Semiconductor fabrication process

A local oxidation of silicon (LOCOS) in a region 30 of SOI substrate 4 partially oxidises 34 the silicon first epitaxial layer 10. Etching the SOI substrate 4 in region 30 creates a trench 16 through the buried oxide (BOX) layer 8 to the bulk silicon substrate 6. A second epitaxial layer 40 is formed on the bulk silicon substrate 6 in the trench 16. Semiconductor devices are formed in the first & second epitaxial layers 10 & 40. The devices may form a BiCMOS where a SiGe or SiGeC BJT such as a HBT is in the second epitaxial silicon layer 40 and a CMOS is in the first epitaxial silicon layer 10. Further lateral isolation may be provided by shallow trench isolation (STI) 50 & 52.