A backside illuminated image sensor comprises a photodiode and a first transistor located in a first chip, wherein the first transistor is electrically coupled to the photodiode. The backside illuminated image sensor further comprises a second transistor formed in a second chip and a plurality of logic circuits formed in a third chip, wherein the second chip is stacked on the first chip and the third chip is stacked on the second chip. The logic circuit, the second transistor and the first transistor are coupled to each other through a plurality of boding pads and through vias.