Revolutionizing electronics with oxide thin-film transistor technology

Oxide thin-film transistor (TFT) technology represents a significant advancement in the field of electronics and displays, continuously finding new opportunities for device applications in sensors, memory, processors, and more. In this Future Tech in Retrospect, we challenge the outlook on past advances and future promises to unlock further potential for oxide TFT technology, revolutionizing the landscape of modern electronics through entirely new application areas. In the rapidly evolving landscape of technology, advancements in thin-film transistor (TFT) technology have continuously pushed the boundaries of what is achievable in the semiconductors and display industries. TFTs are three-terminal semiconductor devices with a thin-film structure. They act as switches in electronic circuits, controlling the flow of current and facilitating the operation of various electronic components. Among the innovations, metal oxide TFTs have emerged as a transformative force, evolving from a promising concept to a cornerstone of modern display technology. They serve as switching and driving units, reshaping the way we interact with electronic products (Figure 1). These oxide TFTs, crafted from semiconductor materials such as amorphous indium gallium zinc oxide alloy (a-IGZO),1Nomura K. Ohta H. Takagi A. Kamiya T. Hirano M. Hosono H. Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors.Nature. 2004; 432: 488-492Crossref PubMed Scopus (6840) Google Scholar emerged as a star player in the mid-2000s. As a successor to traditional low-performance amorphous silicon TFTs, oxide semiconductors offer superior characteristics including high electron mobility, high stability and optical transparency, and large-area uniformity. These features facilitate high-performance flat-panel displays (FPDs) with enhanced resolution, faster response times, and improved energy efficiency, enabling a successful transition from liquid-crystal displays (LCDs) to large-sized organic light-emitting diode (OLED) displays. Advancements in the late 2000s and early 2010s brought about substantial improvements in oxide TFT performance, reshaping consumers' viewing experience with sharper images, smoother video playback, and more vibrant colors on their smartphones, tablets, laptops, and televisions. Specifically, the low processing temperature and extremely low-off current of oxide TFT compared to low-temperature polycrystalline silicon (LTPS) facilitated the advent of flexible and transparent displays in the mid-2010s. This marked a paradigm shift, unveiling new possibilities for foldable and wearable gadgets on plastics, innovative automotive displays, and smart sensors. In the late 2010s and early 2020s, oxide TFTs made their foray into printed electronics.2Zhu H. Shin E.-S. Liu A. Ji D. Xu Y. Noh Y.-Y. Printable semiconductors for backplane TFTs of flexible OLED displays.Adv. Funct. Mater. 2020; 301904588Google Scholar These manufacturing process innovations promote cost effectiveness and scalability, enabling oxide TFTs to be mass produced and seamlessly integrated into a diverse array of devices. These devices include electronic paper, radio-frequency identification (RFID) tags, and Internet of Things (IoT) devices equipped with sensors, actuators, communication modules, and more. Looking ahead, the high transparency and low power consumption of oxide TFTs will shine even brighter in the fields of augmented reality (AR) and virtual reality (VR). We also foresee a future where these devices play a central role in revolutionizing next-generation IoT devices and a spectrum of cutting-edge applications, such as in-memory computing for artificial intelligence (AI) and neuromorphic processing.3Portilla L. Loganathan K. Faber H. Eid A. Hester J.G.D. Tentzeris M.M. Fattori M. Cantatore E. Jiang C. Nathan A. et al.Wirelessly powered large-area electronics for the Internet of Things.Nat. Electron. 2023; 6: 10-17Google Scholar Their capabilities to process high-speed data and support complex algorithms enhance intelligence within real-time autonomous systems, making them well suited for microprocessor applications. The lower processing temperature of oxide TFTs than silicon renders them compatible with back-end-of-line (BEOL) processing for monolithic 3D integration on existing silicon CMOS chips.4Son Y. Frost B. Zhao Y. Peterson R.L. Monolithic integration of high-voltage thin-film electronics on low-voltage integrated circuits using a solution process.Nat. Electron. 2019; 2: 540-548Crossref Scopus (58) Google Scholar This allows increased packing density of transistors and other components compared to traditional 2D layouts, thereby enabling the development of more powerful and compact electronic devices within a smaller footprint. To fully realize the potential applications of oxide TFT technology, it is imperative to advance performance metrics such as mobility, enhance energy efficiency, and broaden the functionality of advanced complementary circuits. The complementary metal-oxide-semiconductor (CMOS) structure, comprising n-type (electron) and p-type (hole) semiconductors, plays a pivotal role in modern advanced integrated circuits.5Myny K. The development of flexible integrated circuits based on thin-film transistors.Nat. Electron. 2018; 1: 30-39Crossref Scopus (389) Google Scholar This configuration not only facilitates high scalability and transistor integration density but also mitigates power dissipation, enhances noise immunity, and furnishes versatile functionalities in complex systems-on-chip. It's worth noting, however, that most oxide semiconductors only exhibit superior electron transport (n-type) due to their intrinsic electrical structure, in which the conduction band minimum (electron transport path) comprises spatially spread metal s-orbitals with an isotropic shape. The substantial overlap among neighboring metal ns-orbitals facilitates high electron mobility even in the amorphous state. On the other hand, the corresponding hole transport path—i.e., valence band maximum (VBM)—consists of anisotropic oxygen 2p-orbitals with strong directivity, resulted in non-dissipative band structure. This leads to a large hole effective mass and low hole mobility. Consequently, the development of high performance p-type oxide TFTs significantly lags behind that of their n-type counterparts. The development of device-quality p-type oxide semiconductors is widely recognized as a significant challenge in the oxide semiconductor community, and innovative breakthroughs in material design and film-device process are required to overcome the intrinsic nature of oxides (Figure 2). Since the first reports on p-channel oxide TFT using copper oxide (Cu2O) and tin monoxide (SnO) in 2008,6Matsuzaki K. Nomura K. Yanagi H. Kamiya T. Hirano M. Hosono H. Epitaxial growth of high mobility Cu2O thin films and application to p-channel thin film transistor.Appl. Phys. Lett. 2008; 93202107Crossref Scopus (230) Google Scholar,7Ogo Y. Hiramatsu H. Nomura K. Yanagi H. Kamiya T. Hirano M. Hosono H. p-channel thin-film transistor using p-type oxide semiconductor, SnO.Appl. Phys. Lett. 2008; 93032113Crossref Scopus (615) Google Scholar several efforts have been made in the development of new p-type oxides. However, p-type material is still very limited. Despite some improvements in device performance achieved through material design, film processing, and device engineering, a comprehensive understanding of the fundamental properties and potential of p-type oxide semiconductors for transistor application is still lacking. One of the drawbacks of existing p-channel oxide TFTs is their high off-current characteristics, attributed to high-density subgap defect. Therefore, there is a need for the development of effective defect termination strategies coupled with a deeper understanding of electronic and defect structures. In light of this situation, low-temperature polycrystalline oxide (LTPO) technology, which compares p-type LTPS and n-type oxide TFTs for CMOS circuit integration and OLED backplane applications, has been adopted. However, polycrystalline silicon faces persistent challenges such as process inhomogeneity, grain boundary effects with poor scaling capability, manufacturing complexity, limited substrate compatibility, and thermal budget constraints. Over the past decades, extensive efforts have been made to explore alternative p-type semiconductors, including organics,8Klauk H. Organic thin-film transistors.Chem. Soc. Rev. 2010; 39: 2643-2666Crossref PubMed Scopus (1163) Google Scholar low-dimensional nanomaterials,9Das S. Sebastian A. Pop E. McClellan C.J. Franklin A.D. Grasser T. Knobloch T. Illarionov Y. Penumatcha A.V. Appenzeller J. et al.Transistors based on two-dimensional materials for future integrated circuits.Nat. Electron. 2021; 4: 786-799Crossref Scopus (351) Google Scholar and metal halides,10Liu A. Zhu H. Kim M.-G. Kim J. Noh Y.-Y. Engineering copper iodide (CuI) for multifunctional p-type transparent semiconductors and conductors.Adv. Sci. 2021; 82100546Google Scholar,11Liu A. Zhu H. Bai S. Reo Y. Caironi M. Petrozza A. Dou L. Noh Y.-Y. High-performance metal halide perovskite transistors.Nat. Electron. 2023; 6: 559-571Crossref Scopus (12) Google Scholar for transistor applications. The success of oxide TFT technology can be attributed to its distinctive material processing properties, including outstanding ambient stability, a wide range of processing compatibility, and excellent uniformity across large areas. Despite ongoing advances, maintaining these qualities simultaneously in emerging semiconductors appears challenging, as they often exhibit inherent issues such as complex synthesis processes, large-area non-uniformity, low stability, and limited process compatibility. Despite significant challenges, there is a prospect for the advancement of p-type oxide TFT technology, expediting the convergence of large-area TFT-based electronics and CMOS-integrated circuits. To enhance hole transport in oxide semiconductors, it is crucial to devise valence band modulation approaches that extend the electronic states of orbitals. The initial endeavor, since 1997, has involved demonstrating a series of Cu+-based delafossites.12Kawazoe H. Yasukawa M. Hyodo H. Kurita M. Yanagi H. Hosono H. P-type electrical conduction in transparent thin films of CuAlO2.Nature. 1997; 389: 939-942Crossref Scopus (1580) Google Scholar This was achieved by introducing ternary cations with occupied d or s states near the VBM to enhance VB hybridization and increase and the bandgap. The design principle was subsequently expanded to incorporate chalcogen anions' p orbitals in lieu of oxygen by forming layered Cu+-based oxychalcogenides.13Ueda K. Inoue S. Hirose S. Kawazoe H. Hosono H. Transparent p-type semiconductor: LaCuOS layered oxysulfide.Appl. Phys. Lett. 2000; 77: 2701-2703Crossref Scopus (268) Google Scholar Due to the increased covalency between Cu and chalcogen atoms, it was anticipated that a more dispersed VBM would result from the hybridization between Cu d and chalcogen p orbitals. Additionally, tellurium (Te)-based semiconductors represent another possible candidate for enabling high-mobility, stable p-type semiconductors. However, application as a transistor channel is challenging due to the narrow bandgap of elemental tellurium, which is approximately 0.35 eV, leading to high off-state current and consequently increased power consumption.14Liu A. Zhu H. Zou T. Reo Y. Ryu G.-S. Noh Y.-Y. Evaporated nanometer chalcogenide films for scalable high-performance complementary electronics.Nat. Commun. 2022; 13: 6372Crossref Scopus (9) Google Scholar Therefore, future in-depth research on the appropriate control of optical and electrical properties, such as through the mixing of oxygen and other chalcogens, is highly interesting. Recent theoretical studies on electronic structure indicate the potential of Te-based oxides as transparent p-type oxide semiconductors.15Shi J. Sheng Z. Zhu L. Xu X. Gao Y. Tang D. Zhang K.H. The electronic structure of β-TeO2 as wide bandgap p-type oxide semiconductor.Appl. Phys. Lett. 2023; 122: 101901Crossref Scopus (3) Google Scholar For next-generation amorphous p-type oxide semiconductors and devices, considering the intricate challenges associated with hole limitation in oxide semiconductors, the hybrid phase combining high-mobility p-type materials dispersed in an amorphous oxide matrix may present an alternative design principle.16Liu A. Kim Y.-S. Kim M.G. Reo Y. Zou T. Choi T. Bai S. Zhu H. Noh Y.-Y. Selenium alloyed tellurium oxide for amorphous p-channel transistors.Nature. 2024; https://doi.org/10.1038/s41586-024-07360-wCrossref Google Scholar The transformative journey from early research to widespread adoption underscores the resilience and adaptability of oxide TFT technology. While the narrative of oxide TFT technology is rich in accomplishments, it is far from concluded, promising an exciting chapter in the ever-evolving landscape of electronic advancements. Looking ahead, we recognize several challenges that can be addressed to push the boundaries of oxide TFT technology, ensuring its continued relevance in displays, electronic devices, and emerging technologies across various applications (Box 1).Box 1Challenges in further developing oxide TFT technologies(1)Scaling and integration: as technology continues to evolve, there is a rising demand for the integration of oxide TFTs into high-resolution displays and high-integration circuits. Consequently, scaling down device dimensions while preserving performance and reliability becomes paramount. Amorphous semiconductors offer a downsizing roadmap when compared to polycrystalline ones; short-channel TFTs maintain excellent device performance and reliability without the negative impact from crystal grains. Recently, through the use of atomic layer deposition processes, ultra-thin (0.5 nm) amorphous indium oxide has shown highly promising potential for achieving high-performance, nano-scale TFTs.17Si M. Lin Z. Chen Z. Sun X. Wang H. Ye P.D. Scaled indium oxide transistors fabricated using atomic layer deposition.Nat. Electron. 2022; 5: 164-170Crossref Scopus (105) Google Scholar(2)Stability and reliability: ongoing research is focused on achieving comprehensive high electrical performance and operational stability of oxide TFTs, particularly in demanding environments such as high temperatures, light illumination, humidity, or mechanical stress. Advancements in material exploration, film manufacturing processes, defect mitigation strategies, interface modification, film quality improvement, and device engineering are essential for striking the balance between electrical performance and stability.(3)Power consumption and efficiency: despite the high electron mobility of oxide TFTs, reducing the energy consumption of oxide-TFT-based devices is essential, particularly for battery-driven portable products. This can be achieved by minimizing leakage currents, optimizing driving schemes, and exploring novel device designs. The development of high-performance p-type oxide semiconductors and TFTs still holds great promise for realizing viable all-oxide low-power dissipation CMOS circuits. The emergence of Te-based materials presents a new opportunity for designing high-mobility and stable p-type semiconductors, indicating a promising future in this exciting research direction.(4)Enhanced functionality: The high mobility of oxide semiconductors enables their application in small-size TFTs, holding significant potential for light detection and imaging. This characteristic opens avenues for integration with light-sensing diodes or direct detection materials, leading to low noise, improved spatial resolution, high operating speed, and on-pixel signal amplification. Additionally, combining oxide TFTs with quantum dot technology shows promise in enhancing display performance, including improved color reproduction and efficiency. Furthermore, research into the development of biologically compatible oxide TFT technology holds potential for applications beyond biocompatible implantable electronics. It extends to healthcare monitoring and diagnostics, including biosensors and medical imaging devices. Moreover, vertical stacking of heterogeneous semiconductors and devices allows for the integration of various technologies (e.g., memory, logic, sensors) within the same chip, enhancing functionality and versatility in electronic systems.(5)Sustainability and recycling: it is estimated that more than 50 million tonnes of electronic waste are produced globally every year, less than 20% of which can be collected and recycled. This problem highlights the importance of considering how electronic devices are ultimately dismantled, destroyed, and recycled. Given the ongoing exploration of novel materials and manufacturing techniques, attention toward environmentally friendly materials and recyclable, sustainable oxide TFT technologies is crucial to address electronic waste concerns. (1)Scaling and integration: as technology continues to evolve, there is a rising demand for the integration of oxide TFTs into high-resolution displays and high-integration circuits. Consequently, scaling down device dimensions while preserving performance and reliability becomes paramount. Amorphous semiconductors offer a downsizing roadmap when compared to polycrystalline ones; short-channel TFTs maintain excellent device performance and reliability without the negative impact from crystal grains. Recently, through the use of atomic layer deposition processes, ultra-thin (0.5 nm) amorphous indium oxide has shown highly promising potential for achieving high-performance, nano-scale TFTs.17Si M. Lin Z. Chen Z. Sun X. Wang H. Ye P.D. Scaled indium oxide transistors fabricated using atomic layer deposition.Nat. Electron. 2022; 5: 164-170Crossref Scopus (105) Google Scholar(2)Stability and reliability: ongoing research is focused on achieving comprehensive high electrical performance and operational stability of oxide TFTs, particularly in demanding environments such as high temperatures, light illumination, humidity, or mechanical stress. Advancements in material exploration, film manufacturing processes, defect mitigation strategies, interface modification, film quality improvement, and device engineering are essential for striking the balance between electrical performance and stability.(3)Power consumption and efficiency: despite the high electron mobility of oxide TFTs, reducing the energy consumption of oxide-TFT-based devices is essential, particularly for battery-driven portable products. This can be achieved by minimizing leakage currents, optimizing driving schemes, and exploring novel device designs. The development of high-performance p-type oxide semiconductors and TFTs still holds great promise for realizing viable all-oxide low-power dissipation CMOS circuits. The emergence of Te-based materials presents a new opportunity for designing high-mobility and stable p-type semiconductors, indicating a promising future in this exciting research direction.(4)Enhanced functionality: The high mobility of oxide semiconductors enables their application in small-size TFTs, holding significant potential for light detection and imaging. This characteristic opens avenues for integration with light-sensing diodes or direct detection materials, leading to low noise, improved spatial resolution, high operating speed, and on-pixel signal amplification. Additionally, combining oxide TFTs with quantum dot technology shows promise in enhancing display performance, including improved color reproduction and efficiency. Furthermore, research into the development of biologically compatible oxide TFT technology holds potential for applications beyond biocompatible implantable electronics. It extends to healthcare monitoring and diagnostics, including biosensors and medical imaging devices. Moreover, vertical stacking of heterogeneous semiconductors and devices allows for the integration of various technologies (e.g., memory, logic, sensors) within the same chip, enhancing functionality and versatility in electronic systems.(5)Sustainability and recycling: it is estimated that more than 50 million tonnes of electronic waste are produced globally every year, less than 20% of which can be collected and recycled. This problem highlights the importance of considering how electronic devices are ultimately dismantled, destroyed, and recycled. Given the ongoing exploration of novel materials and manufacturing techniques, attention toward environmentally friendly materials and recyclable, sustainable oxide TFT technologies is crucial to address electronic waste concerns. Addressing these challenges will require collaboration among researchers, engineers, and industry stakeholders to advance oxide TFT technology and unlock its full potential in shaping the future of electronics, displays, and beyond. By overcoming these obstacles, oxide TFTs can continue to drive innovation and enable exciting new applications in various fields, enhancing our lives in ways we have yet to imagine. Huihui Zhu is a Professor at the School of Physics, University of Electronic Science and Technology of China (UESTC). She received her PhD from the Pohang University of Science and Technology and subsequently worked as a postdoctoral researcher in the Department of Chemistry at Northwestern University. Her research focuses on the development of perovskite semiconductors and applications in electronic devices. Kenji Nomura is an Associate Professor of the Department of Electrical and Computer Engineering at the University of California San Diego. He developed a key semiconductor material and invented high-performance oxide TFTs. He received his PhD from the Tokyo Institute of Technology. Yong-Young Noh is Chair Professor of the Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH). He received his PhD from GIST in 2005. His research interest is the field of developing novel semiconductors for field-effect transistors. Ao Liu is a Full Professor at the Institute of Fundamental and Frontier Sciences, UESTC. He received his PhD from POSTECH in 2022 and subsequently worked as a postdoctoral researcher at Northwestern University. His research focuses on the exploration of innovative semiconductor materials and their applications in transistors and integrated circuits. This study was supported by the Ministry of Science and ICT through the National Research Foundation, funded by the Korean government (2021R1A2C3005401 and RS-2023-00260608). The authors declare no competing interests.