We have developed a maximum applied voltage detector using an alpha-IGZO TFT exposed to ozone annealing. This TFT has an interesting property; the I-ds-V-gs characteristic shifts positively and becomes steep when gate voltage is applied, whereas it recovers the initial one with a small threshold voltage and a large subthreshold swing upon light illumination. Therefore, this TFT can be used as a maximum applied voltage detector; first, light is irradiated to initialize the I-ds-V-gs characteristic, next, changing external voltage is applied to the gate electrode, and finally, the maximum applied voltage is obtained from the final I-ds-V-gs characteristic. (C) 2012 Elsevier Ltd. All rights reserved.