Semiconductor device and method for manufacturing semiconductor device

A method for manufacturing a semiconductor device, comprising the steps of:

• forming a first oxide layer;
• forming an oxide semiconductor layer over and in contact with the first oxide layer;
• forming a first insulating layer over the oxide semiconductor layer;
• forming a second insulating layer by etching the first insulating layer so that a first part of the oxide semiconductor layer is exposed;
• forming a first conductive layer over the first part of the oxide semiconductor layer and the second insulating layer;
• forming a second conductive layer by etching the first conductive layer so that part of the first part of the oxide semiconductor layer is exposed, wherein the second conductive layer is along and in contact with a side surface of the second insulating layer;
• removing the second insulating layer;
• etching the first oxide layer and the oxide semiconductor layer with the second conductive layer as a mask;
• forming a third insulating layer over the first oxide layer, the oxide semiconductor layer and the second conductive layer after etching the oxide semiconductor layer and the first oxide layer;
• forming a groove in the third insulating layer so that part of the second conductive layer is exposed;
• forming a source electrode layer and a drain electrode layer by removing the part of the second conductive layer;
• and
• forming a second oxide layer in contact with the oxide semiconductor layer, a gate insulating layer over the second oxide layer and a gate electrode layer over the gate insulating layer in the groove.