Transistor, clocked inverter circuit, sequential circuit, and semiconductor device including sequential circuit

A semiconductor device comprising:

• a first gate electrode over a substrate;
• a first insulating layer over the first gate electrode;
• a first metal oxide layer including a channel formation region over the first insulating layer;
• a second metal oxide layer over the first metal oxide layer;
• a source electrode and a drain electrode, each of the source electrode and the drain electrode comprising a first conductive layer and a second conductive layer over the first conductive layer,
• a second insulating layer over the second metal oxide layer, the source electrode, and the drain electrode;
• and
• a second gate electrode over the second insulating layer,
• wherein the first gate electrode overlaps with the first metal oxide layer,
• wherein the second gate electrode is in contact with the first gate electrode in at least one opening in the first insulating layer and the second insulating layer,
• wherein the first conductive layer of the source electrode is in contact with a first side surface of the first metal oxide layer;
• wherein the first conductive layer of the drain electrode is in contact with a second side surface of the first metal oxide layer;
• and
• wherein a third surface of the first metal oxide layer faces the second gate electrode with the second insulating layer interposed therebetween and without the source electrode and the drain electrode interposed therebetween.