Probing the DOS Band Tail in Amorphous Thin-Film Transistors via Capacitance–Voltage Analysis

Charge carrier transport in disordered semiconductors is critically influenced by the shape of the band tail in the density of states (DOS). To minimize energetic disorder and suppress band tails, deposition processes and post-treatment methods of semiconducting thin films must be carefully optimized. While capacitance–voltage (CV) measurements are routinely employed to extract doping densities and flatband voltages, no standardized procedure currently exists to quantitatively determine the DOS from such measurements. In this work, we address this gap by introducing a novel method to extract quantitative DOS information from CV data. Our approach relies on an analytical solution for charge accumulation in an exponential DOS distribution. We apply the method to Indium Gallium Zinc Oxide (IGZO) thin-film transistors and systematically investigate how measurement frequency and channel geometry affect the results. Comparison with alternative optical and electrical techniques confirms that CV measurements can provide reliable and straightforward access to DOS parameters, provided that the transistor channel dimensions exceed L × W = 20 µm × 100 µm. Additionally, CV measurements offer practical advantages, as they are fully compatible with standard transistor architectures, including encapsulation and light shielding commonly used in technological applications.