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 factordη/dVGSon behalf of the ideality factor itself. Contrary to the subthreshold current method which requires an accurate threshold voltage(VT), the DIFT is free fromVTitself and considerably useful to TFTs with a nonuniform distribution of DOS over the bandgap. Through the DIFT applied to an a-IGZO TFT withW/L = 200\ μ m/30\, the subgap DOS is extracted to be a superposition of exponential deep and tail states withNDA = 7.1 × 10¹⁵\ cm⁻³ · eV⁻¹,kTDA = 0.6\ eV,NTA = 1.5 × 10¹⁶\ cm⁻³ · eV⁻¹, andkTTA = 0.024\ eV.