Photofield-Effect in Amorphous In-Ga-Zn-O (a-IGZO) Thin-Film Transistors

We 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 similar to 660 nm (visible range), the off-state drain current (I-DS_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 similar to 3.05eV extracted from the absorption measurement. The a-IGZO TFT properties under monochromatic illumination (lambda = 420nm) with different intensity was also investigated and I-DS_off was found to increase with the light intensity. Throughout the study, the field-effect mobility (mu(eff)) is almost unchanged. But due to photo-generated charge trapping, a negative threshold voltage (V-th) 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.