Method for manufacturing semiconductor device

A semiconductor device comprising:

• an oxide semiconductor layer comprising a channel formation region over an insulating surface;
• a source electrode and a drain electrode connected to the oxide semiconductor layer;
• a first layer over and in contact with the source electrode, the drain electrode, and the channel formation region;
• a second layer over the first layer;
• and
• a wiring over the second layer;
• wherein the first layer comprises gallium and oxygen,
• wherein the wiring is connected to the source electrode or the drain electrode through a contact hole in the first layer and the second layer, and
• wherein a diameter of the contact hole in the first layer is smaller than a diameter of the contact hole in the second layer.

A semiconductor device comprising:

• an oxide semiconductor layer comprising a channel formation region over an insulating surface, the oxide semiconductor layer comprising indium and zinc;
• a source electrode and a drain electrode connected to the oxide semiconductor layer;
• a first layer over and in contact with the source electrode, the drain electrode, and the channel formation region;
• a gate insulating layer over the first layer;
• a gate electrode over the gate insulating layer;
• a second layer over the gate electrode;
• and
• a wiring over the second layer;
• wherein the first layer comprises gallium and oxygen, and
• wherein the wiring is connected to the source electrode or the drain electrode through a contact hole in the first layer and the second layer.