Semiconductor device

A transistor including an oxide semiconductor and having favorable operation characteristics is provided. Further, by using the transistor, a semiconductor having improved operation characteristics can be provided. In planar view, one of a source electrode and a drain electrode of the transistor is surrounded by a ring-shaped gate electrode. Further, in planar view, one of the source electrode and the drain electrode of the transistor is surrounded by a channel formation region. Accordingly, the source electrode is not electrically connected to the drain electrode through a parasitic channel generated in an end portion of an island-shaped oxide semiconductor layer.