Transparent electronics is today one of the most advanced topics for a wide range of device applications. The key components are wide bandgap semiconductors, where oxides of different origins play an important role, not only as passive component but also as active component, similar to what is observed in conventional semiconductors like silicon. Transparent electronics has gained special attention during the last few years and is today established as one of the most promising technologies for leading the next generation of flat panel display due to its excellent electronic performance. In this paper the recent progress in n- and p-type oxide based thin-film transistors (TFT) is reviewed, with special emphasis on solution-processed and p-type, and the major milestones already achieved with this emerging and very promising technology are summarizeed. After a short introduction where the main advantages of these semiconductors are presented, as well as the industry expectations, the beautiful history of TFTs is revisited, including the main landmarks in the last 80 years, finishing by referring to some papers that have played an important role in shaping transparent electronics. Then, an overview is presented of state of the art n-type TFTs processed by physical vapour deposition methods, and finally one of the most exciting, promising, and low cost but powerful technologies is discussed: solution-processed oxide TFTs. Moreover, a more detailed focus analysis will be given concerning p-type oxide TFTs, mainly centred on two of the most promising semiconductor candidates: copper oxide and tin oxide. The most recent data related to the production of complementary metal oxide semiconductor (CMOS) devices based on n- and p-type oxide TFT is also be presented. The last topic of this review is devoted to some emerging applications, finalizing with the main conclusions. Related work that originated at CENIMAT|I3N during the last six years is included in more detail, which has led to the fabrication of high performance n- and p-type oxide transistors as well as the fabrication of CMOS devices with and on paper.

Oxide Semiconductor Thin‐Film Transistors: A Review of Recent Advances

Review of recent developments in amorphous oxide semiconductor thin-film transistor devices

The field of flexible electronics has rapidly expanded over the last decades, pioneering novel applications, such as wearable and textile integrated devices, seamless and embedded patch-like systems, soft electronic skins, as well as imperceptible and transient implants. The possibility to revolutionize our daily life with such disruptive appliances has fueled the quest for electronic devices which yield good electrical and mechanical performance and are at the same time light-weight, transparent, conformable, stretchable, and even biodegradable. Flexible metal oxide semiconductor thin-film transistors (TFTs) can fulfill all these requirements and are therefore considered the most promising technology for tomorrow's electronics. This review reflects the establishment of flexible metal oxide semiconductor TFTs, from the development of single devices, large-area circuits, up to entirely integrated systems. First, an introduction on metal oxide semiconductor TFTs is given, where the history of the field is revisited, the TFT configurations and operating principles are presented, and the main issues and technological challenges faced in the area are analyzed. Then, the recent advances achieved for flexible n-type metal oxide semiconductor TFTs manufactured by physical vapor deposition methods and solution-processing techniques are summarized. In particular, the ability of flexible metal oxide semiconductor TFTs to combine low temperature fabrication, high carrier mobility, large frequency operation, extreme mechanical bendability, together with transparency, conformability, stretchability, and water dissolubility is shown. Afterward, a detailed analysis of the most promising metal oxide semiconducting materials developed to realize the state-of-the-art flexible p-type TFTs is given. Next, the recent progresses obtained for flexible metal oxide semiconductor-based electronic circuits, realized with both unipolar and complementary technology, are reported. In particular, the realization of large-area digital circuitry like flexible near field communication tags and analog integrated circuits such as bendable operational amplifiers is presented. The last topic of this review is devoted for emerging flexible electronic systems, from foldable displays, power transmission elements to integrated systems for large-area sensing and data storage and transmission. Finally, the conclusions are drawn and an outlook over the field with a prediction for the future is provided.

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Metal oxide semiconductor thin-film transistors for flexible electronics

We report a self-aligned coplanar amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) using an a-IGZO/SiO2 stack layer. From the channel-length dependence of the total resistance for the TFTs, the channel and parasitic resistances were found to be 8.4 kΩ/μm and 9.7 kΩ/sq, respectively. The fabricated a-IGZO TFT exhibits field-effect mobility of 23.3 cm2/V ·s, threshold voltage of 3.6 V, and gate voltage swing of 203 mV/dec. A 23-stage ring oscillator made of the self-aligned TFTs exhibits a propagation delay time of 17 ns/stage at a supply voltage of 22 V.

Self-Aligned Coplanar a-IGZO TFTs and Application to High-Speed Circuits

This paper reviews the mechanisms underlying visible light detection based on phototransistors fabricated using amorphous oxide semiconductor technology. Although this family of materials is perceived to be optically transparent, the presence of oxygen deficiency defects, such as vacancies, located at subgap states, and their ionization under illumination, gives rise to absorption of blue and green photons. At higher energies, we have the usual band-to-band absorption. In particular, the oxygen defects remain ionized even after illumination ceases, leading to persistent photoconductivity, which can limit the frame-rate of active matrix imaging arrays. However, the persistence in photoconductivity can be overcome through deployment of a gate pulsing scheme enabling realistic frame rates for advanced applications such as sensor-embedded display for touch-free interaction.

Amorphous Oxide Semiconductor TFTs for Displays and Imaging

Circuits implemented with high-performance amorphous-indium-gallium-zinc-oxide thin-film transistors (TFTs) are realized on polyimide/polyethylene-terephthalate plastic substrates. The TFTs on plastic exhibit a saturation mobility of 19 cm2/V·s and a gate voltage swing of ~0.14 V/dec. For an input of 20 V, an 11-stage ring oscillator operates at 94.8 kHz with a propagation delay time of 0.48 μs. A shift register, consisting of ten TFTs and one capacitor, operates well with good bias stability. AC driving of pull-down TFTs gives the gate driver an improved lifetime of over ten years.

Transparent Flexible Circuits Based on Amorphous-Indium–Gallium–Zinc–Oxide Thin-Film Transistors

A high-performance inverter implemented with single-gated driving and dual-gated load amorphous-indium-gallium-zinc-oxide thin-film transistors (TFTs) is demonstrated The threshold voltage of the load TFT shifts to the negative gate voltage direction when a constant positive bias is applied on the top gate while sweeping the bottom gate Using a positive top gate bias, the load TFT can be operated in the depletion mode to realize inverters with excellent switching characteristics, such as a wider swing range and a higher noise margin

A Full-Swing a-IGZO TFT-Based Inverter With a Top-Gate-Bias-Induced Depletion Load

We report the effect of the drain-offset length on the performance of amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs). While the field-effect mobility decreases from ~ 40 to 10 cm2/V·s by increasing the drain-offset length from 0 to 5 μm, the threshold voltage (Vth) and swing (S) remain relatively independent of the offset length variation. Because of its high mobility even for large (5 μm ) offset lengths, the drain-offset a-IGZO TFT can be used to eliminate the kickback voltage in active-matrix displays.

High-Performance Drain-Offset a-IGZO Thin-Film Transistors

This paper investigates the origin and reduction of self-heating effects in single-crystal silicon-on-glass (SiOG) thin-film transistors (TFTs). A hump forms in the transfer characteristics of p-channel SiOG TFTs when the temperature of the devices is increased either by direct heating or electrical biasing. The size of the hump proportionally scales with the channel width W, indicating that it is related to the bulk active-layer properties such as conduction through a backchannel. While the hump increases in the positive direction, the main transistor shifts in the negative direction with increasing self-heating stress time, supporting the exclusion of edge effects. The time dependence of the hump shift is well described by the stretched-exponential behavior, indicating that the backchannel is a result of electron trapping into the silica layer that is between the glass and silicon active layer. To mitigate this hump effect, we demonstrate in this paper that TFTs with an active layer divided into smaller parts along the W direction (in order to increase heat dissipation) show better stability to self-heating stress (i.e., no hump formation) than TFTs with full active layers. Split devices have more channel edges, compared with those with a full active layer, supporting the idea that the hump is indeed not due to edge effects.

Degradation Model of Self-Heating Effects in Silicon-on-Glass TFTs

A high performance amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT)-based inverter is demonstrated using the dual gate TFT structure. The results indicate that the load of the inverter behaves like a depletion-mode TFT when the top gate is under a positive bias. The proposed inverter shows much improved switching characteristics such as a wider swing range and higher noise margins, which are all achieved without the requirement of an additional process step to make the depletion load.

Min Hyuk Choi

Papers

Transparent Flexible Circuits Based on Amorphous-Indium–Gallium–Zinc–Oxide Thin-Film Transistors

A Full-Swing a-IGZO TFT-Based Inverter With a Top-Gate-Bias-Induced Depletion Load

High-Performance Drain-Offset a-IGZO Thin-Film Transistors

Degradation Model of Self-Heating Effects in Silicon-on-Glass TFTs

P‐13: A Full‐Swing a‐IGZO TFT‐Based Inverter with a Top Gate‐Induced Depletion Load