Cmos image sensor with asymmetric well structure of source follower

A method, comprising:

• disposing a well of a first conductivity type on a substrate of the first conductivity type;
• disposing a first impurity region of a drive transistor in the well;
• forming a first gate electrode of a second conductivity type for the drive transistor such that the first gate electrode is connected to the first impurity region;
• forming a second gate electrode of the second conductivity type for a select transistor such that the second gate electrode is formed in parallel to the first gate electrode;
• and
• disposing a second impurity region of the second conductivity type in the substrate such that the second impurity region is coupled to the second gate electrode and an output node of a unit pixel, wherein a width of the well disposed under the second impurity region is in a range from approximately −0.1 μm to approximately −0.33 μm.

A method, comprising:

• disposing a well in an active region of a substrate;
• forming a drive transistor including:
• a terminal coupled to a power voltage;
• and
• a first gate electrode formed across the well;
• and
• forming a select transistor including:
• a drain-source junction between another terminal of the drive transistor and an output node of a unit pixel;
• a second gate electrode formed in parallel to the first gate electrode;
• and
• an impurity region, wherein a width of the well disposed under the impurity region is in a range from approximately −0.1 μm to approximately −0.33 μm.

A method, comprising:

• disposing a well in an active region of a substrate;
• forming a drive transistor including:
• a drain region connected to a power voltage, and
• a first gate electrode formed on the well;
• and
• forming a select transistor including:
• a source region connected to an output node of a unit pixel;
• and
• a second gate electrode formed in parallel to the first gate electrode;
• wherein said forming a drive transistor and said forming a select transistor asymmetrically arrange the drain region of the drive transistor and the source region of the select transistor in the well;
• and
• wherein said forming a select transistor forms the second gate electrode such that an end of the well is disposed below and within a footprint of the second gate electrode.

A method, comprising:

• forming a photodiode in a substrate;
• forming in the substrate a floating diffusion node configured to receive charge from the photodiode;
• forming a drive transistor including a first gate electrode coupled to the floating diffusion node and formed across a well of the substrate;
• and
• forming a select transistor coupled between the drive transistor and an output node of a unit pixel and including a second gate electrode formed across the well;
• wherein said forming a select transistor comprises forming the second gate electrode such that an end of the well is disposed below and within a footprint of the second gate electrode.

A method, comprising:

• forming a photodiode in a substrate;
• disposing, in the substrate, a floating diffusion node configured to receive charge from the photodiode;
• forming a drive transistor including a first gate electrode that is coupled to the floating diffusion node and that is formed across a well of the substrate;
• and
• forming a select transistor coupled between the drive transistor and an output node of a unit pixel and including a second gate electrode formed across the well;
• wherein said forming a select transistor forms the second gate electrode such that at least a portion of the second gate electrode extends beyond an end of the well.