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We propose a light irradiation history sensor using an amorphous In-Ga-Zn-O (α-IGZO) thin-film transistor (TFT) fabricated by high oxygen partial pressure sputtering. The α-IGZO TFT has an interesting property. The Ids-Vgs characteristic shifts positively, and the subthreshold slope becomes steep when the gate bias is applied, whereas the Ids-Vgs characteristic shifts negatively and the subthreshold slope becomes gradual when the light is irradiated. Therefore, the α-IGZO TFT can be used as a light irradiation history sensor by the following steps. First, the gate bias is applied to reset the Ids-Vgs characteristic. Next, the light is irradiated, and finally, the Ids-Vgs characteristic is measured, which depends on the light irradiation history.