Image sensors including a gate electrode surrounding a floating diffusion region

1. An image sensor comprising:

first and second stacked impurity regions having different conductivity types;

a floating diffusion region in the first impurity region; and

a transfer gate electrode surrounding the floating diffusion region in the first impurity region and not extending through the first impurity region, the transfer gate electrode and the floating diffusion region overlapping the second impurity region.

2. The image sensor of claim 1 , wherein the transfer gate electrode has a closed-loop shape around the floating diffusion region.

3. The image sensor of claim 1 , wherein the transfer gate electrode is substantially centered with respect to the second impurity region in a plan view.

4. The image sensor of claim 3 , wherein:

the first and second impurity regions provide a photoelectric conversion device; and

first and second distances from the transfer gate electrode to respective first and second edges of the photoelectric conversion device are substantially equal.

5. The image sensor of claim 1 , wherein:

the first and second impurity regions share a boundary therebetween; and

the transfer gate electrode is spaced apart from the boundary that is between the first and second impurity regions.

6. The image sensor of claim 1 , wherein:

the first and second impurity regions provide a photoelectric conversion device;

the transfer gate electrode extends primarily vertically; and

the transfer gate electrode is substantially centered with respect to the photoelectric conversion device in a plan view.

7. The image sensor of claim 1 , further comprising a local impurity region adjacent the transfer gate electrode,

wherein the local impurity region has the same conductivity type as the first impurity region, and

wherein the local impurity region has an impurity concentration higher than that of the first impurity region.

8. The image sensor of claim 1 , wherein:

the first impurity region further comprises first and second layers having different impurity concentrations;

the floating diffusion region is in the first layer; and

the transfer gate electrode is in the first and second layers.

9. The image sensor of claim 1 , wherein a portion of the first impurity region is configured to provide a channel region of a charge transmission element.

10. The image sensor of claim 1 , wherein the first impurity region comprises:

a surface impurity region; and

an electric potential barrier region between the surface impurity region and the second impurity region,

wherein the surface impurity region has an impurity concentration higher than that of the electric potential barrier region.

11. An image sensor comprising:

a semiconductor layer having a first surface and a second surface that are opposite each other;

a first impurity region of a first conductivity type formed in the semiconductor layer, the first impurity region adjacent to the first surface;

a second impurity region of a second conductivity type formed in the semiconductor layer, the second impurity region adjacent to the second surface;

a floating diffusion region in the first impurity region; and

a transfer gate electrode surrounding the floating diffusion region in the first impurity region,

wherein the transfer gate electrode is overlapped with the second impurity region, in a plan view.

12. The image sensor of claim 11 , wherein the transfer gate electrode is spaced apart from the second surface of the semiconductor layer.

13. The image sensor of claim 11 , wherein the transfer gate electrode has an opening exposing a portion of the first impurity region, and the floating diffusion region is formed in the portion of the first impurity region to be self-aligned with the opening of the transfer gate electrode.

14. The image sensor of claim 11 , further comprising a gate insulating layer conformably covering a surface of the transfer gate electrode,

wherein the floating diffusion region is in contact with a portion of the gate insulating layer.

15. The image sensor of claim 11 , further comprising an isolation layer formed in the semiconductor layer to define a first active region and a second active region,

wherein the transfer gate electrode is disposed on the first active region, and the first impurity region is in contact with the isolation layer.

16. The image sensor of claim 15 , further comprising at least one reading device disposed on the second active region, the reading device electrically connected to the transfer gate electrode and the floating diffusion region.