Photodiode with different electric potential regions for image sensors
An image sensor pixel is disclosed. The pixel may include a photodiode having a first region with a first potential and a second region with a second, higher potential, with the second region being offset in depth from the first region in a semiconductor chip. A storage node may be positioned at substantially the same depth as the second region of the photodiode. A storage gate may be operable to transfer charge between the photodiode and the storage node.
1. An image sensor pixel comprising:
a photosensitive region formed along a surface of a semiconductor chip;
a storage node below and at least partially overlapping with the photosensitive region; and
a second region configured to transfer charge from the photosensitive region to the storage node;
wherein:
the photosensitive region is doped with a first concentration of a dopant; and
the second region is doped with a second concentration of the dopant that is greater than the first concentration.
2. The image sensor pixel of claim 1 , wherein the storage node is positioned entirely beneath the photosensitive region.
3. The image sensor pixel of claim 1 , further comprising a floating diffusion node configured to receive a charge from the storage node.
4. The image sensor pixel of claim 3 , wherein:
the floating diffusion node is coupled to the storage node by a transfer gate that is configured to reset the storage node when activated; and
the storage node is coupled to the photosensitive region by a storage gate operable to selectively transfer charge from the photosensitive region to the storage node.
5. The image sensor pixel of claim 3 , wherein:
the surface is a first surface of the semiconductor chip;
the storage node is formed along a second surface of the semiconductor chip opposite to the first surface; and
the floating diffusion node is positioned proximate to the second surface of the semiconductor chip.
6. The image sensor pixel of claim 3 , wherein the floating diffusion node is positioned on a separate chip that is distinct from the semiconductor chip.
7. The image sensor pixel of claim 1 , further comprising a shielding region configured between the photosensitive region and the storage node.
8. An image sensor comprising:
a photodiode having a photosensitive first region formed along a first surface of a semiconductor chip and a second region formed below a portion of the first region and extending to proximate a second surface opposite to the first surface of the semiconductor chip;
a storage node formed in the semiconductor chip at a depth below and at least partially overlapping with the first region; and
a storage gate coupled to a second surface of the semiconductor chip opposite to the first surface and configured to pump a charge from the second region to the storage node;
wherein:
the first region is doped with a first concentration of a dopant; and
the second region is doped with a second concentration of the dopant, the second concentration being greater than the first concentration.
9. The image sensor of claim 8 , wherein the storage gate is configured to selectively increase a potential of the second region in response to an applied voltage.
10. The image sensor of claim 8 , wherein a first inherent potential of the first region is approximately equal to a second inherent potential of the second region.
11. The image sensor of claim 8 , wherein the storage node is bifurcated into two regions having different levels of doping.
12. The image sensor of claim 8 , wherein the surface is a first surface of the semiconductor chip; and
the storage node is pinned to the second surface.
13. The image sensor of claim 12 , wherein:
pinning the storage gate creates a virtual barrier for charge entering from the storage gate until an applied voltage to the storage gate is reduced.
14. The image sensor of claim 8 , wherein the second region is not pinned to the second surface of the semiconductor chip.
15. The image sensor of claim 8 , wherein:
the storage node is a first storage node;
the depth of the first storage node is a first depth;
the storage gate is first storage gate; and
the image sensor further comprises:
a second storage node formed in the semiconductor chip at a second depth below and at least partially overlapping with the first region; and
a second storage gate positioned on the second surface of the semiconductor chip and configured to pump a charge from the second region to the second storage node.
16. The image sensor of claim 15 , wherein the second storage node is located opposite to the first storage node with respect to the second region.
17. An image pixel comprising:
a photodiode having a photosensitive first region positioned along a first surface of a semiconductor chip and a second region positioned below a portion of the first region; and
a storage node positioned proximate to a second surface of the semiconductor chip opposite to the first surface and at least partially overlapping the first region of the photodiode,
wherein the storage node is bifurcated into a first region having a first doping level and a second region having a second doping level that is different than the first doping level.
18. The image pixel of claim 17 , wherein the storage node is coupled to the photodiode by a storage gate that is configured to pump a charge from the photodiode to the storage node in response to an applied voltage.
19. The image pixel of claim 17 , wherein the bifurcated storage node is pinned to the second surface and forms a virtual barrier that resists charge from entering the storage node until the applied voltage is reduced.
20. The image pixel of claim 17 , further comprising:
a floating diffusion node that is configured to receive a charge from the storage node; and
a transfer gate coupling the storage node to the floating diffusion node and configured to reset the storage mode when activated.