A diode includes an anode of a first conductivity type; a first cathode of the first conductivity type; and a second cathode of a second conductivity type opposite the first conductivity type. A lightly-doped region of the first conductivity type is under and vertically overlaps the anode and the first and the second cathodes. The portion of the lightly-doped region directly under the second cathode is fully depleted at a state when no bias voltage is applied between the anode and the second cathode.