The deep well 411 is formed in the semiconductor layer 41′, and has a first conductivity type. The deep well 411 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 41′ in the form of accelerated ions, to form the deep well 411. The buried layer 412 is formed below the deep well 411 in the semiconductor layer 41′, and has a second conductivity type. The buried layer 412 encompasses the lower boundary of the deep well 411, and is in contact with the deep well 411. The buried layer 412 can be formed by, for example but not limited to, anion implantation process step which implants second conductivity type impurities into the substrate 41 and/or the semiconductor layer 41′ in the form of accelerated ions, to form the buried layer 412. For example, when the semiconductor layer 41′ is an epitaxial layer formed on the substrate 41, the second conductivity type impurities are implanted into the substrate 41 in the form of accelerated ions, and thereafter the epitaxial layer is formed on the substrate 41 by an epitaxial growth process step to form the semiconductor layer 41′, and then the buried layer 412 is formed at or around an interface between the substrate 41 and the semiconductor layer 41′ by a thermal process step, wherein part of the second conductivity type impurities diffuse from the substrate 41 to the semiconductor layer 41′ in the thermal process step to form the buried layer 412.