The electrical properties of amorphous indium-gallium-zinc oxide thin film transistors are measured in the temperature range from 70 to 300 K. The device shows normal enhancement mode operation with significantly reduced drain current at low temperature. Its turn-on voltage and subthreshold swing decrease as temperature increases. The transport mechanisms of channel electrons are analyzed based on the evolution of field-effect mobility and channel conductance as a function of temperature and gate bias. It is suggested that in low temperature range, the dominant carrier transport mechanism is hopping between localized band-tail states. As temperature increases, multiple trapping and release plays a role in the whole carrier transport process. Meanwhile, in high gate bias range when the Fermi level moves above the mobility edge, band transport starts to dominate. (C) 2013 Elsevier Ltd. All rights reserved.