The current conduction mechanism in radio frequency sputtered amorphous indium gallium zinc oxide (a-IGZO) films was investigated using model devices designed to mimic the carrier injection from an electrode to an a-IGZO channel in thin-film transistors. Interface-limited mechanisms, such as thermionic emission and Fowler-Nordheim tunneling, clearly fail to fit the current-voltage (I-V) curves. Instead, the I-V characteristics of the a-IGZO devices fit well within the framework of space-charge-limited current, whereas the conduction is enhanced by the Frenkel effect at high field (>0.1 MV/cm). (c) 2007 The Electrochemical Society.