Top-gate ZnO thin-film transistors with a polymer dielectric designed for ultraviolet optical gating

We report on the fabrication of ultraviolet (UV)-sensing top-gate ZnO thin-film transistors (TFTs) with a poly-4-vinylphenol (PVP) polymer gate dielectric on glass substrate. Our top-gate ZnO-TFT showed a field-effect mobility of 0.05 cm²/V s, maximum saturation current of 0.11 μA at a gate bias of 10 V and an on/off ratio of ∼10³ in the dark. Under UV illumination with a wavelength of 364 nm the ZnO-TFT exhibited ∼4.7 μA for a drain current (at the same gate bias of 10 V), which is ∼50 times higher than without UV. Such photo-transistor action appeared more pronounced under a depletion regime of 0 V gate bias and the photo-to-dark current ratio was more than about 10⁴. By adopting this high UV-sensitivity, our inverter device with the top-gate ZnO-TFT and a load resistance well demonstrated its optical gating behavior.