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.