(EN) A field effect transistor has: a gate electrode for applying a gate voltage; a source electrode and a drain electrode for transmitting electrical signals; an active layer formed between the source electrode and the drain electrode; and a gate insulating layer formed between the gate electrode and the active layer, the field effect transistor being characterized in that the active layer includes at least two types of oxide layer, an A layer and a B layer; and the active layer satisfies condition (1) and/or (2) noted below. Condition (1): The active layer is composed of three or more oxide layers, including two or more of the A layer. Condition (2): The bandgap of the A layer is smaller than the bandgap of the B layer, and the oxygen affinity of the A layer is equal to or greater than the oxygen affinity of the B layer.
