  We investigated the microscopic and electronic structures amorphous oxide
semiconductors InGaZnO$_{4}$ (a-IGZO) and the role of O-deficiency through the
first-principle calculations. The structure of the amorphous oxide is
complicated by the admixture of many different kinds of substructures, however
it is surprisingly found that the band tail states, which are well-known to be
present in the amorphous semiconductors, are few generated for the conduction
band minimum (CBM). The electronic structure around CBM is little affected by
the disorder and also by the O-deficiency. Free electron carriers can be
generated without a creation of donor-level in the O-deficient amorphous oxide.
