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 d eta/dV(GS) on behalf of the ideality factor itself. Contrary to the subthreshold current method which requires an accurate threshold voltage (V-T), the DIFT is free from V-T itself and considerably 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 mu m/30 mu m, the subgap DOS is extracted to be a superposition of exponential deep and tail states with N-DA = 7.1 x 10(15) cm(-3) . eV(-1), kT(DA) = 0.6 eV, N-TA = 1.5 x 10(16) cm(-3) . eV(-1), and kT(TA) = 0.024 eV.