A semiconductor device adopts an isolation scheme to protect low voltage transistors from high voltage operations. The semiconductor device includes a substrate, a buried layer, a transistor well region, a first trench, and a second trench. The substrate has a top surface and a bottom surface. The buried layer is positioned within the substrate, and the transistor well region is positioned above the buried layer. The first trench extends from the top surface to penetrate the buried layer, and the first trench has a first trench depth. The second trench extending from the top surface to penetrate the buried layer. The second trench is interposed between the first trench and the transistor well region. The second trench has a second trench depth that is less than the first trench depth.