Highly Bendable In-Ga-ZnO Thin Film Transistors by Using a Thermally Stable Organic Dielectric Layer

Organic/Inorganic Hybrid Top-Gate Transistors with Ultrahigh Electron Mobility via Enhanced Electron Pathways

The top-gate structure is currently adopted in various flat-panel displays because of its diverse advantages such as passivation from the external environment and process compatibility with industries. However, the mobility of the currently commercialized top-gate oxide thin-film transistors (TFTs) is insufficient to drive ultrahigh-resolution displays. Accordingly, this work suggests metal-capped Zn-Ba-Sn-O transistors with top-gate structures for inducing mobility-enhancing effects. The fabricated top-gate device contains para-xylylene (PPx), which is deposited by a low-temperature chemical vapor deposition (CVD) process, as a dielectric layer and exhibits excellent interfacial and dielectric properties. A technology computer-aided design (TCAD) device simulation reveals that the mobility enhancement in the Al-capped (Zn,Ba)SnO₃ (ZBTO) TFT is attributed not only to the increase in the electron concentration, which is induced by band engineering due to the Al work function but also to the increased electron velocity due to the redistribution of the lateral electric field. As a result, the mobility of the Al-capped top-gate ZBTO device is 5 times higher (∼110 cm²/Vs) than that of the reference device. These results demonstrate the applicability of top-gate oxide TFTs with ultrahigh mobility in a wide range of applications, such as for high-resolution, large-area, and flexible displays.

Chemically Tunable Organic Dielectric Layer on an Oxide TFT: Poly(p-xylylene) Derivatives

Inorganic materials such as SiO_x and SiN_x are commonly used as dielectric layers in thin-film transistors (TFTs), but recent advancements in TFT devices, such as inclusion in flexible electronics, require the development of novel types of dielectric layers. In this study, CVD-deposited poly(p-xylylene) (PPx)-based polymers were evaluated as alternative dielectric layers. CVD-deposited PPx can produce thin, conformal, and pinhole-free polymer layers on various surfaces, including oxides and metals, without interfacial defects. Three types of commercial polymers were successfully deposited on various substrates and exhibited stable dielectric properties under frequency and voltage sweeps. Additionally, TFTs with PPx as a dielectric material and an oxide semiconductor exhibited excellent device performance; a mobility as high as 22.72 cm²/(V s), which is the highest value among organic gate dielectric TFTs, to the best of our knowledge. Because of the low-temperature deposition process and its unprecedented mechanical flexibility, TFTs with CVD-deposited PPx were successfully fabricated on a flexible plastic substrate, exhibiting excellent durability over 10000 bending cycles. Finally, a custom-synthesized functionalized PPx was introduced into top-gated TFTs, demonstrating the possibility for expanding this concept to a wide range of chemistries with tunable gate dielectric layers.

Performance Enhancement and Bending Restoration for Flexible Amorphous Indium Gallium Zinc Oxide Thin-Film Transistors by Low-Temperature Supercritical Dehydration Treatment

For high-performance and high-lifetime flexible and wearable electronic applications, a low-temperature posttreatment method is highly expected to enhance the device performance and repair the defects induced by the low-temperature fabrication process intrinsically. Particularly, if the method can repair the traces induced by the multiple cycles of bending or deforming, it would overcome current fatal obstacles and provide a vital solution to the rapid development of flexible electronics. In this work, we propose a method to apply low-temperature supercritical CO₂ fluid with a dehydration function to improve or even restore the performance of flexible amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs). After the treatment, the a-IGZO TFT exhibits 3 times improvement drivability up to 0.24 μA/μm, a smaller subthreshold swing of 0.18 V dec^-1, a smaller V_th of 0.25 V, and a larger I_on/I_off ratio of 3.8 × 10⁷. Additionally, the posttreated a-IGZO TFTs possess relatively good uniformity and reproducibility with an on-current standard deviation of 0.047 μA/μm, and the performance of the a-IGZO TFT after the treatment remains almost unchanged even after bending 2500 times at a bending radius of 5 mm. These characteristics are attributed to the improved quality of the channel and gate dielectric. It is worth noting that when this is applied to a flexible TFT-driven organic light-emitting diode lighting system, this treatment method can restore the performance of not only the TFT but also the lighting system, even after the system has been bent more than 600 times and has failed. To date, this is the first time that the bending-track erasing function of the supercritical fluid for flexible systems has been reported, which has the potential to prolong the lifetime of flexible electronics.

Self-Aligned Top-Gate Metal-Oxide Thin-Film Transistors Using a Solution-Processed Polymer Gate Dielectric

For high-speed and large-area active-matrix displays, metal-oxide thin-film transistors (TFTs) with high field-effect mobility, stability, and good uniformity are essential. Moreover, reducing the RC delay is also important to achieve high-speed operation, which is induced by the parasitic capacitance formed between the source/drain (S/D) and the gate electrodes. From this perspective, self-aligned top-gate oxide TFTs can provide advantages such as a low parasitic capacitance for high-speed displays due to minimized overlap between the S/D and the gate electrodes. Here, we demonstrate self-aligned top-gate oxide TFTs using a solution-processed indium-gallium-zinc-oxide (IGZO) channel and crosslinked poly(4-vinylphenol) (PVP) gate dielectric layers. By applying a selective Ar plasma treatment on the IGZO channel, low-resistance IGZO regions could be formed, having a sheet resistance value of ~20.6 kΩ/sq., which can act as the homojunction S/D contacts in the top-gate IGZO TFTs. The fabricated self-aligned top-gate IGZO TFTs exhibited a field-effect mobility of 3.93 cm²/Vs and on/off ratio of ~10⁶, which are comparable to those fabricated using a bottom-gate structure. Furthermore, we also demonstrated self-aligned top-gate TFTs using electrospun indium-gallium-oxide (IGO) nanowires (NWs) as a channel layer. The IGO NW TFTs exhibited a field-effect mobility of 0.03 cm²/Vs and an on/off ratio of >10⁵. The results demonstrate that the Ar plasma treatment for S/D contact formation and the solution-processed PVP gate dielectric can be implemented in realizing self-aligned top-gate oxide TFTs.

Effect of Simultaneous Mechanical and Electrical Stress on the Electrical Performance of Flexible In-Ga-Zn-O Thin-Film Transistors

We investigated the effect of simultaneous mechanical and electrical stress on the electrical characteristics of flexible indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs). The IGZO TFTs exhibited a threshold voltage shift (∆VTH) under an application of positive-bias-stress (PBS), with a turnaround behavior from the positive ∆VTH to the negative ∆VTH with an increase in the PBS application time, whether a mechanical stress is applied or not. However, the magnitudes of PBS-induced ∆VTH in both the positive and negative directions exhibited significantly larger values when a flexible IGZO TFT was under mechanical-bending stress than when it was at the flat state. The observed phenomena were possibly attributed to the mechanical stress-induced interface trap generation and the enhanced hydrogen diffusion from atomic layer deposition-grown Al2O3 to IGZO under mechanical-bending stress during PBS. The subgap density of states was extracted before and after an application of PBS under both mechanical stress conditions. The obtained results in this study provided potent evidence supporting the mechanism suggested to explain the PBS-induced larger ∆VTHs in both directions under mechanical-bending stress.

Research Progress on Flexible Oxide-Based Thin Film Transistors

Oxide semiconductors have drawn much attention in recent years due to their outstanding electrical performance, such as relatively high carrier mobility, good uniformity, low process temperature, optical transparency, low cost and especially flexibility. Flexible oxide-based thin film transistors (TFTs) are one of the hottest research topics for next-generation displays, radiofrequency identification (RFID) tags, sensors, and integrated circuits in the wearable field. The carrier transport mechanism of oxide semiconductor materials and typical device configurations of TFTs are firstly described in this invited review. Then, we describe the research progress on flexible oxide-based TFTs, including representative TFTs fabricated on different kinds of flexible substrates, the mechanical stress effect on TFTs and optimized methods to reduce this effect. Finally, an outlook for the future development of oxide-based TFTs is given.

Self-Powered Biosensors Using Various Light Sources in Daily Life Environments: Integration of p-n Heterojunction Photodetectors and Colorimetric Reactions for Biomolecule Detection

Electronic biosensors operating without power supply are high in demand owing to increasing interest in point-of-care (POC) coupled with portable and wearable electronic devices for smart healthcare services. Although self-powered electronic sensors have emerged with the promise of resolving the energy supply problems, achieving sufficient sensitivity to targets in real samples is highly challenging because of the matrix effect caused by electroactive species. In this study, we developed a self-powered biosensor platform by combining n-indium gallium zinc oxide (IGZO)/p-Si heterojunction photodetectors and physically separated colorimetric reactions. The self-powered biosensors were applied to glucose detection in real human samples using light sources from daily life environments such as fluorescent light and sunlight. The sensors showed high sensitivity and stability from 0.01 to 10 mg mL^-1 of glucose in human saliva and urine without matrix effect from the electroactive species in real samples. In addition, a small change in glucose concentration in human serum was distinguishable with a resolution of 0.01 mg mL^-1. Notably, these results were obtained using well-developed and widely used materials like Si and IGZO with simple deposition techniques. Moreover, this self-powered biosensing platform can be universally applied for the detection of all biomolecules being detected by colorimetric assays. To the best of our knowledge, this is the first report on such self-powered biosensors, which could be a promising candidate for future POC biosensors integrated with portable and wearable electronic devices.

Plasma Polymerization Enabled Polymer/Metal-Oxide Hybrid Semiconductors for Wearable Electronics

A facile fabrication of polymer/metal-oxide hybrid semiconductors is introduced to overcome the intrinsically brittle nature of inorganic metal-oxide semiconductors. The fabrication of the hybrid semiconductors was enabled by plasma polymerization of polytetrafluoroethylene (PTFE) via radio frequency magnetron sputtering process which is highly compatible with metal-oxide semiconductor manufacturing facilities. Indium-gallium-zinc oxide (IGZO) and PTFE are cosputtered to fabricate PTFE-incorporated IGZO thin-film transistors (IGZO:PTFE TFTs) and they exhibit a field-effect mobility of 10.27 cm² V^-1 s^-1, a subthreshold swing of 0.38 V dec^-1, and an on/off ratio of 1.08 × 10⁸. When compared with conventional IGZO TFTs, the IGZO:PTFE TFTs show improved stability results against various electrical, illumination, thermal, and moisture stresses. Furthermore, the IGZO:PTFE TFTs show stable electrical characteristics with a threshold voltage ( V_th) shift of 0.89 V after 10 000 tensile bending cycles at a radius of 5 mm.