Complementary metal-oxide-semiconductor image sensors

1. A complementary-metal-oxide-semiconductor (CMOS) image sensor comprising:

a substrate having a first surface and a second surface facing each other;

a first device isolation layer extending from the first surface to the second surface of the substrate and defining a first pixel region and a second pixel region;

a second device isolation layer disposed in the substrate in each of the first and second pixel regions and defining a first active portion and a second active portion in each of the first and second pixel regions, wherein the second active portion spaced apart from the first active portion;

first and second photoelectric conversion layers disposed in the substrate of the first and second pixel regions, respectively;

first and second transfer gates disposed on the first active portions of the first and second pixel regions, respectively;

a reset gate and a selection gate disposed on the second active portion of the first pixel region; and

a source follower gate disposed on the second active portion of the second pixel region,

wherein a length of the source follower gate is greater than a length of the reset gate or a length of the selection gate.

2. The CMOS image sensor of claim 1 , wherein the second device isolation layer is adjacent to the first surface of the substrate, and

wherein the first and second conversion layers are adjacent to the second surface of the substrate.

3. The CMOS image sensor of claim 1 , wherein the first and second isolation layers comprise an insulating material.

4. The CMOS image sensor of claim 1 , wherein a vertical length of the second device isolation layer is shorter than a vertical length of the first device isolation layer.

5. The CMOS image sensor of claim 1 , wherein the first device isolation layer surrounds each of the first and second conversion layers.

6. The CMOS image sensor of claim 1 , wherein the first photoelectric conversion layer overlaps with the first and second active portions of the first pixel region when viewed from a plan view, and

wherein the second photoelectric conversion layer overlaps with the first and second active portions of the second pixel region when viewed from a plan view.

7. The CMOS image sensor of claim 1 , wherein the first active portions of the first and second pixel regions are disposed between the second active portions of the first and second pixel regions, in one direction.

8. The CMOS image sensor of claim 1 , further comprising at least one floating diffusion region disposed in at least one of the first active portions of the first and second pixel regions.

9. The CMOS image sensor of claim 1 , further comprising:

first and second floating diffusion regions disposed in the first active portions of the first and second pixel regions, respectively; and

a first interconnection conductor electrically connecting the first and second floating diffusion regions to the source follower gate.

10. The CMOS image sensor of claim 9 , further comprising:

a first dopant region disposed in the second active portion beside the selection gate in the first pixel region;

a second dopant region disposed in the second active portion beside the source follower gate in the second pixel region; and

a second interconnection conductor electrically connecting the first and second dopant regions.

11. The CMOS image sensor of claim 10 , further comprising:

first contact plugs electrically connecting the first dopant region to the second interconnection conductor; and

second contact plugs electrically connecting the second dopant region to the second interconnection conductor.