Field effect transistor using oxide semicondutor and method for manufacturing the same

A field effect transistor which comprises, on a substrate, at least a semiconductor layer, a passivation layer, a source electrode, a drain electrode, a gate insulating film and a gate electrode,

• the source electrode and the drain electrode being connected to each other through the semiconductor layer,
• the gate insulating film being present between the gate electrode and the semiconductor layer,
• the passivation layer being at least on one surface side of the semiconductor layer, and
• the semiconductor layer comprising a composite oxide which comprises In (indium), Zn (zinc) and Ga (gallium) in the following atomic ratios (1) to (3):
• In/(In+Zn)=0.2 to 0.8 (1)
• In/(In+Ga)=0.59 to 0.99 (2)
• Zn/(Ga+Zn)=0.29 to 0.99 (3),
• wherein the semiconductor layer is an amorphous film which has an energy width (E0) on the non-localized level of 14 meV or less.