The deep well 211 is formed in the semiconductor layer 21′, and has a first conductivity type. The deep well 211 can be formed by, for example but not limited to, an ion implantation process step which implants first conductivity type impurities into the semiconductor layer 21′ in the form of accelerated ions, to form the deep well 211. The buried layer 212 is formed below the deep well 211 in the semiconductor layer 21′, and has a second conductivity type. The buried layer 212 encompasses the lower boundary of the deep well 211, and is in contact with the deep well 211. The buried layer 212 can be formed by, for example but not limited to, anion implantation process step which implants second conductivity type impurities into the substrate 21 and/or the semiconductor layer 21′ in the form of accelerated ions, to form the buried layer 212. For example, when the semiconductor layer 21′ is an epitaxial layer formed on the substrate 21, the second conductivity type impurities are implanted into the substrate 21 in the form of accelerated ions, and thereafter the epitaxial layer is formed on the substrate 21 by an epitaxial growth process step to form the semiconductor layer 21′, and then the buried layer 212 is formed at or around an interface between the substrate 21 and the semiconductor layer 21′ by a thermal process step, wherein part of the second conductivity type impurities diffuse from the substrate 21 to the semiconductor layer 21′ in the thermal process step to form the buried layer 212.