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.
We discuss the role of aluminum oxide (i.e. Al2O3 when stoichiometric) for transistors and sensors based on oxide semiconductors such as InGaZnO (IGZO) and two-dimensional (2D) semiconductors, such as monolayer MoS2. Aluminum oxide is a well-known capping and dielectric layer in semiconductor technology typically deposited by atomic-layer deposition (ALD), which offers a dense and high-quality film with low gas permeability even when deposited on flexible substrates. However, when deposited at low temperature (< 200°C), aluminum oxide can include a significant amount of fixed charges and defects, which lead to unusual charge trapping and doping effects in semiconductor devices. For example, such charge trapping can cause (apparent) sub-60 mV/decade subthreshold swing at room temperature in IGZO transistors, but can also lead to potential applications in neuromorphic computing. We also discuss effective doping (~1013 cm-2) of 2D semiconductors by thin ALD-grown non-stoichiometric AlOx capping layers. This is achieved with an aluminum seed layer, which enables uniform growth of the subsequently deposited ALD film. This approach leads to a negative shift in threshold voltage, record on-state current (~700 μA/μm) in a monolayer semiconductor, and drastic reduction in contact resistance. Finally, we investigate the passivation effects of Al2O3 capping, which limits the interaction of the underlying semiconductors with ambient air and moisture. We demonstrate improved response in MoS2 temperature sensors and long-term stability in flexible MoS2 transistors (8 months). Further, we evaluate the effects of Al2O3 passivation on IGZO transistors after aging for 80 months.
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