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In this paper, the photosensitive effect of n-type low-temperature polycrystalline-silicon thin-film transistors (TFTs) is investigated. A novel layout is adopted to demonstrate that the photo leakage current occurs in the depletion region at the drain junction. Based on the Poole-Frenkel effect lowering of a coulombic barrier and phonon-assisted tunneling, it is discovered that the photosensitivity behavior for poly-Si TFT is dependent on the gate and drain bias. However, this photoinduced leakage current behavior is not included in the present SPICE device model. Therefore, a new parameter, unit-lux-current (ULC), is proposed to depict the photoinduced current. Its dependence on the gate/drain bias and temperature is discussed, and the equation of ULC is further derived, which has good agreement with the experimental data. A qualitative deduction is developed to account for the photo leakage mechanism. ULC variation with respect to defect states in the drain region is also discussed.
Date of Publication: Jan. 2009