A metal grid within a trench isolation structure on the back side of an image sensor is coupled to a contact pad so that a voltage on the metal grid is continuously variable with a voltage on the contact pad. One or more conductive structures directly couple the metal grid to a contact pad. The conductive structures may bypass a front side of the image sensor. A bias voltage on the metal grid may be varied through the contact pad whereby a trade-off between reducing cross-talk and increasing quantum efficiency may be adjusted dynamically in accordance with the application of the image sensor, its environment of use, or its mode of operation.