Methods and structures for forming a localized silicon-on-insulator (SOI) finFET are disclosed. Fins are formed on a bulk substrate. Nitride spacers protect the fin sidewalls. A shallow trench isolation region is deposited over the fins. An oxidation process causes oxygen to diffuse through the shallow trench isolation region and into the underlying silicon. The oxygen reacts with the silicon to form oxide, which provides electrical isolation for the fins. The shallow trench isolation region is in direct physical contact with the fins and/or the nitride spacers that are disposed on the fins. Structures comprising bulk-type fins, SOI-type fins, and planar regions are also disclosed.