Carrier transport properties in amorphous oxide semiconductor InGaZnO4 (a-IGZO) thin films were investigated in detail using temperature dependence of Hall measurements. It was found that Hall mobility increased distinctly as carrier concentration increased. Unlikely conventional amorphous semiconductors such as a-Si/H, definite normal Hall voltage signals were observed on the films with carrier concentrations (Nc)> 10(16) cm(-3), and Hall mobilities as large as 15 cm(2) (Vs)(-1) were attained in the films with Ne > 10(20) cm(-3). When N, was less than 10 19 cm(-3), the temperature dependence of Hall mobility showed thermally-activated behavior in spite that carrier concentration was independent of temperature. While, it changed to almost degenerate conduction at N-c> 10(18) cm(-3). These behaviors are similar to those observed in single-crystal line IGZO, and are explained by percolation conduction through distributed potential barriers which are formed in the vicinity of the conduction band bottom due to the randomness of the amorphous structure. The effective mass of a-IGZO was estimated to be similar to 0.34 m(e) (m(e) is the mass of free electron) from optical data, which is almost the same as that of crystalline IGZO (-0.32 m(c)). (c) 2004 Published by Elsevier B.V.