A semiconductor device and its manufacturing method, relating to semiconductor techniques. The semiconductor device manufacturing method comprises: forming a patterned first hard mask layer on a substrate to define a position for buried layers; conducting a first ion implantation using the first hard mask layer as a mask to form a first buried layer and a second buried layer both having a first conductive type and separated from each other at two sides of the first hard mask layer in the substrate; conducting a second ion implantation to form a separation region with a second conductive type opposite to the first conductive type in the substrate between the first and the second buried layers; removing the first hard mask layer; and forming a semiconductor layer on the substrate. This inventive concept reduces an area budget of a substrate and simplifies the manufacturing process.