Field effect transistors including variable width channels and methods of forming the same

A method of forming a field effect transistor comprising:

• forming a field insulation layer defining a first substrate region and a second substrate region spaced apart from each other in a substrate;
• forming a lightly doped region in the substrate;
• forming a gate on the first substrate region;
• and
• forming a heavily doped region in the second substrate region,
• wherein a channel region defined in the first substrate region under the gate comprises a first portion having a first width and a second portion having a second width in a channel width direction that is transverse to a direction from the first substrate region to the second substrate region, the first portion is interposed between the second substrate region and the second portion and is adjacent to the field insulation layer between the first and the second substrate regions, the first width is smaller than the second width, and the lightly doped region surrounds a sidewall and a bottom surface of the heavily doped region.

A field effect transistor comprising:

• a substrate;
• a field insulation layer in the substrate that is configured to define first and second substrate regions that are spaced apart from one another, the field insulation layer that defines the first substrate region having different widths at corresponding different lengths away from the second substrate region so that the first substrate region also has different widths at corresponding different lengths away from the second substrate region;
• a gate on at least a portion of the first substrate region that has different widths at corresponding different lengths away from the second substrate region;
• and
• a source/drain region in the second substrate region.