Extraction of Subgap Density of States in Amorphous InGaZnO Thin-Film Transistors by Using Multifrequency Capacitance–Voltage Characteristics

An extraction technique for subgap density of states (DOS) in an n-channel amorphous InGaZnO thin-film transistor (TFT) by using multifrequency capacitance-voltage (<i>C</i> -<i>V</i>) characteristics is proposed and verified by comparing the measured <i>I</i>- <i>V</i> characteristics with the technology computer-aided design simulation results incorporating the extracted DOS as parameters. It takes on the superposition of exponential tail states and exponential deep states with characteristic parameters for <i>N</i> _TA = 1.1 × 10¹⁷ cm^-3 · eV^-1, <i>N</i> _DA = 4 × 10¹⁵ cm^-3 · eV^-1, <i>kT</i> _TA = 0.09 eV, and <i>kT</i> _DA = 0.4 eV. The proposed technique allows obtaining the frequency-independent <i>C</i>-<i>V</i> curve, which is very useful for oxide semiconductor TFT modeling and characterization, and considers the nonlinear relation between the energy level of DOS and the gate voltage <i>V</i> _GS. In addition, it is a simple, fast, and accurate extraction method for DOS in amorphous InGaZnO TFTs without optical illumination, temperature dependence, and numerical iteration.