Image sensor with transfer gate having multiple channel sub-regions

A method of fabricating a complementary metal-oxide-semiconductor (“CMOS”) image sensor pixel, the method comprising:

• forming a transfer transistor channel region within an epitaxial layer disposed over a substrate, wherein the transfer transistor channel region includes a first channel sub-region having a first doping concentration and a second channel sub-region having a second doping concentration different from the first doping concentration;
• fabricating a transfer transistor gate over at least a portion of the transfer transistor channel region and over a portion of a floating diffusion region;
• and
• forming a photosensitive element within the epitaxial layer, wherein the first channel sub-region extends over at least a portion of the photosensitive element and beneath a portion of the transfer transistor gate.

An image sensor comprising:

• a complementary metal-oxide-semiconductor (“CMOS”) array of image sensor pixels disposed on a substrate, wherein each of the image sensor pixels includes:
• a photosensitive element;
• a floating diffusion region;
• and
• a transfer transistor channel region disposed between the photosensitive element and the floating diffusion region, wherein the transfer transistor channel region includes a first channel sub-region having a first doping concentration and a second channel sub-region having a second doping concentration different from the first doping concentration;
• and
• readout circuitry coupled to the CMOS array to readout image data from each of the image sensor pixels.