Capacitance-voltage (C-V) characteristics of amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) thin film transistors (TFTs) are comparatively investigated with two different measurement configurations. Normal gate-to-source/drain (S/D) C-V and quasi-static C-V curves are employed to characterize physical mechanisms with equivalent circuit models for a-IGZO TFTs. The difference between the normal C-V and the quasi-static C-V (QSCV) characteristics is investigated by the dependence on the gate voltage (V-G), measurement configuration, and optical illumination. The discrepancy is analyzed to be due to a high hole barrier in the S/D contact region and a slow response of active bulk charges (Q(loc) and Q(free)) in the a-IGZO active layer. (C) 2010 Elsevier Ltd. All rights reserved.