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.

Functional oxide materials currently represent a key challenge as well as a promising powerful tool for both fundamental understanding and technological development of the next generation of transparent electronics, such as field-effect transistors.[1] Here, we report a fully transparent ZnO thin-film transistor (ZnO-TFT) with a transmittance above 80 % in the visible part of the spectrum, including the glass substrate, fabricated by radiofrequency (rf) magnetron sputtering at room temperature, with a bottom gate configuration. ), the opacity of polysilicon TFTs limits the aperture ratio for active matrix arrays; this is highly important, for instance, when OLEDs have to be addressed. Also, if flexible substrates based on polymers are intended to be used, the processing temperature is quite a limiting factor.One possible way to overcome such problems is the utilization of efficient and reliable oxide-based thin-film transistors. Transparent-oxide-semiconductor-based transistors have recently been proposed, using intrinsic zinc oxide (ZnO) as an active channel.[3±8] One of the main advantages exhibited by these transistors lies in the magnitude of the electron-channel mobility, leading to higher drive currents and faster device operating speeds. The mobility reported in the literature ranges from 0.2±7 cm 2 V ±1 s ±1, with an on/off current ratio from 10 5 ±10 7 , and a threshold voltage (V TH ) between ±1 V and 15 V. Until now, most ZnO channel layers have been deposited using substrate heating or subjected to post-thermal annealing, mainly in order to increase the crystallinity of the ZnO layer and thus the mobility in the film. The purpose of this work is to demonstrate the possibility of fabricating high-mobility ZnO thin-film transistors at room temperature by rf magnetron sputtering with improved performances and highly compatible with the fabrication technologies used for flexible electronics. By doing so, we overcome processing-temperature limitations, making it possible for the devices to be used in a wide range of applications where the mobility is no longer a limitation, such as for use in so-called fast and invisible electronics. Figure 1 shows the dependence of the electrical resistivity (r) and the average optical transmittance in the visible spectra (between 400±700 nm) as a function of rf power density (P). The highest resistivity (» 10 8 X cm) was obtained for, the films were close to being stoichiometric with few structural defects and a consequently higher resistivity. As we decreased or increased the rf power density from 5 W cm ±2 , a deviation from stoichiometry was obtained, accompanied by a decrease in electrical resistivity due to a lower carrier concentration and/or electron mobility. This was also confirmed by a decrease in the optical transmittance, especially for rf power densities lower than 5 W cm ±2 .COMMUNICATIONS 590

We report high-performance ZnO thin-film transistor (ZnO-TFT) fabricated by rf magnetron sputtering at room temperature with a bottom gate configuration. The ZnO-TFT operates in the enhancement mode with a threshold voltage of 19V, a saturation mobility of 27cm2∕Vs, a gate voltage swing of 1.39V∕decade and an on/off ratio of 3×105. The ZnO-TFT presents an average optical transmission (including the glass substrate) of 80% in the visible part of the spectrum. The combination of transparency, high mobility, and room-temperature processing makes the ZnO-TFT a very promising low-cost optoelectronic device for the next generation of invisible and flexible electronics.