Semiconductor device and fabrication method thereof

A method of forming a semiconductor device, comprising:

• 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;
• 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, wherein the undercut structure has a stepped sidewall including a first step portion on the base substrate and a second step portion on the first step portion, wherein the first step portion has a smaller size than the second step portion in a direction parallel to a surface of the base substrate;
• and
• 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 doped region in the base substrate on both sides of the second gate electrode wherein:
• the base substrate includes a substrate and a plurality of fins on the substrate;
• the first gate electrode is formed across the fin in the first region, covering a portion of a top and sidewall of the fin in the first region;
• the first gate electrode is formed across the fin in the first region, covering a portion of a top and sidewall of the fin in the first region;
• the second gate electrode is formed across the fin in the second region, covering a portion of a top and sidewall of the fin in the second region;
• the first source/drain doped region is formed within the fin in the first region on both sides of the first gate electrode;
• and the second source/drain doped region is formed within the fin in the second region on both sides of the second gate electrode.