Bendable metal oxide thin-film transistors and circuits for analog electronics applications

The tremendous advances flexible electronics has experienced have led to the demonstration and - in some cases - commercialization of a plethora of devices, such as foldable displays, ubiquitously integrated sensor systems, and imperceptible implants. All these systems call for specialized analog circuits capable to transmit and receive data, condition sensors, drive actuators or control powering devices. Nevertheless, the current availability of materials and processes compatible with flexible foils imposes limitations to the realization of high-performance flexible analog systems. Among state-of-the-art technologies, amorphous metal oxides – and especially Indium-Gallium Zinc-Oxide (IGZO) – thin-film-transistors (TFTs), are extremely suited due to their electrical and mechanical performance. Here, we present TFTs based on IGZO semiconductor and Al2O3 insulating layers on polyimide substrates. First, we summarize different approaches to reduce the transistor channel length (down to 160 nm), together with their influence on the AC performance. Even though sub-500 nm lengths are demonstrated for TFTs fabricated using vertical structures, direct laser writing and focused ion beam milling, the highest transit and oscillation frequencies of respectively 135 MHz and 398 MHz, are achieved by 500 nm long self-aligned TFTs. We then show how flexible IGZO TFTs enable the realization of complex, flexible analog circuits operating at frequencies up to 20 MHz. However, even if remarkable performances are demonstrated for flexible NMOS circuits, their unipolar characteristics results in limited gain, high power consumption and complex design. To overcome this, we also show how the complementation of IGZO with ptype carbon nanotubes results in flexible common-source CMOS amplifiers with gain of 28.7 dB.