This letter studies the hot-carrier effect in indium-gallium-zinc oxide (IGZO) thin film transistors with symmetric and asymmetric source/drain structures. The different degradation behaviors after hot-carrier stress in symmetric and asymmetric source/drain devices indicate that different mechanisms dominate the degradation. Since the C-V measurement is highly sensitive to trap states compared to the I-V characterization, C-V curves are utilized to analyze the hot-carrier stress-induced trap state generation. Furthermore, the asymmetric C-V measurements C-GD (gate-to-drain capacitance) and C-GS (gate-to-source capacitance) are used to analyze the trap state in channel location. The asymmetric source/drain structure under hot-carrier stress induces an asymmetric electrical field and causes different degradation behaviors. In this work, the on-current and subthreshold swing (S.S.) degrade under low electrical field, whereas an apparent V-t shift occurs under large electrical field. The different degradation behaviors indicate that trap states are generated under a low electrical field and the channel-hot-electron (CHE) effect occurs under a large electrical field. (c) 2012 Elsevier B.V. All rights reserved.