Method and apparatus for improving sensitivity in vertical color CMOS image sensors
The invention describes in detail the structure of a CMOS image sensor pixel that senses color of impinging light without having absorbing filters placed on its surface. The color sensing is accomplished by having a vertical stack of three-charge detection nodes placed in the silicon bulk, which collect electrons depending on the depth of their generation. The small charge detection node capacitance and thus high sensitivity with low noise is achieved by using fully depleted, potential well forming, buried layers instead of undepleted junction electrodes. Two embodiments of contacting the buried layers without substantially increasing the node capacitances are presented.
1. A light-sensing pixel, having a p type doped region, in a CMOS image sensor, comprising:
a first doped charge collecting region buried within the p type doped region and configured to operate as a depleted potential well;
a first n+ type doped plug extending from near the surface of the image sensor to the first charge collecting region;
a second doped charge collecting region buried within the p type doped region, the second charge collecting region vertically separated from the first charge collecting region by the p type doped region and configured to operate as a depleted potential well;
a second n+ type doped plug extending from near the surface of the image sensor to the second charge collecting region;
a first extension with n+ type doping coupled to and between the first charge collecting region and the first plug, the first extension having a different doping concentration than the first charge collecting region; and
a second extension with n+ type doping coupled to and between the second charge collecting region and the second plug, the second extension having a different doping concentration than the second charge collecting region.
2. The pixel of claim 1 wherein the first and second extensions are configured to operate such that they are not fully depleted of mobile charge.
3. The pixel of claim 1 further including:
a first reset transistor having a source electrically coupled to the first n+ type doped plug, a drain coupled to a reference voltage, and a gate coupled to a reset signal line; and
a second reset transistor having a source electrically coupled to the second n+ type doped plug, a drain coupled to a reference voltage, and a gate coupled to a reset signal line.
4. The pixel of claim 1 further including:
a first amplifier transistor electrically coupled to the first n+ type doped plug, the first amplifier transistor having an output coupled to a first output node; and
a second amplifier transistor electrically coupled to the second n+ type doped plug, the second amplifier transistor having an output coupled to a second output node.
5. The pixel of claim 4 , wherein:
the first amplifier transistor is a source follower amplifier having a gate coupled to the first n+ type doped plug, a drain coupled to a supply potential, and a source coupled the first output node; and
the second amplifier transistor is a source follower amplifier having a gate coupled to the second n+ type doped plug, a drain coupled to a supply potential, and a source coupled the second output node.
6. The pixel of claim 1 , further including:
a third doped charge collecting region formed within the p type doped region and vertically separated from the first charge collecting region by the p type doped region, the third doped charge collecting region separated from an upper surface of the p type doped region by a p+ type surface region; and
a third n+ type doped plug extending from near the surface of the image sensor to the third charge collecting region.
7. The pixel sensor of claim 6 , further including:
a first reset transistor having a source electrically coupled to the first n+ type doped plug, a drain coupled to a reference voltage, and a gate coupled to a reset signal line;
a second reset transistor having a source electrically coupled to the second n+ type doped plug, a drain coupled to a reference voltage, and a gate coupled to a reset signal line; and
a third reset transistor having a source electrically coupled to the third n+ type doped plug, a drain coupled to a reference voltage, and a gate coupled to a reset signal line.
8. The pixel of claim 6 further including:
a first amplifier transistor electrically coupled to the first n+ type doped plug, the first amplifier transistor having an output coupled to a first output node;
a second amplifier transistor electrically coupled to the second n+ type doped plug, the second amplifier transistor having an output coupled to a second output node;
a third amplifier transistor electrically coupled to the third n+ type doped plug, the third amplifier transistor having an output coupled to a third output node.
9. The pixel of claim 8 , wherein:
the first amplifier transistor is a source follower amplifier having a gate coupled to the first n+ type doped plug, a drain coupled to a supply potential, and a source coupled the first output node;
the second amplifier transistor is a source follower amplifier having a gate coupled to the second n+ type doped plug, a drain coupled to a supply potential, and a source coupled the second output node; and
the third amplifier transistor is a source follower amplifier having a gate coupled to the third n+ type doped plug, a drain coupled to a supply potential, and a source coupled the third output node.