Device structures, design structures, and fabrication methods for a silicon controlled rectifier. A well of a first conductivity type is formed in a device region, which may be defined from a device layer of a semiconductor-on-insulator substrate. A doped region of a second conductivity type is formed in the well. A cathode of a silicon controlled rectifier and a cathode of a diode are formed in the device region. The silicon controlled rectifier comprises a first portion of the well and an anode comprised of a first portion of the doped region. The diode comprises a second portion of the well and an anode comprised of a second portion of the doped region.