Fabrication and AC performance of flexible Indium-Gallium-Zinc-Oxide thin-film transistors

The internet of things or foldable phones call for a variety of flexible sensor conditioning and transceiver circuits. However, the realization of high-performance, large-area, and deformable analog circuits is limited by the materials and the processes compatible with mechanically flexible substrates. Among the different semiconductors, InGaZnO is one of the most promising materials to realize high-frequency flexible thin-film transistors (TFTs) and circuits. In this work, the effect of different geometries, including self-aligned, vertical, and double-gate structures on the AC behaviour of flexible IGZO TFTs is presented. All TFTs are based on Al2O3 insulating layers, InGaZnO semiconductor, and polyimide substrates. The presented TFTs exhibit state-of-the-art performance including a field-effect mobility up to 15 cm2 /Vs and a mechanical bendability down to radii of 3.5 mm. Due to different trade-offs required in the fabrication, flexible IGZO TFTs with the shortest channel length of 160 nm do not exhibit the highest measured frequency, whereas exceptional maximum oscillation and transit frequencies of 304 MHz and 135 MHz are demonstrated for 500 nm long self-aligned TFTs. Such optimized transistors can be used to realize entirely flexible analog circuits leading towards imperceptible electronic systems.