Ultraviolet photodetectors (UVPDs) based on diode-connected In-Ga-ZnO (IGZO) thin-film transistors (TFTs), called field-effect diodes (FEDs), are presented. A patterned NiO capping layer (CL) deposited on the top surface of the IGZO channel forming a heterojunction (HJ) was employed to further improve UV detection performance. Experimental results show that FED-based UVPDs have advantages over traditional TFT-based UVPDs, achieving a minimum dark current without additional gate bias and a wider bias range for detection measurements. UVPDs based on the 30-nm-thick IGZO FED with NiO CL show a photoresponsivity and photosensitivity of up to 1376 A/W and 9.10×107 A/A at 275 nm under VD = -1.5 V, which is about 25× and 1480× higher than the case without NiO CL. These improvements are due to the fact that the NiO CL/IGZO HJ reduces the effective channel thickness to suppress dark current and provide additional optically generated electrons in the channel and an additional negative threshold voltage shift to enhance the photocurrent.
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