Carrier transport properties and electronic structure of an n-type transparent oxide semiconductor, InGaO3(ZnO)5, were investigated using single-crystalline thin films. Room-temperature Hall mobility strongly depends on carrier concentration, and rapidly increased from ∼ 2 cm2(Vs)-1 to > 10 cm2(Vs)-1 around the carrier concentration (Nth ∼3 × 1018 cm−3. This change is associatedwith insulator-metal transition. These results are explained by a model similar to Anderson localization, in which shallow semi-localized states are formed originating from random distribution of Ga3+ and Zn2+ ions in the intrinsic crystal structure of InGaO3(ZnO)5. The present conclusion suggests that electron densities larger than Nth are necessary to attain high performances in drift carrier devices fabricated using InGaO3(ZnO)5. It was demonstrated that transparent filed-effect transistors exhibited good performances such as a “normally-offcharacteristics”, an on/off current ratios as large as 105 and a field-effect mobility ∼80 cm2(Vs)-1when high-k material, amorphous HfOx, was used as a gate insulator.