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 ($N_{\rm TA} = \hbox{2} \times \hbox{10}^{18}\ \hbox{eV}^{-1} \cdot \hbox{cm}^{-3}$, $N_{\rm DA} = \hbox{4} \times \hbox{10}^{15}\ \hbox{eV}^{-1} \cdot \hbox{cm}^{-3}$, $kT_{\rm TA} = \hbox{0.085} \ \hbox{eV}$, $kT_{\rm DA} = \hbox{0.5}\ \hbox{eV}$ , $E_{O} = \hbox{1}\ \hbox{eV}$). 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.