Semiconductor device

A method, comprising:

• etching a substrate to form a core structure protruding out of a plane of the substrate;
• forming shallow trench isolation (STI) features on opposite sides of the core structure;
• doping the substrate and a lower portion of the core structure to form a first source/drain region;
• epitaxially growing a barrier layer on a top surface of an upper portion of the core structure, wherein the barrier layer is made of a semiconductor material and is in contact with the top surface the upper portion of the core structure;
• forming a shell wrapping the upper portion of the core structure and the barrier layer;
• and
• forming a gate around the shell.

A method, comprising:

• etching a substrate to form a core structure protruding out of a plane of the substrate;
• forming shallow trench isolation (STI) features on opposite sides of the core structure;
• doping the substrate and a lower portion of the core structure to form a first source/drain region;
• growing a barrier layer over an upper portion of the core structure;
• forming a shell wrapping the upper portion of the core structure and the barrier layer;
• and
• forming a second source/drain region over a top of the shell, wherein the second source/drain region and the first source/drain region have different conductivity types.