Image sensors with vertically stacked photodiodes and vertical transfer gates

An image sensor comprising:

• a substrate;
• a n-type epitaxial layer formed on the substrate;
• a p-type layer formed over the n-type epitaxial layer;
• a deep n-well formed in the p-type layer over the n-type epitaxial layer;
• a floating diffusion region;
• and
• a charge transfer gate coupled between the deep n-well and the floating diffusion region.

An image sensor having a front surface and a back surface, the image sensor comprising:

• a substrate with first and second opposing surfaces, the first surface of the substrate forming the back surface of the image sensor;
• a n-type epitaxial layer adjacent to the substrate;
• a p-type layer formed adjacent to the n-type epitaxial layer, wherein the p-type layer has first and second opposing surfaces, and wherein the second surface of the p-type layer forms the front surface of the image sensor;
• an n-well formed in the p-type layer, wherein the n-well overlaps the n-type epitaxial layer;
• a floating diffusion region formed at the second surface of the p-type layer;
• and
• a charge transfer gate coupled between the n-well and the floating diffusion region, wherein the charge transfer gate is configured to transfer charge along the second surface of the p-type layer from the n-well to the floating diffusion region.

An image sensor comprising:

• a substrate of a first type;
• an epitaxial layer of a second type that is different than the first type formed on the substrate;
• a layer of the first type formed over the epitaxial layer;
• a region of the second type formed in the layer of the first type;
• a floating diffusion region;
• and
• a charge transfer gate coupled between the region of the second type and the floating diffusion region.