A device includes an image sensor chip having an image sensor therein. A read-out chip is underlying and bonded to the image sensor chip, wherein the read-out chip includes a logic device selected from the group consisting essentially of a reset transistor, a source follower, a row selector, and combinations thereof therein. The logic device and the image sensor are electrically coupled to each other, and are parts of a same pixel unit. A peripheral circuit chip is underlying and bonded to the read-out chip, wherein the peripheral circuit chip includes a logic circuit.