Dual gate trench mosfet with shielded gate electrode in step trench

A power device having two types of gate trenches in each unit cell comprising:

• an epitaxial layer of a first conductivity type grown on a substrate;
• a first body region of a second conductivity type having a first source region of said first conductivity type thereon;
• a first type gate trench penetrating said first body region and said first source region and stopping in said epitaxial layer;
• a first gate electrode disposed in said first type gate trench and surrounded with a first gate oxide along sidewalls of said first type gate trench;
• a first channel region of a first MOSFET formed in said first body region along sidewalls of said first type gate trench connecting to said first source region;
• a second type gate trench formed in said epitaxial layer has one-step gate trench structure comprising a second top gate trench and a second bottom gate trench, wherein said second top gate trench is above said second bottom gate trench and has a trench width less than that of said second bottom gate trench;
• and said second type gate trench has a trench depth deeper than that of said first type gate trench;
• and
• a first shielded gate electrode disposed in at least said second bottom gate trench surrounded with a first insulating film on a bottom region and along sidewalls of said second bottom gate trench;
• said first insulating film having a thickness greater than that of said first gate oxide;
• said first body and said first source regions, and said shielded gate electrode being shorted to a source metal through source contact trenches and a shielded gate contact trench respectively.