Transistor, method for manufacturing transistor, semiconductor device, and electronic device

A method for manufacturing a transistor comprising the steps of:

• forming a second oxide layer over a first oxide layer;
• processing the first oxide layer and the second oxide layer into an island shape;
• forming a third oxide layer to cover the second oxide layer;
• forming a first insulating layer to cover the third oxide layer;
• forming a first electrode over the first insulating layer;
• removing part of the third oxide layer and part of the first insulating layer using the first electrode as a mask to expose part of the second oxide layer;
• introducing elements to the part of the second oxide layer;
• forming a second insulating layer;
• processing the second insulating layer to form a structure body covering a side surface of the first electrode;
• forming a second electrode and a third electrode to be in contact with the exposed region of the second oxide layer;
• forming a third insulating layer to cover the second electrode and the third electrode;
• introducing oxygen to the third insulating layer when a fourth insulating layer is formed to cover the third insulating layer;
• and
• performing a heat treatment after the step of introducing oxygen to the third insulating layer when the fourth insulating layer is formed to cover the third insulating layer,
• wherein the second oxide layer is an oxide semiconductor.