Bulk semiconductor substrates configured to exhibit semiconductor-on-insulator (SOI) behavior, and corresponding methods of fabrication, are disclosed herein. An exemplary bulk substrate configured to exhibit SOI behavior includes a first isolation trench that defines a channel region of the bulk substrate and a second isolation trench that defines an active region that includes the channel region. The first isolation trench includes a first isolation trench portion and a second isolation trench portion disposed over the first isolation trench portion. A first isolation material fills the first isolation trench portion, and an epitaxial material fills the second isolation trench portion. The epitaxial material is disposed on the first isolation material. A second isolation material fills the second isolation trench. A portion of the bulk substrate underlying the first isolation trench and the channel region is configured to have a higher resistance than the bulk substrate.