Semiconductor device and fabrication method are provided. The method includes: providing a base substrate; forming an isolation structure film on the base substrate, a top portion of the isolation structure film containing a plurality of first openings; forming a second opening at a bottom of each first opening by removing a portion of the isolation structure film to expose a surface of the base substrate, where the second opening has a size larger than a corresponding first opening along a direction in parallel with the surface of the base substrate; forming fins in the first and second openings; and forming an isolation structure by removing a portion of an isolation material film, where a top surface of the isolation structure is lower than a top surface of the fins and the isolation structure covers a portion of the sidewalls of the fins. The semiconductor devices formed by the method may reduce the self-heating effect and improve the performance of semiconductor devices.