode and the application of an ultrathin CuPc interfacial layer in VOFETs may open up opportunities for future organic electronics.

High-Frequency Operation of Vertical Organic Field-Effect Transistors.

The high‐frequency and low‐voltage operation of organic thin‐film transistors (OTFTs) is a key requirement for the commercial success of flexible electronics. Significant progress has been achieved in this regard by several research groups highlighting the potential of OTFTs to operate at several tens or even above 100 MHz. However, technology maturity, including scalability, integrability, and device reliability, is another crucial point for the semiconductor industry to bring OTFT‐based flexible electronics into mass production. These requirements are often not met by high‐frequency OTFTs reported in the literature as unconventional processes, such as shadow‐mask patterning or alignment with unrealistic tolerances for production, are used. Here, ultra‐short channel vertical organic field‐effect transistors (VOFETs) with a unity current gain cut‐off frequency (f T) up to 43.2 MHz (or 4.4 MHz V−1) operating below 10 V are shown. Using state‐of‐the‐art manufacturing techniques such as photolithography with reliable fabrication procedures, the integration of such devices down to the size of only 12 × 6 µm2 is shown, which is important for the adaption of this technology in high‐density circuits (e.g., display driving). The intrinsic channel transconductance is analyzed and demonstrates that the frequencies up to 430 MHz can be reached if the parasitic electrode overlap is minimized.

Ultralow‐Power and Multisensory Artificial Synapse Based on Electrolyte‐Gated Vertical Organic Transistors

AbstractBioinspired electronics have shown great potential in the field of artificial intelligence and brain‐like science. Low energy consumption and multifunction are key factors for its application. Here, multisensory artificial synapse and neural networks based on electrolyte‐gated vertical organic field‐effect transistors (VOFETs) are first developed. The channel length of the organic transistor is scaled down to 30 nm through cross‐linking strategy. Owing to the short channel length and extremely large capacitance of the electric double layer formed at the electrolyte–channel interface, the minimum power consumption of one synaptic event is 0.06 fJ, which is significantly lower than that required by biological synapses (1–10 fJ). Moreover, the artificial synapse can be trained to learn and memory images in a 5 × 5 synapse array and emulate the human brain's spatiotemporal information processing and sound azimuth detection. Finally, the artificial tongue is designed using the synaptic transistor that can discriminate acidity. Overall, this study provides new insights into realizing energy‐efficient artificial synapses and mimicking biological sensory systems.

Influence of Injection Barrier on Vertical Organic Field Effect Transistors

Organic field-effect transistors (OFETs) have shown great potential for applications that require low temperature deposition on large and flexible substrates. To increase their performance, in particular a high transconductance and transit frequency, the transistor channel length has to be scaled into the submicrometer regime, which can be easily achieved in vertical organic field effect transistors (VOFETs). However, despite high performance observed in VOFETs, these transistors usually suffer from short channel effects like weak saturation of the drain current and direct source-drain leakage resulting in large off currents. Here, we study the influence of the injection barrier at the source electrode on the OFF currents, on/off ratio, and transconductance of vertical OFETs. We use two semiconducting materials, 2,6-diphenyl anthracene (DPA), and C60 to vary the injection barrier at the source electrode and are able to show that increasing the Schottky barrier at the source electrode can decrease the direct source/drain leakage by 3 orders of magnitude. However, the increased injection barrier at the source electrode comes at the expense of an increased contact resistance, which in turn will decrease its transconductance and transit frequency. With the help of a 2D drift-diffusion simulation we show that the trade-off between low off currents and high transconductance is inherent to the current VOFET device setup and that new approaches have to be found to design VOFETs that combine good switching properties with high performance.

Stretchable vertical organic transistors and their applications in neurologically systems

Stretchable organic transistors possess great potential in a variety of emerging areas such as biomedical electronics, wearable gadgets and smart skin. Here, stretchable vertical organic field effect transistor (s-VOFET) is developed for the first time, which exhibits low working voltage (−0.5 V), high current density (5.19 mA/cm2) and excellent stretching stability. More interesting, the on state current increased by 68.5% and the sub-threshold swing decreased by 25.2% with the increase of the strain to 20%. It is ascribed to the enhanced face-on orientation of π-π stacking in semiconductor film by stretching strain, providing more efficient pathways for vertical charge transport. Meanwhile, simulation results demonstrate that stretching strain can enhance gate field controllability to charge carriers, resulting in enhanced charge transport and accumulation. Moreover, stretchable vertical synaptic transistor (s-VST) is fabricated by replacing cross-linked PVA with ion gel, which exhibits stable synaptic characteristics even with 60% stretching strain. Noticeably, coupling triboelectric nanogenerator (TENG) and synaptic transistor leads to a new field of triboelectric, which enables the direct interactions between environmental perception data and human-machine interfacing. Therefore, a stretchable self-powered neurologically integrated artificial tactile pathway is constructed using TENG and s-VST, which successfully conducts touch wireless communication and sensory skin touch-interaction. This work provides critical guidelines for the development of high performance stretchable electronic devices and a new platform for enabling neurological functions in stretchable electronics.

High-Performance Vertical Organic Transistors of Sub-5 nm Channel Length.

Miniaturization of electronic circuits increases their overall performance. So far, electronics based on organic semiconductors has not played an important role in the miniaturization race. Here, we show the fabrication of liquid electrolyte gated vertical organic field effect transistors with channel lengths down to 2.4 nm. These ultrashort channel lengths are enabled by using insulating hexagonal boron nitride with atomically precise thickness and flatness as a spacer separating the vertically aligned source and drain electrodes. The transistors reveal promising electrical characteristics with output current densities of up to 2.95 MA cm-2 at -0.4 V bias, on-off ratios of up to 106, a steep subthreshold swing of down to 65 mV dec-1 and a transconductance of up to 714 S m-1. Realizing channel lengths in the sub-5 nm regime and operation voltages down to 100 μV proves the potential of organic semiconductors for future highly integrated or low power electronics.

Solution-processed pseudo-vertical organic transistors based on TIPS-pentacene

Powerful and versatile transistors are indispensable to realize the vision of future flexible electronics, e.g., for wearable active-matrix displays, smart sensors, or radio-frequency identification tags. Organic thin-film transistors are considered to be a perfect match for flexible electronics. However, conventional organic thin-film transistors still do not match the demanding performance targets of many applications. Furthermore, often complex and expensive fabrication steps are employed for the fabrication of hero-devices, which is not compatible with the paradigm of low-cost production of flexible electronics.In this contribution, we present the first solution-processed vertical organic field-effect transistor (VOFET) with good on-state performance comparable to vacuum-processed VOFETs. This approach unites the advantages of a low-temperature, low-cost solution processing with an ultra-short channel transistor concept possibly enabling large-area, low-cost flexible electronics. We examine the influence of different crystal morphologies (spherulitic and ribbons) on the transistor performance by using spin- and shear-coating as solution-based deposition methods. The solution-processed VOFETs reach channel width-normalized transconductances of up to 0.26 μS/μm with a charge carrier mobility of 4.8 cm2/V. S-parameter measurements finally verify that transition frequencies up to 6 MHz are reachable with shear-coated TIPS-pentacene. However, compared to small molecule-based VOFETs, the solution-processed VOFETs show an unfavorably high off-state current and hysteresis, which are explained by background doping and charge carrier trapping. Hence, in order to advance with, further optimization of the semiconductor material and the insulator–semiconductor interface is thus required to qualify solution-processed VOFETs for commercial applications.

Photonic Synapses with Ultra‐Low Energy Consumption Based on Vertical Organic Field‐Effect Transistors

AbstractArtificial synapses have shown great potential in the research of artificial intelligence and brain‐like computing. Artificial synaptic devices based on vertical organic field‐effect transistors (VOFETs) exhibit shorter carrier transmission distances and more stable source–drain currents than conventional planar organic transistors due to their smaller channel lengths. By taking advantage of the vertical structure, low working voltage can be achieved. Here, vertical synaptic devices with working voltage as low as 10 µV and ultra‐low power consumption (≈1.3 fJ per spike) are proposed. VOFET‐based artificial photonic synaptic devices can successfully simulate typical synaptic behavior, including excitatory post‐synaptic current, short‐term plasticity, and long‐term plasticity. Furthermore, VOFET arrays are demonstrated to enable image contrast enhancement function. This work provides a new approach for the realization of artificial synapses through a simple manufacturing process.

Fabrication and electrical characterization of vertical electrolyte transistor

Here, it is reported a vertical electrolyte transistor (VET) whose structure is based on stacked layers: bottom contact → channel → permeable intermediate electrode → ion gel (electrolyte gate dielectric) → gate top contact. This VET depicts versatility to work with two different mode of operation, as: Electrolyte-Gated Vertical Organic Field Effect Transistor (Electrolyte-Gated VOFET) or Vertical Organic Electrochemical Transistor (VOECT). The difference is regarding to the transistor transconductance that occurs due to induced charge carriers or ionic current, respectively. Here, the focus is on giving special attention to the electrical characterization of the VET diode cell in comparison to the VET full architecture. These measurements are able to provide the baseline output current from a VET and consequently distinguish it from possibles different modes of operation created by ionic species. Both modes of operation show that this VET is able to work at very low voltage range and drive a high current density.

Anodization for Simplified Processing and Efficient Charge Transport in Vertical Organic Field‐Effect Transistors

AbstractVertical organic transistors are an attractive alternative to realize short channel transistors, which are required for powerful electronic devices and flexible electronic circuits operating at high frequencies. Unfortunately, the vertical device architecture comes along with an increased device fabrication complexity, limiting the potential of this technology for application. A new design of vertical organic field‐effect transistors (VOFETs) with superior electrical performance and simplified processing is reported. By using electrochemical oxidized aluminum oxide (AlOx) as a pseudo self‐aligned charge‐blocking structure in vertical organic transistors, direct leakage current between the source and drain can be effectively suppressed, enabling VOFETs with very low off‐current levels despite the short channel length. The anodization technique is easy to apply and can be surprisingly used on both n‐type and p‐type organic semiconductor thin films with significant signs of degradation. Hence, the anodization technique enables a simplified process of high‐performance p‐type and n‐type VOFETs, paving the road toward complementary circuits made of vertical transistors.

Application of graphene vertical field effect to regulation of organic light-emitting transistors**Project supported by the National Natural Science Foundation of China (Grant No. 31872901) and the National Key Research and Development Program of China (Grant No. 2016YFA0501602).

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.

Enhanced mobility for increasing on-current and switching ratio of vertical organic field-effect transistors by surface modification with phosphonic acid self-assembled monolayer

Abstract For typical organic transistors with a lateral source/drain contact geometry, self-assembled monolayer (SAM) has been extensively applied to modify the organic/dielectric interface state and to improve the in-plane charge transport. Here, we demonstrate that SAM modification can also be applied to improve the out-of-plane charge transport in vertical organic field-effect transistors (VOFETs), leading to a simultaneous increase in the on-current and ON/OFF ratio. We investigate a series of phosphonic acid (PA) SAMs with different alkyl chain length for modification of p-type dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) VOFETs and their corresponding lateral transistors for comparison. The perforated source electrode of VOFETs is fabricated using surfactant-assisted colloidal lithography, which provides excellent control of the perforations fill factor and allows the introduction of an insulating cap layer to eliminate the off-current. Overall, the VOFET modified with dodecyl-PA SAM shows the optimal performance, with a 6 times enhanced mobility (and on-current) and a larger ON/OFF ratio compared to the unmodified device. Similar performance enhancement is also observed in the lateral transistors with SAM modifications. From detailed analyses of DNTT film morphology, crystallinity, and molecular orientation, we demonstrate that SAM modification exerts a strong impact on film morphology not only near the interface but also in the bulk, which plays a major factor for enhancing the mobilities in two types of transistors. The highest mobility obtained in the dodecyl-PA SAM modified VOFET is primarily attributed to the formation of largest grains with a resultant decrease of boundary defect density, despite that the crystallinity and molecular packing are not much improved than the other devices.