The invention provides a semiconductor device and a forming method. The semiconductor device comprises a semiconductor substrate, a dielectric layer and a conductive plug, wherein the dielectric layeris located on the semiconductor substrate. The dielectric layer comprises a dopant, and the total doping concentration of the dopant in the dielectric layer is gradually reduced in the direction faraway from the surface of the semiconductor substrate. A through hole for exposing the conductive structure is formed in the dielectric layer, and the conductive plug is disposed in the through hole. The total doping concentration of the dopant in the dielectric layer of the semiconductor device provided by the invention is gradually reduced in the direction far away from the semiconductor substrate; due to the fact that the dielectric layer with the large doping concentration can be etched and removed more easily, when the dielectric layer is etched to form the through hole, the etching rate cannot be obviously reduced along with the increase of the etching depth, the bottom appearance of the through hole can be improved, the through hole meeting the requirement is formed, and then the performance of the semiconductor device is improved.