A method for forming a semiconductor device is provided, wherein a step of forming an S/D region comprises: determining an interface region comprising an active region of a partial width abutting an isolation region, and forming an auxiliary layer covering the interface region; removing a semiconductor substrate of a partial thickness in the active region using the auxiliary layer, a gate stack structure and the isolation region as a mask, so as to form a groove; and growing a semiconductor material in the groove for filling into the groove. A semiconductor device having a material of the semiconductor substrate sandwiched between an S/D region and an isolation region is further provided. The present invention is beneficial to reduce current leakage.