Semiconductor fabrication process

A method of making a semiconductor structure, the method comprising:

• providing a silicon on insulator (SOI) substrate comprising a first epitaxial layer and a bulk silicon substrate separated by a buried oxide layer;
• performing one or more local oxidation of silicon (LOCOS) processes in a region of said SOI substrate to oxidize said first epitaxial silicon layer in said region and thereby form a second oxide layer on said buried oxide layer;
• locally etching the SOI substrate in said region to create a trench through said second oxide layer and said buried oxide layer and to said bulk silicon substrate;
• forming a second epitaxial layer on said bulk silicon substrate in said trench wherein said second epitaxial layer is isolated from said first epitaxial silicon layer by said second oxide layer;
• and
• forming one or more semiconductor devices in said first and second epitaxial layers.