The electrical and optical properties of direct current and radio frequency (RF) sputtered amorphous indium gallium zinc oxide (a-IGZO) films are compared. It is found that the RF sputtered a-IGZO films have better stoichiometry (In:Ga:Zn: O = 1:1:1:2.5-3.0), lower electrical conductivity (sigma<8 S/cm), higher refractive index (n = 1.9-2.0) and larger band gap (E-g = 3.02-3.29 eV), and show less shift of Fermi level (Delta E-F similar to 0.26 eV) and increased concentration of electrons (Delta N-e similar to 10(4)) in the conduction band with the reduction concentration of oxygen vacancy (V-O). Although a-IGZO has intensively been studied for a semiconductor channel material of thin film transistors in next-generation flat panel displays, its fundamental material parameters have not been thoroughly reported. In this work, the work function (phi) of a-IGZO films is tested with the ultraviolet photoelectron spectroscopy. It is found that the phi of a-IGZO films is in the range of 4.0-5.0 eV depending on the V-O. (C) 2012 Elsevier B. V. All rights reserved.