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We report a unified subthreshold coupling factor technique for a simultaneous extraction of the surface potential (ψ S ) and the subgap density-of-states [DOS: g(E)] over the bandgap in amorphous semiconductor thin film transistors (TFTs). It is fully based on the experimental gate bias-dependent coupling factor [m(V GS )] under subthreshold bias. Through the proposed technique only with current-voltage data under subthreshold operation, a unified extraction of the DOS with a consistent mapping of the gate bias (V GS ) to the subgap energy is obtained. Applying to amorphous InGaZnO TFTs, g(E) is obtained to be a superposition of two exponential functions with N TA = 1.62 × 10 17 eV −1 cm −3 and kT TA = 0.026 eV for the tail states while N DA = 6.5 × 10 16 eV −1 cm −3 and kT DA = 0.22 eV for the deep states. Index Terms-Amorphous InGaZnO (a-IGZO), coupling factor, subgap density-of-states (DOS), subthreshold current, surface potential, thin film transistors (TFT).

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Thin film transistors using In-Ga-Zn-O (IGZO) semiconductors were evaluated under current stress by applying positive voltages to the gate and drain electrodes. Initially, the transfer characteristics exhibit identical threshold voltages (V T ) when the source and drain electrodes are interchanged during measurement ( forward and reverse V DS sweep). However as stress time increases, larger shifts in V T are observed under forward V DS sweep than under reverse V DS sweep conditions. Sub-gap states analyses based on the photoresponse of capacitance-voltage (C-V) curves suggest that local annihilation of donor-like traps occurs near the drain electrode. Hump-like features are clearly observed in the C-V curves collected between the drain and gate electrodes,while they do not appear in the C-V data obtained between the source and the gate. Based on the above, a local charge trapping model is introduced in order to interpret the device degradation. In this model, the major carrier electrons are trapped more abundantly near the source electrode due to the presence of a Schottky junction between IGZO and the source/drain electrodes.