Photoresponse was investigated for an amorphous oxide semiconductor, In-Ga-Zn-O, by the steady-state photoconductivity (SSPC) method All the films exhibited extremely slow reversible photoresponses. Analysis of the transient photocurrent at varied temperatures provided similar activation energies of similar to 0.5 eV for both the time constants and the photoconductivity. Mobility-lifetime (mu tau) products were estimated from the photoconductivity spectra measured at the sweep rate of 2 nm/s, which monotonically increased with increasing dark conductivity sigma(D) (i e the Fermi level E(F) becomes shallower) The obtained mu tau values are larger than those of hydrogenated amorphous silicon even if the E(F) dependence is considered (C) 2009 Elsevier B.V. All rights reserved