Semiconductor device with impurity doped oxide semiconductor layer and method for manufacturing the semiconductor device thereof

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

• forming an oxide semiconductor layer;
• forming an oxide film comprising a region in contact with a side surface of the oxide semiconductor layer and a region in contact with a top surface of the oxide semiconductor layer;
• forming a first conductive layer over the oxide film;
• forming an insulating layer comprising a region in contact with a top surface of the first conductive layer, a region in contact with a side surface of the first conductive layer, and a region in contact with a top surface of the oxide film;
• performing heat treatment to supply oxygen to the oxide semiconductor layer from the oxide film;
• and
• performing an impurity element doping treatment to the oxide semiconductor layer through the oxide film before the formation of the insulating layer,
• wherein a channel formation region of the oxide semiconductor layer and the first conductive layer overlap with each other,
• wherein the impurity element is phosphorous, boron, magnesium, aluminum, or silicon, and
• wherein the impurity element is doped to achieve a region with a concentration distribution where a concentration of the impurity element in the oxide film becomes higher as the impurity element becomes closer to the oxide semiconductor layer.