Recent Advances in Metal−Oxide Thin−Film Transistors: Flexible/Stretchable Devices, Integrated Circuits, Biosensors and Neuromorphic Applications

Electrical Characteristics of Solution-Based Thin-Film Transistors with a Zinc-Tin Oxide/Carbon Nanotube Stacked Nanocomposite Active Layer

A stacked nanocomposite zinc-tin oxide/single-walled carbon nanotubes (ZTO/SWNTs) active layer was fabricated for thin-film transistors (TFTs) as an alternative to the conventional single-layer structure of mixed ZTO and SWNTs. The stacked nanocomposite of the solution-processed TFTs was prepared using UV/O3 treatment and multiple annealing steps for each layer. The electrical properties of the stacked device were superior to those of the single-layer TFT. The ZTO/SWNT TFT, fabricated using a stacked structure with ZTO on the top and SWNT at the bottom layer, showed a significant improvement in the field-effect mobility of 15.37 cm2/V·s (factor of three increase) and an Ion/Ioff current ratio of 8.83 × 108 with improved hysteresis. This outcome was attributed to the surface treatment and multiple annealing of the selected active layer, resulting in improved contact and a dense structure. This was also attributed to the controlled dispersion of SWNT, as electron migration paths without dispersants. This study suggests the potential expansion of applications, such as flexible electronics and low-cost fabrication of TFTs.

Flexible Printed Ultraviolet-to-Near-Infrared Broadband Optoelectronic Carbon Nanotube Synaptic Transistors for Fast and Energy-Efficient Neuromorphic Vision Systems

To simulate biological visual systems and surpass their functions and performance, it is essential to develop high-performance optoelectronic neuromorphic electronics with broadband response, low power consumption, and fast response speed. Among these, optoelectronic synaptic transistors have emerged as promising candidates for constructing neuromorphic visual systems. In this work, flexible printed broadband (from 275 to 1050 nm) optoelectronic carbon nanotube synaptic transistors with good stability, high response speed (3.14 ms), and low-power consumption (as low as 0.1 fJ per event with the 1050 nm pulse illumination) using PbS quantum dots (QDs) modified semiconducting single-walled carbon nanotubes (sc-SWCNTs) as active layers are developed. In response to optical pulses within the ultraviolet to near-infrared wavelength range, the optoelectronic neuromorphic devices exhibit excitatory postsynaptic current, paired-pulse facilitation, and a transition from short-term plasticity to long-term plasticity, and other optical synaptic behaviors. Furthermore, a simplified neural morphology visual array is developed to simulate integrated functions such as image perception, memory, and preprocessing. More importantly, it can also emulate other complicated bionic functions, such as the infrared perception of salmon eyes and the warning behavior of reindeer in different environments. This work holds immense significance in advancing the development of artificial neural visual systems.

Hydrogel-Based Artificial Synapses for Sustainable Neuromorphic Electronics

Hydrogels find widespread applications in biomedicine because of their outstanding biocompatibility, biodegradability, and tunable material properties. Hydrogels can be chemically functionalized or reinforced to respond to physical or chemical stimulation, which opens up new possibilities in the emerging field of intelligent bioelectronics. Here, the state-of-the-art in functional hydrogel-based transistors and memristors is reviewed as potential artificial synapses. Within these systems, hydrogels can serve as semisolid dielectric electrolytes in transistors and as switching layers in memristors. These synaptic devices with volatile and non-volatile resistive switching show good adaptability to external stimuli for short-term and long-term synaptic memory effects, some of which are integrated into synaptic arrays as artificial neurons; although, there are discrepancies in switching performance and efficacy. By comparing different hydrogels and their respective properties, an outlook is provided on a new range of biocompatible, environment-friendly, and sustainable neuromorphic hardware. How potential energy-efficient information storage and processing can be achieved using artificial neural networks with brain-inspired architecture for neuromorphic computing is described. The development of hydrogel-based artificial synapses can significantly impact the fields of neuromorphic bionics, biometrics, and biosensing.

Quantum-Dot Light-Emitting Fiber Toward All-In-One Clothing-Type Health Monitoring

In the Fourth Industrial Revolution, as the connection between objects and people becomes increasingly important, interest in wearable optoelectronic device-based medical diagnosis is on the rise. Pulse oximetry sensors based on a fiber platform, which is the smallest unit of clothing, could be considered an attractive candidate for this application. In this study, red and green quantum-dot light-emitting fibers (QDLEFs) based on a 250 μm-diameter 1-dimensional fiber were successfully implemented, achieving high current efficiencies of approximately 22.46 mW/sr/A and 23.6 mW/sr/A and narrow full-width at half-maximum (FWHM) of about 33 nm, respectively. In addition, its omnidirectional flexibility was confirmed through a vertical and lateral bending test with 0.92% strain. By employing a transparent and flexible elastomer, a wearable pulse oximeter incorporating QDLEFs was successfully demonstrated for oxygen saturation level (SpO2) monitoring on finger and wrist. It was demonstrated to be washable, and could be operated for up to about 18 h. Due to the elastomer and bottom emission, it exhibited excellent wear resistance characteristics in a 50 cycle reciprocating test conducted at about 2180.43 kPa with 220-grit abrasive paper sheet. A theoretical investigation based on modified photon diffusion analysis (MPDA) modeling also determined that using narrow FWHM light sources, such as QDLEFs, improves the resolution and accuracy of SpO2 monitoring. Accordingly, the proposed QDLEF showed distinguished potential as an all-in-one clothing type pulse oximetry.

Advancing Fab-Compatible Color-Selective Organic Photodiodes: Tailored Molecular Design and Nanointerlayers

High-performance organic photodiodes (OPDs) and OPD-based image sensors are primarily realized using solution processes based on various additives and coating methods. However, vacuum-processed OPDs, which are more compatible with large-scale production, have received little attention, thereby hindering their integration into advanced systems. This study introduces innovations in the material and device structures to prepare superior vacuum-processed OPDs for commercial applications. A series of vacuum-processable, low-cost p-type semiconductors is developed by introducing an electron-rich cyclopentadithiophene core containing various electron-accepting moieties to fine-tune the energy levels without any significant structural or molecular weight changes. An additional nanointerlayer strategy is used to control the crystalline orientation of the upper-deposited photoactive layer, compensating for device performance reduction in inverted, top-illuminated OPDs. These approaches yielded an external quantum efficiency of 70% and a specific detectivity of 2.0 × 1012 Jones in the inverted structures, which are vital for commercial applications. These OPDs enabled visible-light communications with extremely low bit error rates and successful X-ray image capture.

Synthesis and Characterization of a Semiconductor Diodic Bilayer PbS/CdS Made by the Chemical Bath Deposition Technique

In this work, we report a heterojunction formed by a PbS/CdS bilayer using the chemical bath deposition (CBD) technique because it is a relatively simple, fast, and low-cost technique; is permitted to obtain high-quality thin films (TFs); and also covers large areas. Some characterizations have been carried out to confirm the identity of the involved bilayer. For the cadmium sulfide (CdS) film, optical properties such as absorption, transmission, reflection, extinction coefficient, and refractive index were measured. Moreover, the bandgap was calculated, and morphology was obtained by scanning electron microscopy (SEM). Also, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM) were performed for the synthesis of CdS films. On the other hand, for the synthesis of lead sulfide (PbS) films, we performed TEM, energy-dispersive spectroscopy, and XRD. A surface morphological SEM image of the PbS film synthesized was also taken. The multiheterojunction PbS/CdS bilayer was characterized by the current-voltage (I-V) curve, and the behavior of the bilayer was evaluated under the conditions of darkness and controlled fixed lighting, detecting a very slight photosensitivity of the complete diodic device through those measurements. The calculated bandgap for the CdS TF was E g = 2.55 eV, while after a chosen thermal annealing, the bandgap decreased to 2.38 eV. On the other hand, the PbS film presented a cubic structure.

Imitating Synapse Behavior: Exploiting Off-Current in TPBi-Doped Small Molecule Phototransistors for Broadband Wavelength Recognition

Phototransistors have gained significant attention in diverse applications such as photodetectors, image sensors, and neuromorphic devices due to their ability to control electrical characteristics through photoresponse. The choice of photoactive materials in phototransistor research significantly impacts its development. In this study, we propose a novel device that emulates artificial synaptic behavior by leveraging the off-current of a phototransistor. We utilize a p-type organic semiconductor, dinaphtho[2,3-b:2',3'- f]thieno[3,2-b]thiophene (DNTT), as the channel material and dope it with the organic semiconductor 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) on the DNTT transistor. Under light illumination, the general DNTT transistor shows no change in off-current, except at 400 nm wavelength, whereas the TPBi-doped DNTT phototransistor exhibits increased off-current across all wavelength bands. Notably, DNTT phototransistors demonstrate broad photoresponse characteristics in the wavelength range of 400-1000 nm. We successfully simulate artificial synaptic behavior by differentiating the level of off-current and achieving a recognition rate of over 70% across all wavelength bands.

Enhancement-Mode Ambipolar Thin-Film Transistors and CMOS Logic Circuits using Bilayer Ga2O3/NiO Semiconductors

Recent advancements in power electronics have been driven by Ga2O3-based ultrawide bandgap (UWBG) semiconductor devices, enabling efficient high-current switching. However, integrating Ga2O3 power devices with essential silicon CMOS logic circuits for advanced control poses fabrication challenges. Researchers have introduced Ga2O3-based NMOS and pseudo-CMOS circuits for integration, but these circuits may either consume more power or increase the design complexity. Hence, this article proposes Ga2O3-based CMOS realized using heterogeneous 3D-stacked bilayer ambipolar transistors. These ambipolar transistors consist of HfO2/NiO/Ga2O3/NiO/HfO2 heterostructures that are wrapped around by the Ti/Au gate electrode, resulting in record high electron and hole current on/off ratios of 109 and 107. The threshold voltage, subthreshold swing, and current density measured from 100 ambipolar devices (across 5 batches) are around -7.99 ± 0.92 V (p-channel) and 7.81 ± 0.81 V (n-channel), 0.59 ± 0.07 V/dec (p-channel) and 0.61 ± 0.06 V/dec (n-channel), and 0.99 ± 0.26 mA/mm (p-channel) and 58.23 ± 12.99 mA/mm (n-channel), respectively. All the 100 ambipolar devices showed decent long-term stability over a period of 200 days, exhibiting reliable electrical performance. The threshold voltage shift (ΔVTH) after negative bias stressing for a period of 3500 s is around 11.52 V (p-channel) and 10.21 V (n-channel), respectively. Notably, the n-channels exhibit ∼2 orders higher on/off ratio than the best Ga2O3 unipolar transistors at 300 °C. Moreover, the polarities of ambipolar transistors are reconfigurable into p- or n-MOS, which are integrated to demonstrate CMOS inverter, NOR, and NAND logic gates. The switching periods from "0" to "1" and from "1" to "0" of NOR are 0.12 and 0.17 μs, and those of NAND are 0.16 and 0.13 μs. This work lays the foundation of oxide-semiconductor-based CMOS for future integrated electronics.

Piezoresistive Effect of Conductive and Non-Conductive Fillers in Bi-Layer Hybrid CNT Composites under Extreme Strain

Polymers mixed with conductive fillers hold significant potential for use in stretchable and wearable sensor devices. Enhancing the piezoresistive effect and mechanical stability is critical for these devices. To explore the changes in the electrical resistance under high strains, typically unachievable in single-layer composites, bi-layer structures were fabricated from carbon nanotubes (CNTs) and EcoFlex composites to see unobservable strain regions. Spherical types of non-conductive fillers composed of polystyrene and conductive filler, coated with Ni and Au on non-conductive fillers, were used as secondary fillers to improve the piezoresistive sensitivity of composites, and their respective impact on the conductive network was compared. The electrical and mechanical properties were examined in the static state to understand the impact of these secondary fillers. The changes in the electrical resistance under 100% and 300% tensile strain, and their dependence on the inherent electrical properties of the secondary fillers, were also investigated. Single-layer CNT composites proved incapable of withstanding 300% strain, whereas the bi-layer structures proved resilient. By implementing cyclic stretching tests, contrary to non-conductive fillers, reduced piezoresistive influence of the conductive secondary filler under extreme strain conditions could be observed.

From Enzymatic Dopamine Biosensors to OECT Biosensors of Dopamine

Neurotransmitters are an important category of substances used inside the nervous system, whose detection with biosensors has been seriously addressed in the last decades. Dopamine, a neurotransmitter from the catecholamine family, was recently discovered to have implications for cardiac arrest or muscle contractions. In addition to having many other neuro-psychiatric implications, dopamine can be detected in blood, urine, and sweat. This review highlights the importance of biosensors as influential tools for dopamine recognition. The first part of this article is related to an introduction to biosensors for neurotransmitters, with a focus on dopamine. The regular methods in their detection are expensive and require high expertise personnel. A major direction of evolution of these biosensors has expanded with the integration of active biological materials suitable for molecular recognition near electronic devices. Secondly, for dopamine in particular, the miniaturized biosensors offer excellent sensitivity and specificity and offer cheaper detection than conventional spectrometry, while their linear detection ranges from the last years fall exactly on the clinical intervals. Thirdly, the applications of novel nanomaterials and biomaterials to these biosensors are discussed. Older generations, metabolism-based or enzymatic biosensors, could not detect concentrations below the micro-molar range. But new generations of biosensors combine aptamer receptors and organic electrochemical transistors, OECTs, as transducers. They have pushed the detection limit to the pico-molar and even femto-molar ranges, which fully correspond to the usual ranges of clinical detection of human dopamine in body humors that cover 0.1 ÷ 10 nM. In addition, if ten years ago the use of natural dopamine receptors on cell membranes seemed impossible for biosensors, the actual technology allows co-integrate transistors and vesicles with natural receptors of dopamine, like G protein-coupled receptors. The technology is still complicated, but the uni-molecular detection selectivity is promising.

Analyzing Acceptor-like State Distribution of Solution-Processed Indium-Zinc-Oxide Semiconductor Depending on the In Concentration

Understanding the density of state (DOS) distribution in solution-processed indium-zinc-oxide (IZO) thin-film transistors (TFTs) is crucial for addressing electrical instability. This paper presents quantitative calculations of the acceptor-like state distribution of solution-processed IZO TFTs using thermal energy analysis. To extract the acceptor-like state distribution, the electrical characteristics of IZO TFTs with various In molarity ratios were analyzed with respect to temperature. An Arrhenius plot was used to determine electrical parameters such as the activation energy, flat band energy, and flat band voltage. Two calculation methods, the simplified charge approximation and the Meyer-Neldel (MN) rule-based carrier-surface potential field-effect analysis, were proposed to estimate the acceptor-like state distribution. The simplified charge approximation established the modeling of acceptor-like states using the charge-voltage relationship. The MN rule-based field-effect analysis validated the DOS distribution through the carrier-surface potential relationship. In addition, this study introduces practical and effective approaches for determining the DOS distribution of solution-processed IZO semiconductors based on the In molarity ratio. The profiles of the acceptor-like state distribution provide insights into the electrical behavior depending on the doping concentration of the solution-processed IZO semiconductors.

Fiber-based quantum-dot pulse oximetry for wearable health monitoring with high wavelength selectivity and photoplethysmogram sensitivity

Increasing demand for real-time healthcare monitoring is leading to advances in thin and flexible optoelectronic device-based wearable pulse oximetry. Most previous studies have used OLEDs for this purpose, but did not consider the side effects of broad full-width half-maximum (FWHM) characteristics and single substrates. In this study, we performed SpO₂ measurement using a fiber-based quantum-dot pulse oximetry (FQPO) system capable of mass production with a transferable encapsulation technique, and a narrow FWHM of about 30 nm. Based on analyses we determined that uniform angular narrow FWHM-based light sources are important for accurate SpO₂ measurements through multi-layer structures and human skin tissues. The FQPO was shown to have improved photoplethysmogram (PPG) signal sensitivity with no waveguide-mode noise signal, as is typically generated when using a single substrate (30-50%). We successfully demonstrate improved SpO₂ measurement accuracy as well as all-in-one clothing-type pulse oximetry with FQPO.

Flexible strategy of epitaxial oxide thin films

Applying functional oxide thin films to flexible devices is of great interests within the rapid development of information technology. The challenges involve the contradiction between the high-temperature growth of high-quality oxide films and low melting point of the flexible supports. This review summarizes the developed methods to fabricate high-quality flexible oxide thin films with novel functionalities and applications. We start from the fabrication methods, e.g. direct growth on flexible buffered metal foils and layered mica, etching and transfer approach, as well as remote epitaxy technique. Then, various functionalities in flexible oxide films will be introduced, specifically, owing to the mechanical flexibility, some unique properties can be induced in flexible oxide films. Taking the advantages of the excellent physical properties, the flexible oxide films have been employed in various devices. Finally, future perspectives in this research field will be proposed to further develop this field from fabrication, functionality to device.

Investigation on Atomic Bonding Structure of Solution-Processed Indium-Zinc-Oxide Semiconductors According to Doped Indium Content and Its Effects on the Transistor Performance

The atomic composition ratio of solution-processed oxide semiconductors is crucial in controlling the electrical performance of thin-film transistors (TFTs) because the crystallinity and defects of the random network structure of oxide semiconductors change critically with respect to the atomic composition ratio. Herein, the relationship between the film properties of nitrate precursor-based indium-zinc-oxide (IZO) semiconductors and electrical performance of solution-processed IZO TFTs with respect to the In molar ratio was investigated. The thickness, morphological characteristics, crystallinity, and depth profile of the IZO semiconductor film were measured to analyze the correlation between the structural properties of IZO film and electrical performances of the IZO TFT. In addition, the stoichiometric and electrical properties of the IZO semiconductor films were analyzed using film density, atomic composition profile, and Hall effect measurements. Based on the structural and stoichiometric results for the IZO semiconductor, the doping effect of the IZO film with respect to the In molar ratio was theoretically explained. The atomic bonding structure by the In doping in solution-processed IZO semiconductor and resulting increase in free carriers are discussed through a simple bonding model and band gap formation energy.