An image sensor includes a first substrate having a first transistor integrated therein, and a first plurality of wiring structures on the first substrate. The first plurality of wiring structures include a first wiring structure electrically connected to the first transistor. A second substrate extends on the first plurality of wiring structures, and has a second transistor integrated therein, which is electrically connected to a second wiring structure within the first plurality of wiring structures. A second plurality of wiring structures extend on the second substrate. A third substrate is provided on the second plurality of wiring structures. A microlens extends on a light receiving surface of the third substrate. A light sensing element extends within the third substrate. A transfer gate (TG) extends into a portion of the third substrate, extends adjacent the light sensing element, and is electrically connected to a first wiring structure within the second plurality of wiring structures. A floating diffusion (FD) region extends within the third substrate and adjacent the TG. The FD region is electrically connected to a second wiring structure within the second plurality of wiring structures.