Method of fabricating FinFETs

A method of making a fin field effect transistor (FinFET) comprising:

• forming a plurality of first trenches having a first width and a first depth in a substrate, each first trench of the plurality of first trenches extending downward from a surface of the substrate to a first height, wherein a first space between adjacent first trenches defines a first fin;
• forming a plurality of second trenches having a second width and a second depth in the substrate, the second width less than the first width, each second trench of the plurality of second trenches extending downward from the surface of the substrate to a second height greater than the first height, wherein a second space between adjacent second trenches defines a second fin;
• forming a first isolation structure partially filling each first trench of the plurality of first trenches;
• forming a second isolation structure partially filling each second trench of the plurality of second trenches, wherein a difference between a height of the first isolation structure and the first depth is substantially equal to a different between a height of the second isolation structure and the second depth;
• forming a first gate electrode over a top surface and sidewalls of the first fin;
• and
• forming a second gate electrode over a top surface and sidewalls of the second fin.