Characterization and Modeling of a-IGZO TFTs

In this paper, we present a systematic approach to the characterization and modeling of amorphous Indium Gallium Zinc Oxide (a-IGZO) thin-film transistors (TFTs), where the key parameters are determined from the analysis of both I- V and C- V characteristics, in a step-by-step fashion, without complex interdependences that may affect the accuracy of the results. Flat band voltage VFB and carrier concentration nFB are extracted by a method we have previously developed, validated here by applying it to simulated data. Next, the density of deep gap states is extracted, followed by the determination, by a new method, of the shallow donor concentration. The tail states parameters are determined last, by matching the calculated nFB to the experimental one. Simulations are then performed without any adjustable parameters. The approach is applied to the study of device to device variations, indicating that the material is strongly compensated. As for the analysis of Negative Bias under Illumination Stress (NBIS), this work confirms that the effect is due to creation of a double donor, with a shallow level close to the conduction band (positive correlation energy). Oxygen vacancies are the likely candidates. These defects are not detected in unstressed devices, where the characteristics can be accurately simulated by incorporating donors with a single shallow level.