A semiconductor device and a fabrication method are provided. The fabrication method includes providing a base substrate including a first region for forming a first transistor and a second region for forming a second transistor, the first transistor having a working current less than the second transistor. The fabrication method further includes forming a gate electrode layer on the base substrate; etching the gate electrode layer to form a first gate electrode in the first region; after forming the first gate electrode, etching the gate electrode layer to form a second gate electrode in the second region, with the second gate electrode having an undercut structure; forming a first source/drain doped region in the base substrate on both sides of the first gate electrode and forming a second source/drain region in the base substrate on both sides of the second gate electrode.