Transparent Field-effect Transistors Research Articles

Transparent and High-Performance Extended Gate Ion-Sensitive Field-Effect Transistors Using Electrospun Indium Tin Oxide Nanofibers

Herein, we propose a transparent high-performance extended-gate ion-sensitive field-effect transistor (EG-ISFET) using an electrospun indium-tin-oxide (ITO) nanofiber sensing membrane with a high specific surface area. Electrospinning is a simple and effective technique for forming nanofibers. Nevertheless, one-step calcination, such as conventional thermal annealing or microwave annealing, cannot sufficiently eliminate the inherent defects of nanofibers. In this study, we efficiently removed residual polymers and internal impurities from nanofibers via a two-step calcination process involving combustion and microwave annealing. Moreover, Ar plasma treatment was performed to improve the electrical characteristics of ITO nanofibers. Conformally coated thin-film sensing membranes were prepared as a comparative group and subjected to the same calcination conditions to verify the effect of the nanofiber sensing membrane. The characteristics of the ITO nanofiber and ITO thin-film sensing membranes were evaluated using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), optical transmittance, and conductivity. Moreover, the sensor operation of the EG-ISFETs is evaluated in terms of sensitivity and non-ideal behaviors. The optimized process improves the sensor characteristics and sensing membrane quality. Therefore, the ITO nanofiber sensing membrane improves the sensitivity and stability of the EG-ISFET, suggesting its applicability as a high-performance biochemical sensor.

Interdigitated electrodes enhanced photosensitive monolayer WS2 field effect transistor on glass substrate

Recently, 2D-material-based optoelectronic devices are integrated on optical-fiber as a substrate. However, silicon substrate, usually for optoelectronic device integration, impedes fiber-optic integration due to its much larger refractive index. Here, we fabricated a highly transparent photosensitive field effect transistor (FET) using monolayer WS2 on a fiber-compatible glass substrate with interdigitated electrodes. Experiments shows that the current switching ratio for the FET reach up to 105. Additionally, the interdigitated electrodes could improve photocurrent collection capability, leading to a relative high responsivity of 1.48 mA/W. Meanwhile, the difference in the photocurrent induced by light illumination from the front and back to the device is only 1.25%. This indicates light beam can pass through the device, facilitating online monitoring of optical power. Moreover, it is also shown that the transmitted light beam could be reflected to retransmit thought the device, thus increasing photocurrent by 6.8%. Such glass-substrate provide a simple and feasible strategy for future incorporation of optical-fiber with on-chip device.

Ferroelectric Wide-Bandgap Metal Halide Perovskite Field-Effect Transistors: Toward Transparent Electronics.

Transparent field-effect transistors (FETs) are attacking intensive interest for constructing fancy "invisible" electronic products. Presently, the main technology for realizing transparent FETs is based on metal oxide semiconductors, which have wide-bandgap but generally demand sputtering technique or high-temperature (>350°C) solution process for fabrication. Herein, a general device fabrication strategy for metal halide perovskite (MHP) FETs is shown, by which transparent perovskite FETs are successfully obtained using low-temperature (<150°C) solution process. This strategy involves the employment of ferroelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) as the dielectric, which conquers the challenging issue of gate-electric-field screening effect in MHP FETs. Additionally, an ultra-thin SnO2 is inserted between the source/drain electrodes and MHPs to facilitate electron injection. Consequently, n-type semi-transparent MAPbBr3 FETs and fully transparent MAPbCl3 FETs which can operate well at room temperature with mobility over 10-3 cm2 V-1 s-1 and on/off ratio >103 are achieved for the first time. The low-temperature solution processability of these FETs makes them particularly attractive for applications in low-cost, large-area transparent electronics.

Electrotuneable Radical Polymers for Thin-Film Electronic Device Applications

Polyradicals – organic radical polymers in which each repeating unit contains a singly occupied molecular orbital (i.e. an unpaired electron spin) – are unique alternatives to their π-conjugated and semiconducting counterparts for several applications.[1] Unique of polyradicals are tunable charge states at their repeating units (see figure) which enable multi-stable charge transport regimes at the nanoscale. In this talk, we will present the use of thin films of polyradicals in transparent and flexible thin-film nanoelectronics. Although field-effect thin-film transistors (FETs) and flash memory devices ("memristors") based on radical polymers have been often proposed, memristor stability was frequently limited to a few writing cycles, in spite of the excellent quality of the active layer, and no FETs have been demonstrated, even though evidence of polyradical doping has been offered.[2]Here, the design criteria for flash memory devices are reviewed.It will be shown, using a combination of Kelvin-probe force microscopy (KPFM), electrical transport and optical measurements, that single-layer flash memory devices can be demonstrated from 6-oxoverdazyls, a class of radical polymers from which ultra-thin and ultra-smooth organic thin flims are advantageously processable.[3] As a case study, ultrathin devices in which the active layer is formed by a 15-nm homogeneous film of a poly-norbornene-6-oxoverdazyl (PN-6OV) polyradical synthesized by a dry vacuum polymer deposition technique are presented and compared with the corresponding devices of poly-6-oxoverdazyls (P6OV) synthesized by wet chemistry. [4] We will show that high performance is associated to the presence three tunable charge states in each monomer: positive, neutral, and negative, and also depends on the length of the pendant groups to which the radical repeating units are attached. We will demonstrate that careful engineering of the anode and cathode work functions, specifically aligning them with the negative and positive energy levels of the polyradical, is vital to maximize the on/off current ratio and ensure flash operation. The possibility to achieve electro-tunable poly-6-oxoverdazyl radical polymers by different techniques offer uniques opportunities for their use in a variety of different contexts, for example in transparent and/or flexible electronics, and where compatibility with different substrates is required.In the last part of our talk, we will present how a vertical device architecture, with drain-source contacts sandwiching the active layer of a strongly correlated 6-oxoverdazyl polyradical, leads to on/off ratios >103 in p-type PR-FETs. [4] Hole injection thus occurs by contact doping via tunable charge states at the polyradical-electrode interface. PRFETs are superior to existing organic FETs as they combine memristor and transistor functions in one mem-transistor device, offering unique potential for synaptic and spintronic applications.[1] Joo, Y.; Agarkar, V.; Sung, S. H.; Savoie, B. M.; Boudouris, B. W. A Nonconjugated Radical Polymer Glass with High Electrical Conductivity. Science 2018, 359, 1391-1395[2] Nguyen, T. P.; Easley, A. D.; Kang, N.; Khan, S.; Lim,S-M.; Rezenom, Y. H.; Wang, S.; Tran, D. K.; Fan, J.; Letteri, R. A.; He, X.; Su, L.; Yu, C-H.; Lutkenhaus, J. L.; Wooley K, L. Polypeptide organic radical batteries. Nature, 2021, 593, 61[3] Ezugwu, S.; Paquette, J. A.; Yadav, V.; Gilroy, J. B.; Fanchini, G. Design Criteria for Ultrathin Single-Layer Flash Memristors from an Organic Polyradical. Adv. Electron. Mater. 2016, 2, 1600253[4] Singh, D.; Magnan, F.; Gilroy, J. B.; Fanchini, G. Transparent and flexible field-effect transistors and mem-transistors with electroactive layers of solution-processed organic polyradicals, https://arxiv.org/abs/1910.10212 Figure 1

Tailored Uniaxial Alignment of Nanowires Based on Off-Center Spin-Coating for Flexible and Transparent Field-Effect Transistors.

The alignment of nanowires (NWs) has been actively pursued for the production of electrical devices with high-operating performances. Among the generally available alignment processes, spin-coating is the simplest and fastest method for uniformly patterning the NWs. During spinning, the morphology of the aligned NWs is sensitively influenced by the resultant external drag and inertial forces. Herein, the assembly of highly and uniaxially aligned silicon nanowires (Si NWs) is achieved by introducing an off-center spin-coating method in which the applied external forces are modulated by positioning the target substrate away from the center of rotation. In addition, various influencing factors, such as the type of solvent, the spin acceleration time, the distance between the substrate and the center of rotation, and the surface energy of the substrate, are adjusted in order to optimize the alignment of the NWs. Next, a field-effect transistor (FET) incorporating the highly aligned Si NWs exhibits a high effective mobility of up to 85.7 cm2 V−1 s−1, and an on-current of 0.58 µA. Finally, the single device is enlarged and developed in order to obtain an ultrathin and flexible Si NW FET array. The resulting device has the potential to be widely expanded into applications such as wearable electronics and robotic systems.

Effect of oxygen flow rate ratio on crystalline phase and properties of copper oxide films prepared by room-temperature high-power impulse magnetron sputtering

Copper oxide films were prepared at room temperature using high-power impulse magnetron sputtering (HiPIMS). The oxygen flow rate ratio in the Ar/O2 mixture plasma gas is varied from 1.0% to 3.0% and its influences on the plasma radicals, growth mechanism and film properties are systematically investigated. The crystal phase of the film strongly depends on the oxygen flow rate ratio as indicated by the XRD and XPS results. The films consist of mixed (Cu + Cu2O) phase when deposited with an oxygen flow rate ratio of 1.0–2.5%. The Cu2O fraction increases with oxygen flow rate ratio and reaches the greatest value at 2.5%. A n-type to p-type conductivity transition is observed at around 2.0% due to the increasing Cu2O fraction. A phase transition to CuO is observed at a higher oxygen flow rate ratio of 3.0%. The phase transition and conductivity type transition occurred at a much lower oxygen flow rate ratio but similar O2 partial pressure when compared with those reported in literature, implying that the O2 partial pressure is the critical quantity for reactive sputtering rather than the O2 flow rate ratio. Crucially, the film deposited with 2.5% oxygen flow rate ratio exhibits the highest hole mobility of 32 cm2/V·s, the lowest hole concentration of 4.0 × 1015/cm3, the highest transmittance and the largest band gap of 2.62 eV. The film is potential to be used as the channel material of transparent p-type field effect transistors. The results may be helpful for the preparation of copper oxide film by HiPIMS for application in optoelectronic devices.

High-k BaTiO3 nanoparticle films as gate dielectrics for flexible field effect transistors

Highly uniform dielectric and semiconductor layers were incorporated from nanoparticle inks into flexible transparent field-effect transistors (FETs). The FETs showed superior performance characteristics, being promising for the development of silicon-free based electronics.

High transparency and enhanced mobility of field-effect transistors of the semiconductor/insulator polymer blends with ultralow semiconductor content

In this work, we report the achievement of transparent field effect transistors (FETs) with high performance by blending a narrow bandgap polymer PDPP2TBT into inert polystyrene (PS) matrix. Transmittance of nearly 100% across the whole visible range, as well as enhanced FET mobility compared to pure semiconductor films, is available on the blend films with a low PDPP2TBT/PS weight ratio of 0.5–1.0%. From film morphology observation, excellent transport characteristics on the blends of ultra-low semiconductor content is attributed to the formation of interlinked network of the PDPP2TBT aggregates throughout the PS matrix, which provides effective pathways for intra-chain carrier transport. The novel finding will present a promising route on the development of low-cost transparent electronics. • Fully transparent FETs are constructed by PDPP2TBT/polystyrene (PS) blends. • Blend FETs exhibit excellent electrical properties at ultralow copolymer content. • Interlinked network of the PDPP2TBT aggregates formed throughout the blend provides efficient carrier conduction pathways.

Fully transparent field-effect transistor with high drain current and on-off ratio

We report a fully transparent thin-film transistor utilizing a La-doped BaSnO3 channel layer that provides a drain current of 0.468 mA/μm and an on-off ratio of 1.5 × 108. The La-doped BaSnO3 channel is grown on a 100–150 nm thick unintentionally doped BaSnO3 buffer layer on a (001) MgO substrate by molecular-beam epitaxy. Unpatterned channel layers show mobilities of 127–184 cm2 V−1 s−1 at carrier concentrations in the low to mid 1019 cm−3 range. The BaSnO3 is patterned by reactive ion etching under conditions preserving the high mobility and conductivity. Using this patterning method, a sub-micron-scale thin film transistor exhibiting complete depletion at room temperature is achieved.

Improved electrical performance and transparency of bottom-gate, bottom-contact single-walled carbon nanotube transistors using graphene source/drain electrodes

A highly transparent and high-performance random-network single-walled carbon nanotubes (r-SWCNTs) transistor was successfully fabricated by using chemical vapor deposition-grown graphene source/drain (S/D) electrodes. The bottom-gate, bottom-contact geometry was selected for the graphene S/D contact r-SWCNT (Gr-SWCNT) transistor because of its enhanced gate modulation and good sustainability. A palladium S/D contact r-SWCNT (Pd-SWCNT) transistor with the same device geometry was also fabricated for a comparative study. The transmission line method demonstrated that the resistivity of graphene was small enough (∼0.95Ωμm) to be used as S/D electrodes in a single transistor device, and the contact resistance of Gr-SWCNTs was much lower than that of Pd-SWCNTs. Particularly, the correlation between the applied gate voltage and the sheet resistance is strongly dependent on the r-SWCNT film density. The resulting Gr-SWCNT transistor exhibits high mobility and good on/off current ratio compared to the Pd-SWCNT transistor. The high charge injection originated from the ohmic contact behavior and dense r-SWCNT channel formation by the enhancement of selective wetting due to the surface energy matching between the r-SWCNT semiconductor and graphene S/D electrodes. Thus, this approach can encourage creating highly transparent and high-performance carbon-based field effect transistor.

Field-Effect Transistor Based on Zinc Oxide Using Diffusion Technology

Based on the technology developed by authors of the local diffusion doping of certain parts of the ZnO film of the donor (Ga) and acceptor (Li) impurities, a transparent field-effect transistor with the n-type channel was manufactured. The MgF2 films were used as a gate insulator. The field effect and the dark electrical characteristics of the structures were investigated. The field phototransistors based on these structures have been developed. The photoelectric characteristics of the field phototransistors were investigated, and the mechanism of the photoelectric amplification was proposed.

Wedge-Shaped GaN Nanowalls: A Potential Candidate for Two-Dimensional Electronics and Spintronics

Schrödingerand Poisson equations are solved self-consistently in order to obtain the potential and charge density distribution in [Formula: see text]-type GaN nanowalls tapered along [Formula: see text]-axis by different angles. The study shows two-dimensional (2D) quantum confinement of electrons in the central vertical plane of the wall for the entire range of tapering. Calculation of room temperature electron mobility in the 2D channel shows a steady decrease with the increase of the inclination angle of the side facets with respect to the base. However, it is interesting to note that the mobility remains to be much larger than that of bulk GaN even for the inclination angle of 65[Formula: see text]. The properties of high mobility and the vertical orientation of the 2DEG plane in this system can be exploited in fabricating highly conducting transparent interconnects and field effect transistors, which can lead to large scale integration of 2D devices in future.

A highly flexible and sensitive piezoresistive sensor based on MXene with greatly changed interlayer distances

Since the successful synthesis of the first MXenes, application developments of this new family of two-dimensional materials on energy storage, electromagnetic interference shielding, transparent conductive electrodes and field-effect transistors, and other applications have been widely reported. However, no one has found or used the basic characteristics of greatly changed interlayer distances of MXene under an external pressure for a real application. Here we report a highly flexible and sensitive piezoresistive sensor based on this essential characteristics. An in situ transmission electron microscopy study directly illustrates the characteristics of greatly changed interlayer distances under an external pressure, supplying the basic working mechanism for the piezoresistive sensor. The resultant device also shows high sensitivity (Gauge Factor ~ 180.1), fast response (<30 ms) and extraordinarily reversible compressibility. The MXene-based piezoresistive sensor can detect human being’s subtle bending-release activities and other weak pressure.

Transparent field-effect transistors based on AlN-gate dielectric and IGZO-channel semiconductor

The degradation of thin-film transistors (TFTs) caused by the self-heating effect constitutes a problem to be solved for the next generation of displays. Aluminum nitride (AlN) is a viable alternative for gate dielectric of TFTs due to its good thermal conductivity, matching coefficient of thermal expansion to indium–gallium–zinc-oxide, and excellent stability at high temperatures. Here, AlN thin films of different thicknesses were fabricated by a low temperature reactive radio-frequency magnetron sputtering process, using a low cost, metallic Al target. Their electrical properties have been thoroughly assessed. Furthermore, the 200nm and 500nm thick AlN layers have been integrated as gate-dielectric in transparent TFTs with indium–gallium–zinc-oxide as channel semiconductor. Our study emphasizes the potential of AlN thin films for transparent electronics, whilst the functionality of the fabricated field-effect transistors is explored and discussed.

An experimental investigation on the effective electron mass in InGaO3(ZnO)3 by valence electron energy-loss spectroscopy

In2−xGaxO3(ZnO)m are reported to exhibit an extremely high field-effect mobility, and therefore considered as the potential channel materials for transparent field-effect transistor. To gain a deep insight into the high mobility of In2−xGaxO3(ZnO)m, the effective electron mass, which is inherently related to the mobility of charge carriers as well as the density of states, is investigated by valence electron energy-loss spectroscopy. The effective electron masses in InGaO3(ZnO)3 nanobelts with the incident electron beam along the c axis and <112-0> are determined as 0.130m0 and 0.132m0, respectively. These results critically checked by a comparison with calculated values and values measured by others. It not only well validates the results acquired from theoretical investigations, but also shed a new light on the isotropy of the effective electron mass in InGaO3(ZnO)m.

Real-time Monitoring of Cell Activities by Diamond Solution-gated Field Effect Transistors

In this study, we present basic aspects for real-time monitoring of cell growth of human cell lines. We successfully fabricated fully optically transparent diamond-based solution-gated field effect transistor (SG-FET) on glass substrates. The diamond transistor functionalities are demonstrated by the sensitivity of electrical characteristics to solutions with different Na+ and K+ ion concentrations, and pH. Moreover, the diamond SG-FETs exhibited sensitivities to the adhesion of proteins and cells, to the cell delamination, or even to the cell death.

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.

Highly transparent and stretchable field-effect transistor sensors using graphene-nanowire hybrid nanostructures.

Transparent and stretchable electronics with remarkable bendability, conformability, and lightness are the key attributes for sensing or wearable devices. Transparent and stretchable field-effect transistor sensors using graphene-metal nanowire hybrid nanostructures have high mobility (≈3000 cm(2) V(-1) s(-1) ) with low contact resistance, and they are transferrable onto a variety of substrates. The integration of these sensors for RLC circuits enables wireless monitoring.

All-perovskite transparent high mobility field effect using epitaxial BaSnO3 and LaInO3

We demonstrate an all-perovskite transparent heterojunction field effect transistor made of two lattice-matched perovskite oxides: BaSnO3 and LaInO3. We have developed epitaxial LaInO3 as the gate oxide on top of BaSnO3, which were recently reported to possess high thermal stability and electron mobility when doped with La. We measured the dielectric properties of the epitaxial LaInO3 films, such as the band gap, dielectric constant, and the dielectric breakdown field. Using the LaInO3 as a gate dielectric and the La-doped BaSnO3 as a channel layer, we fabricated field effect device structure. The field effect mobility of such device was higher than 90 cm2 V−1 s−1, the on/off ratio was larger than 107, and the subthreshold swing was 0.65 V dec−1. We discuss the possible origins for such device performance and the future directions for further improvement.

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