Structures and methods for trench isolation are disclosed. In one example, a silicon-on-insulator (SOI) structure is disclosed. The SOI structure includes: a substrate, a dielectric layer and a polysilicon region. The substrate includes: a handle layer, an insulation layer arranged over the handle layer, a buried layer arranged over the insulation layer, and a trench extending downward from an upper surface of the buried layer and terminating in the handle layer. The dielectric layer is located on a bottom surface of the trench and contacting the handle layer. The polysilicon region is located in the trench and contacting the dielectric layer.