Imaging pixels having coupled gate structure
An image sensor may include one or more pixels having a coupled gate structure that may selectively route overflow charge from a photodiode to increase the dynamic range of the pixel. The coupled gate structure may include two, three or four transistors. During charge accumulation in the pixel, overflow charge may pass from a photodiode to the coupled gate structure to be selectively routed to one of a plurality of paths. Timing of control signals for a subset of the transistors in the coupled gate structure may alternate such that only one transistor is active to pass charge to one of the plurality of paths at any given time. Depending on the selected path, overflow charge may be routed to a pixel voltage supply or to one or more storage nodes in the pixel. Pixels may also include a dual-gain structure, which may provide additional charge storage capacity.
1. An electronic device comprising:
a first node;
a second node; and
a coupled gate structure comprising:
a storage node;
a threshold transistor coupled between the first node and the storage node;
a first transistor coupled to the storage node; and
a second transistor coupled between the storage node and the second node, wherein a gate of the first transistor and a gate of the second transistor receive alternating timing signals during a charge integration period, such that the first transistor is only active when the second transistor is inactive during the charge integration period, the second transistor is only active when the first transistor is inactive during the charge integration period, and both the first transistor and the second transistor are active during the charge integration period.
2. The electronic device defined in claim 1 , wherein the storage node is a fully depletable pinned storage node, wherein the second node is a floating diffusion node, and wherein the electronic device further comprises:
a photodiode, wherein the first node is coupled between the photodiode and the threshold transistor.
3. The electronic device defined in claim 2 , wherein the first transistor is coupled between the storage node and a pixel voltage supply, and wherein the electronic device further comprises:
a third transistor coupled to the storage node;
a gain select transistor coupled between the third transistor and the floating diffusion node; and
a capacitor having a terminal coupled to the third transistor and to the gain select transistor.
4. The electronic device defined in claim 2 , further comprising:
a reset transistor coupled between the floating diffusion node and a pixel voltage supply;
a gain select transistor coupled between the reset transistor and the floating diffusion node; and
a capacitor having a terminal coupled to the gain select transistor and to the reset transistor.
5. The electronic device defined in claim 4 , wherein the first transistor is coupled between the storage node and the floating diffusion node.
6. The electronic device defined in claim 5 , further comprising:
a third transistor coupled between the floating diffusion node and the first transistor; and
an additional capacitor having a terminal coupled to the first transistor and to the third transistor.
7. The electronic device defined in claim 6 , wherein the third transistor is coupled to the floating diffusion node through the gain select transistor.
8. A method of operating an imaging pixel comprising:
with a photodiode, accumulating charge during a charge accumulation period;
during the charge accumulation period, modulating control signals of at least two parallel transistors such that overflow charge is transferred from the photodiode to one of at least two paths in alternating intervals, wherein modulating the control signals comprises activating each of the at least two parallel transistors at least once during the charge accumulation period.
9. The method defined in claim 8 , wherein a first path of the at least two paths is coupled to a pixel voltage supply, and wherein a second path of the at least two paths is coupled to a floating diffusion node.
10. The method defined in claim 9 , further comprising:
holding a storage gate at an intermediate voltage level during the charge accumulation period such that the overflow charge is transferred from the photodiode to a fully depletable pinned storage node before the overflow charge is transferred to the one of at least two paths.
11. The method defined in claim 8 , wherein modulating control signals further comprises:
during the charge accumulation period, modulating control signals of the at least two parallel transistors and an additional transistor such that overflow charge is transferred to one of at least three paths in alternating intervals, wherein, at most, one of the two parallel transistors and the additional transistor is active at any given time in the charge accumulation period.
12. The method defined in claim 11 , wherein a first path of the at least three paths is coupled to a pixel voltage supply, wherein a second path of the at least three paths is coupled to a floating diffusion node, and wherein a third path of the at least three paths is coupled to a storage node that is different than the floating diffusion node.
13. The method defined in claim 8 , wherein the control signals of the at least two parallel transistors respectively alternate between a low voltage level and an intermediate voltage level.
14. An imaging pixel comprising:
a photosensitive element;
a floating diffusion node;
a pinned storage node that is fully depletable;
an additional storage node;
a first transistor coupled between the photosensitive element and the pinned storage node;
a second transistor coupled between the pinned storage node and the additional storage node;
a third transistor coupled between the floating diffusion node and the photosensitive element; and
a reset transistor coupled between the pinned storage node and a pixel voltage supply, wherein the second transistor and the reset transistor are configured such that reset transistor activation and second transistor activation alternate while charge accumulates in the photosensitive element.
15. The imaging pixel defined in claim 14 , further comprising:
a gain select storage node, wherein the first transistor is coupled between the gain select storage node and the pinned storage node; and
a gain select transistor coupled between the floating diffusion node and the gain-select storage node.
16. The image pixel defined in claim 14 , further comprising:
an additional reset transistor coupled between the additional storage node and the pixel voltage supply.
17. The image pixel defined in claim 16 , wherein the second transistor is coupled to the additional storage node through the floating diffusion node and the third transistor.