Source follower device for enhanced image sensor performance

An image sensor, comprising:

• a photodiode disposed in a semiconductor material to generate image charge in response to incident light;
• a floating diffusion disposed in the semiconductor material proximate to the photodiode;
• a transfer transistor coupled to the photodiode to transfer the image charge from the photodiode into the floating diffusion in response to a transfer signal applied to a transfer gate of the transfer transistor;
• and
• a source follower transistor coupled to the floating diffusion to amplify a charge on the floating diffusion, the source follower transistor, including:
• a gate electrode including a semiconductor material having a first dopant type;
• a source electrode, having a second dopant type, disposed in the semiconductor material:
• a drain electrode, having the second dopant type, disposed in the semiconductor material;
• a channel, having the second dopant type, disposed between the source electrode and the drain electrode, wherein the source electrode and the drain electrode have a higher dopant concentration than the channel;
• and
• a charged layer having the first dopant type disposed in the semiconductor material, wherein the channel is disposed between the charged layer and the gate electrode;
• wherein the charged layer includes a first layer of dopant and a second layer of dopant in-plane with the first layer of dopant, wherein the first layer of dopant and the second layer of dopant are separated by a portion of the semiconductor material.

A method of fabricating an image sensor, comprising:

• implanting a photodiode in a semiconductor material to generate image charge in response to incident light;
• implanting a floating diffusion in the semiconductor material proximate to the photodiode;
• forming a transfer transistor coupled to the photodiode to transfer the image charge from the photodiode into the floating diffusion;
• and
• forming a source follower transistor coupled to the floating diffusion, including:
• forming a gate electrode including a semiconductor material having a first dopant type;
• implanting a source electrode, having a second dopant type, in the semiconductor material;
• implanting a drain electrode, having the second dopant type, in the semiconductor material;
• forming a channel, having the second dopant type, between the source electrode and the drain electrode, wherein the source electrode and the drain electrode have a higher dopant concentration than the channel;
• and
• forming a charged layer having the first dopant type in the semiconductor material proximate to the channel wherein the channel is disposed between the charged layer and the gate electrode, wherein forming the charged layer includes implanting a first layer of dopant in the semiconductor material that is substantially parallel to the channel, and implanting a second layer of dopant in-plane with the first layer of dopant, wherein the first layer of dopant and the second layer of dopant are separated by a portion of the semiconductor material.