Photofield‐effect in amorphous In‐Ga‐Zn‐O (a‐IGZO) thin‐film transistors

AbstractWe studied both the wavelength and intensity dependent photo‐responses (photofield‐effect) in amorphous In‐Ga‐Zn‐O (a‐IGZO) thin‐film transistors (TFTs). During the a‐IGZO TFT illumination with the wavelength range from 460∼660 nm (visible range), the off‐state drain current (IDS_off) only slightly increased while a large increase was observed for the wavelength below 400 nm. The observed results are consistent with the optical gap of ∼3.05eV extracted from the absorption measurement. The a‐IGZO TFT properties under monochromatic illumination (λ=420nm) with different intensity was also investigated and IDS_off was found to increase with the light intensity. Throughout the study, the field‐effect mobility (μeff) is almost unchanged. But due to photo‐generated charge trapping, a negative threshold voltage (Vth) shift is observed. The mathematical analysis of the photofield‐effect suggests that a highly efficient UV photocurrent conversion process in TFT off‐region takes place. Finally, a‐IGZO mid‐gap density‐of‐states (DOS) was extracted and is more than an order of magnitude lower than reported value for hydrogenated amorphous silicon (a‐Si:H), which can explain a good switching properties observed for a‐IGZO TFTs.