A method of making a semiconductor structure, the method comprising: - providing a silicon on insulator (SOI) substrate (4) comprising a first epitaxial layer (10) and a bulk silicon substrate (6) separated by a buried oxide layer (8); - performing a local oxidation of silicon (LOCOS) process in a region (30) of said silicon on insulator substrate (4) to at least partially oxidise said first epitaxial silicon layer (10) in said region (30); - locally etching the silicon on insulator substrate (4) in said region (30) to create a trench (16) through said buried oxide layer (8) and to said bulk silicon substrate (6); - forming a second epitaxial layer (40) on said bulk silicon substrate (6) in said trench (16); and - forming one or more semiconductor devices in said first and second epitaxial layers. A method according to claim 1, wherein said step of forming one or more semiconductor devices comprises forming a SiGe device in said second epitaxial layer (40). A method according to claim 1 or 2, wherein said step of forming one or more semiconductor devices comprises forming a bipolar junction transistor BJT in said second epitaxial layer (40). A method according to claim 1, 2 or 3, wherein said step of forming one or more semiconductor devices comprises forming one or more CMOS devices in said first epitaxial layer (10) located over said buried oxide layer (8). A method according to any one of the preceding claims, further comprising performing a second local oxidation of silicon process to fully oxidise said first epitaxial silicon layer (10) in said region. A method according to any one of the preceding claims, wherein the or each local oxidation of silicon process comprises one or more of the following steps in the following order: - depositing a nitride hard mask layer on said substrate; - depositing an oxide layer on said substrate; - depositing a photoresist on said substrate, and patterning said photoresist to define said region. A method according to any one of the preceding claims, wherein said local oxidation of silicon process partially oxidises said first epitaxial layer (10) in said region and thereby forms an oxide layer on said first epitaxial layer (10) in said region, the method further comprising etching said region to remove said oxide layer on said first epitaxial layer to expose said first epitaxial layer in said region. A method according to any one of the preceding claims, further comprising, after the or each local oxidation of silicon process, depositing a nitride hard mask layer and a photoresist on said substrate, and patterning said nitride hard mask and said photoresist to define a trench region in said region. A method according to claim 8, wherein said step of locally etching comprises performing a dry etch in said trench region (30) to etch through a part but not through the whole of said buried oxide layer (8). A method according to claim 9, wherein said step of locally etching comprises a wet etch to etch through a remaining part of said buried oxide layer in said trench region to expose said bulk silicon substrate (6). A method according to any one of the preceding claims, wherein said step of forming a second epitaxial layer (40) comprises forming said second epitaxial layer (40) to have a thickness greater than a combined thickness of said buried oxide layer (8) and said first epitaxial layer (10). A method according to any one of the preceding claims, further comprising, after forming said second epitaxial layer (40), performing chemical mechanical planarization CMP to level an upper surface of said second epitaxial layer. A method according to any one of the preceding claims, further comprising providing shallow trench isolation STI (50, 52) to provide lateral isolation at least between said first (10) and second (40) epitaxial layers. A method according to any one of the preceding claims, wherein said step of forming one or more semiconductor devices comprises selectively doping said first (10) and second (40) epitaxial layers. A semiconductor structure (2) formed by the method of any one of the preceding claims.