Photo-Field Effect Analysis of Amorphous In-Ga-Zn-O Thin-Film Transistors

Alex Kuo, T.K. Won, and Jerzy Kanicki

The a-IGZO was found to be highly visible light transparent with optical energy band gap (Tauc gap) of ~3eV. We have studied the wavelength dependent a-IGZO TFT photosensitivity under broad-band illumination (λ=365~660nm) and found that the a-IGZO TFT is stable under visible light (λ=460~ 660nm). Under UV illumination (λ<400nm), TFT off-state drain current (IDS__off) increases and the change is consistent with the Tauc gap of the a-IGZO. In this project, we further explored the light intensity dependent a-IGZO TFT response along with the photofield-effect analysis under UV-monochromatic (420nm) photo illumination. The results were also used for estimating of the a-IGZO mid-gap density-of-states (DOS).

The photo-response of a-IGZO TFT under different illumination intensity is found to be similar to the response under different wavelength. The photo-current (I_ph) is further ex-tracted for photo-field effect analysis. The I_ph is found to have power-law dependence (γ) with the irradiance level (I). In addition, γ can approach unity under low V_GS region which suggests TFT photo-current is strongly proportional to the UV light intensity in off-region with a high photo-to-current conversion efficiency. The total mid-gap DOS of a-IGZO TFT is estimated to be ~1017 cm^-3eV^-1 with a linear slope (α) of 7.76×1016 cm^-3eV^-2. The extracted a-IGZO TFT mid-gap DOS is more than a order lower than the pre-viously reported values for a-Si:H TFT. The finding is supporting a good switching prop-erty of the a-IGZO TFTs that is experimentally observed. This work was done in colla-boration with the Tokyo Institute of Technology, Japan.

T. C. Fung, C. S. Chuang, K. Nomura, H.-P. D. Shieh, H. Hosano, and J. Ka-nicki, Journal of Information Display, vol. 9, pp. 21-29, 2008.

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