The present invention relates to an amorphous oxide and a thin film transistor using the amorphous oxide. In particular, the present invention provides an amorphous oxide having an electron carrier concentration less than 10.sup.18/cm.sup.3, and a thin film transistor using such an amorphous oxide. In a thin film transistor having a source electrode 6, a drain electrode 5, a gate electrode 4, a gate insulating film 3, and a channel layer 2, an amorphous oxide having an electron carrier concentration less than 10.sup.18/cm.sup.3 is used in the channel layer 2.