A semiconductor device including a substrate of a first conductivity type, a metal-oxide-semiconductor-field-effect transistor (MOSFET), junction gate field-effect transistors (JFETs), an isolation structure, and a buried layer of a second conductivity type is provided. The MOSFET is located on the substrate and has a first epitaxial layer of the second conductivity type. The JFET is located on the substrate and has a second epitaxial layer of the second conductivity type. The isolation structure is located between the MOSFET and the JFET to separate the first epitaxial layer from the second epitaxial layer. The buried layer is located between the MOSFET and the substrate. The buried layer extends from below the MOSFET to below the isolation structure and below the JFET, so as to electrically connect the MOSFET to the first JFET.