An image sensor includes a substrate having a pixel region and a periphery region. The image sensor further includes a first isolation structure formed in the pixel region; the first isolation structure including a first trench having a first depth. The image sensor further includes a second isolation structure formed in the periphery region; the second isolation structure including a second trench having a second depth greater than the first depth. The pixel region includes only NMOS devices and the periphery region includes both NMOS and PMOS devices.