Sense node capacitive structure for time of flight sensor
View Patent ↗An increased sense node capacitance, mainly for 3D time-of-flight (TOF) applications, includes a storage structure that combines the advantages of gate and diffusion capacitance in order to improve the overall capacitance. The storage structure provides higher capacitance per unit area and accordingly a better fill-factor/sensitivity of the pixel; improved noise behaviour because of the use of gate capacitances, better protection against interacting signals and thus better signal quality.
1. A pixel comprising:
a substrate;
a photosensitive area in the substrate that converts light into photogenerated charges;
a storage structure, comprising an integration gate and a diffusion capacitance, that stores the photogenerated charges, wherein the diffusion capacitance functions as a sense node, and wherein the diffusion capacitance is embedded in the integration gate;
an amplifier for reading out photogenerated charges via the sense node; and
a barrier gate which separates the photogenerated charges in the integration gate and the diffusion capacitance from the photosensitive area, wherein photogenerated charges are transferred to the storage structure by flowing over the barrier gate and into the integration gate and the diffusion capacitance.
2. A pixel as claimed in claim 1 , wherein during reset phase a potential is generated in a substrate below the integration gate that is higher than a potential below the sense node.
3. A pixel as claimed in claim 2 , wherein the higher potential below the integration gate is filled with the photogenerated charges to a reset voltage potential from a reset node for the sense node.
4. A pixel as claimed in claim 1 , wherein the diffusion capacitance is embedded in a middle of the integration gate.
5. A pixel as claimed in claim 1 , further comprising a temporary integration gate that temporarily accumulates the photogenerated charges from the photosensitive area until the photogenerated charges are moved into the storage structure via the barrier gate.
6. A method of operation of a pixel, the method comprising:
converting light into photogenerated charges in a photosensitive area;
flowing the photogenerated charges from the photosensitive area over a barrier gate and into a storage structure comprising an integration gate and a diffusion capacitance, wherein the diffusion capacitance is embedded in the integration gate;
storing the photogenerated charges in the storage structure; and
reading out photogenerated charges from the diffusion capacitance which functions as a sense node.
7. A method as claimed in claim 6 , further comprising generating a potential during reset phase in a substrate below the integration gate that is higher than a potential below the sense node.
8. A method as claimed in claim 7 , further comprising filling the higher potential below the integration gate with the photogenerated charges to a reset voltage potential from a reset node for the sense node.
9. A method as claimed in claim 6 , further comprising temporarily storing the photogenerated charges from the photosensitive area in a temporary integration gate until the photogenerated charges are moved into the storage structure via the barrier gate.