Unified Subthreshold Coupling Factor Technique for Surface Potential and Subgap Density-of-States in Amorphous Thin Film Transistors

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 × 10¹⁷ eV^-1 cm^-3 and kT_TA = 0.026 eV for the tail states while N_DA = 6.5 × 10¹⁶ eV^-1 cm^-3 and kT_DA = 0.22 eV for the deep states.