Effects of operational and geometrical conditions upon photosensitivity of amorphous InZnO thin film transistors

The photosensitivity of an amorphous oxide semiconductor thin film transistor (TFT) with an In-Zn-O (IZO) layer as the active channel passivated with SiO2 is investigated. Under illumination, the photocurrent (IPhoto) in the off regime is greatly increased rather than displaying a negative shift of the threshold voltage. In this way, the photosensitivity (IPhoto/IDark) can be maximized by adjusting a reading bias to be placed in the off regime. Furthermore, the photosensitivity is significantly influenced by operational and geometrical conditions, such as the bias voltages and the active layer thickness. In particular, for a 1500-Å-thick IZO TFT, the photosensitivity is increased by a factor of almost eight orders of magnitude. This suggests back channel conduction of excess electrons generated from the optically ionized oxygen vacancies (Vo ++) where the gate voltage-induced electric field is screened.