Differential Ideality Factor Technique for Extraction of Subgap Density of States in Amorphous InGaZnO Thin-Film Transistors

We propose a differential ideality factor technique (DIFT) for extraction of subgap density of states (DOS) over the bandgap in amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) by using the differential ideality factor dη/dV_GS on behalf of the ideality factor itself. Contrary to the sub threshold current method which requires an accurate threshold voltage (V_T), the DIFT is free from V_T itself and consider ably useful to TFTs with a nonuniform distribution of DOS over the bandgap. Through the DIFT applied to an a-IGZO TFT with W/L = 200 μm/30 μm, the subgap DOS is extracted to be a superposition of exponential deep and tail states with NUA = 7.1 × 10¹⁵ cm^-3 · eV^-1, kT_DA = 0.6 eV, N_TA=1.5 × 10¹⁶ cm^-3 · eV^-1, and kT_TA = 0.024 eV.