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 NTA=1.62× 10¹⁷ eV ^-1cm^-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.