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A technique for extracting the acceptorlike density of states (DOS) of n-channel amorphous GaInZnO (a-GIZO) thin-film transistors based on the combination of subbandgap optical charge pumping and C-V characteristics is proposed. While the energy level is scanned by the photon energy and the gate voltage sweep, its density is extracted from the optical response of C-V characteristics. The extracted DOS shows the superposition of the exponential tail states and the Gaussian deep states (N TA=2times1018 eV-1ldrcm-3, N DA=4times1015 eV-1ldrcm-3, kT TA=0.085 eV, kT DA=0.5 eV , E O=1 eV). The TCAD simulation results incorporated by the extracted DOS show good agreements with the measured transfer and output characteristics of a-GIZO thin-film transistors with a single set of process-controlled parameters.