An image sensor includes a first photodiode formed in a semiconductor substrate at a depth reachable by red light, a second photodiode disposed on or over the first photodiode in the semiconductor substrate at a depth reachable by blue light, a third photodiode disposed adjacent to the second photodiode, a plug connected to the first photodiode, transistor structures on the semiconductor substrate and electrically connected with the first, second and third diodes, an insulating layer covering the transistor structures, and microlenses on the insulating layer.