Flexible, Low-Voltage, Metal Oxide Thin Film Transistors (TFT) and Circuits for Wearables and Internet of Things (IoT)

Multifunctional flexible electronics are booming in many ways. Flexible electronics have opened up a world of possibilities for their widespread use in smart electronics such as artificial electronic-skin (e-skin), flexible displays, consumer electronics, implantable devices. Flexible devices and sensors are paramount in order to realize the true potential of flexible electronics, wearable eco-systems, biomedical and anticipated pervasive Internet of Things (IoT). With special properties like enhanced flexibility, light weight and conformability, flexible technology allows a seamless system integration. Although today's modern electronics are capable of performing everyday needs in existing applications, flexible electronics can provide potential solutions for many as yet unseen applications which requires lower processing capacity, low cost as well as thin, lightweight and environmentally friendly products. To fulfill the needs of a diverse range of applications, next-generation flexible electronics should be engineered with added flexibility and mechanical deformability, by exploring new materials and tactics to overcome the limits of traditional methods.

At the moment, enhanced device performance, greater integration density, fabrication cost and advanced functionality are propelling the traditional silicon semiconductor technology and electronics industry forward, servicing traditional macro-electronic sectors (e.g., mobile phones, laptops, routers, automotive). This unquenchable need for higher performing electronic devices necessitates new technological advances and keeps costs high. The overall performance of modern silicon-based devices and technologies are reaching new levels. With the ballooning consumer electronics needs; the global semiconductor industry is expected to remain growing robustly. But rather than competing with well-developed rigid silicon devices, the goal for flexible electronics is to improve the efficiency of current stateof- the-art thin film transistors (TFTs), required for low energy flexible wearables and Internet of Things (IoT), and mature these designs for low-cost, printable manufacturing.

Thin film transistors (TFTs) are among the most important thin film devices and can form the foundation of any electronic system and application. In this regard, metal oxide semiconductors are viewed as a potential candidate for flexible electronics, due to a broad range of properties, such as a large band gap, high optical transparency, high mobility, and solution processable deposition at low temperatures compared to CMOS processes. However, despite a dedicated effort by the research community and industry, metal oxide thin film transistors still face great challenges being used in flexible electronics, wearables, and Internet of Things (IoT), such as operating voltage, switching speed, on-off ratio, process temperature and gate dielectric deposition compatible for flexible substrates.

The present thesis investigates solution-processed metal-oxide thin film transistors (TFTs) and inverter circuits for flexible electronics. The low-voltage (< 3 V) thin film transistors using indium oxide (In2O3), gallium oxide (Ga2O3) and indium gallium zinc oxide (IGZO) as the active channel semiconductor were fabricated on solid (glass) and flexible (polyimide) substrates. The low-voltage operating thin film transistors were accomplished by following low temperature solution processing and room-temperature anodization routes for metal oxide semiconductor and gate oxide dielectric deposition, respectively. The incorporation of anodized high-κ aluminum oxide (Al2O3) for gate dielectric into the TFT fabrication process has significantly helped to reduce operating and threshold voltages while also improving carrier mobility. On the other hand, solution processing allows low-temperature, large-area deposition while lowering fabrication costs.

The solution-processed metal-oxide TFTs show excellent electrical performance, at a low gate voltage (Vg), which could enable novel applications in flexible electronics. Along with electrical characterization, bending performance and SPICE simulation results are also presented. Furthermore, thorough thin film characterization and interface analysis between oxide semiconductor and gate dielectric were performed, using various characterization techniques, such as C-V (Capacitance-voltage) profiling, AFM (atomic force microscopy), TEM (transmission electron microscopy) and TLM (transfer length measurement). Additionally, a low-voltage flexible inverter circuit using solution-processed metal-oxide thin film transistors was fabricated and characterized. The results indicate that flexible, low voltage operating devices and circuits enable a viable alternative to silicon dominated semiconductor devices, and hence a path forward, for wearables and Internet of Things (IoT).