A method and structure of improving the robustness of an electrostatic discharge (ESD) protection device is disclosed. One aspect of the instant disclosure provides a semiconductor structure that comprises: a first well structure; a second well structure arranged adjacent to the isolation structure in the substrate, a diffusion region respectively disposed in the first and the second well structures; an isolation structure arranged between the well structures and laterally separating the diffusion regions; and a partition structure arranged in the isolation structure. The partition structure affects a steeper slope on a lateral surface of the isolation structure bordering at least one of the diffusion regions, thereby modifying a ballasting characteristic of the isolation structure.