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We report a unified subthreshold coupling factor technique for a simultaneous extraction of the surface potential (ψS) 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(VGS)] 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 (VGS) to the subgap energy is obtained. Applying to amorphous InGaZnO TFTs, g(E) is obtained to be a superposition of two exponential functions with NTA = 1.62 × 1017 eV-1 cm-3 and kTTA = 0.026 eV for the tail states while NDA = 6.5 × 1016 eV-1 cm-3 and kTDA = 0.22 eV for the deep states.