A semiconductor device is provided, comprising a substrate; a first well having a first conductive type and extending down from a surface of the substrate; a diffusion region doped with impurity of the first conductive type and extending down from a surface of the first well; and a plurality of active devices formed within the diffusion region, and the active devices arranged separately from each other. The active devices are electrically isolated from each other by the diffusion region. The active device is self-isolated by a conductive guarding structure, and the semiconductor device comprising embodied STI-free active devices solves STI edge issues.