The hysteresis and off-current of amorphous indium-gallium-zinc oxide thin film transistors with various active layer thicknesses under the light illumination

We investigated the photo-induced hysteresis and off-current (I_off) of amorphous In-Ga-Zn-O (IGZO) TFT, of which the active layer thickeness varies from 400 Å to 700 Å, under 400 nm wavelength illumination. As the active layer thicknesses are increased, the hysteresis and off-current under the light illumination increases. Because the photo-induced holes in the active layer contribute to the increase in the ionized oxygen vacancy at the interface, the hysteresis phenomenon is observed under 400 nm wavelength light illumination. When the transfer curve was measured under the reverse sweep (from V_GS = 20 V to V_GS = -20 V), I_off was increased and independent on V_GS in the off-region. Due to wide band gap of IGZO (~3 eV), the photo-induced hole cannot cross the barrier between the valence band of IGZO and source/drain. Increase of I_off might be caused by the photo-induced electrons at the back side of the active layer. When the active layer thickness increases from 400 Å to 500 Å, I_off was barely changed, while it is increases linearly when the active layer thickness increases from 500 Å to 700 Å. In our device, the debye length of IGZO might be about 500 Å, so that the photo-induced electrons can contribute to I_off even though it is under the off-region.