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

• Published in: Journal of Information Display Vol. 9; no. 4; pp. 21 - 29
• Main Authors: Fung, Tze-Ching, Chuang, Chiao-Shun, Nomura, Kenji, Shieh, Han-Ping David, Hosono, Hideo, Kanicki, Jerzy
• Format: Journal Article
• Language: Korean
• Published: 2008
• Subjects: Photofield effect; TFT; Metal oxide semiconductor; InGaZnO
• ISSN: 1598-0316; 2158-1606

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 $460\sim660$ 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 $\sim$3.05eV extracted from the absorption measurement. The a-IGZO TFT properties under monochromatic illumination ($\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.