A bipolar junction semiconductor device and associated method of manufacturing, the bipolar junction semiconductor device has a P type substrate, a N type buried layer formed in the substrate, a P− type first epitaxial layer formed on the buried layer, a P− type second epitaxial layer formed on the first epitaxial layer, a PNP BJT unit formed in the first and second epitaxial layers at a first active area, a NPN BJT unit formed in the first and second epitaxial layers at a second active area and a first isolation structure of N type formed in the first and second epitaxial layers at an isolation area. The isolation area is located between the first active area and the second active area, the first isolation structure connected with the buried layer forms an isolation barrier.