Abstract
We have examined the effects of oxygen content and thickness in sputtered InSnO (ITO) electrodes, especially for the application of imperceptible amorphous-InGaZnO (a-IGZO) thin-film transistors (TFTs) in humidity sensors. The imperceptible a-IGZO TFT with 50-nm ITO electrodes deposited at Ar:O2 = 29:0.3 exhibited good electrical performances with Vth of −0.23 V, SS of 0.34 V/dec, µFE of 7.86 cm2/V∙s, on/off ratio of 8.8 × 107, and has no degradation for bending stress up to a 3.5-mm curvature. The imperceptible oxide TFT sensors showed the highest sensitivity for the low and wide gate bias of −1~2 V under a wide range of relative humidity (40–90%) at drain voltage 1 V, resulting in low power consumption by the sensors. Exposure to water vapor led to a negative shift in the threshold voltage (or current enhancement), and an increase in relative humidity induced continuous threshold voltage shift. In particular, compared to conventional resistor-type sensors, the imperceptible oxide TFT sensors exhibited extremely high sensitivity from a current amplification of >103.
Highlights
Imperceptible devices that are ultra-thin, ultra-light, and transparent have been spotlighted as next-generation new-concept electronic devices, because they can be fashioned to be undetectable to human touch and sight [1,2,3,4,5]
We developed oxide thin-film transistors (TFTs) in the order of rigid → separately transparent and flexible →
For the realization of complete imperceptible TFTs with high performance comparable to those on rigid substrates, we surveyed optimized ITO coating conditions with relatively small thicknesses at room temperature. Based on these technical developments, we proposed humidity sensor devices with extremely high responsivity and imperceptible physical characteristics
Summary
Imperceptible devices that are ultra-thin, ultra-light, and transparent have been spotlighted as next-generation new-concept electronic devices, because they can be fashioned to be undetectable to human touch and sight [1,2,3,4,5]. The ultra-light slim devices on polymer substrates can be attached to objects with round shapes, and transparent sensors can be embedded on commercial glasses such as building windows and tables without blocking visibility. More developed semiconductor sensors are based on Schottky or p-n junction diodes, and these exhibited highly responsible and enhanced sensitivity due to their reduced reverse current level. The sensibility performance of semiconductor sensors can be determined by off-state current values; the diode is shown to have very adequate electrical performance. Resistor-type sensors comprise nanoparticles or polycrystalline to enhance sensibility by enlarging the reactive surface area [15,16,17,18]
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