A circuit structure includes a first isolation region, and a first dummy gate electrode over and vertically overlapping the first isolation region. First pickup regions of a diode are formed on opposite sides of the first isolation region, wherein sidewalls of the first pickup regions contact opposite sidewalls of the first isolation region. Second pickup regions of the diode are formed on opposite sides of a combined region of the first pickup regions and the first isolation region, wherein the first and the second pickup regions are of opposite conductive types. A well region is under the first and the second pickup regions and the first isolation region, wherein the well region is of a same conductivity type as the second pickup regions.