Image sensors having imaging pixels with ring-shaped gates
An image sensor may include a plurality of imaging pixels with high dynamic range. Each imaging pixel may have a photodiode, a floating diffusion region, and a transfer transistor configured to transfer charge from the photodiode to the floating diffusion region. Each imaging pixel may also include an overflow capacitor and an overflow transistor interposed between the photodiode and the overflow capacitor. A dual conversion gain transistor may be interposed between the overflow capacitor and the floating diffusion region. To reduce noise associated with operation of the pixel, a ring-shaped conductive layer may form a gate for both the overflow transistor and the dual conversion gain transistor. This common gate may be set to an intermediate level during integration to allow charge to overflow past the overflow transistor to the overflow capacitor. The common gate may also be used to assert the dual conversion gain transistor.
1. An imaging pixel comprising:
a photodiode configured to generate charge in response to incident light;
an overflow capacitor;
a first transistor that is configured to allow charge above a given threshold to overflow from the photodiode to the overflow capacitor;
a floating diffusion region;
a second transistor that is interposed between the overflow capacitor and the floating diffusion region; and
a ring-shaped conductive layer that forms a gate for the first transistor and a gate for the second transistor.
2. The imaging pixel defined in claim 1 , further comprising:
a transfer transistor that is configured to transfer charge from the photodiode to the floating diffusion region.
3. The imaging pixel defined in claim 2 , further comprising:
an enclosed diffusion region, wherein the ring-shaped conductive layer has a central opening that overlaps the enclosed diffusion region.
4. The imaging pixel defined in claim 3 , further comprising:
a conductive layer that electrically connects the enclosed diffusion region to the overflow capacitor.
5. The imaging pixel defined in claim 4 , wherein a first portion of the ring-shaped conductive layer is interposed between the enclosed diffusion region and the photodiode and a second portion of the ring-shaped conductive layer is interposed between the enclosed diffusion region and the floating diffusion region.
6. The imaging pixel defined in claim 5 , wherein the first portion of the ring-shaped conductive layer forms the gate for the first transistor and the second portion of the ring-shaped conductive layer forms the gate for the second transistor.
7. The imaging pixel defined in claim 6 , further comprising:
a bias voltage supply terminal contact; and
a reset transistor, wherein the reset transistor has a gate that is interposed between the floating diffusion region and the bias voltage supply terminal contact.
8. The imaging pixel defined in claim 7 , further comprising:
a source follower transistor having a gate;
an additional conductive layer that electrically connects the floating diffusion region to the gate of the source follower transistor;
a column output line;
an output contact coupled to the column output line;
a row select transistor having a gate that is interposed between the gate of the source follower transistor and the output contact; and
a control path that is configured to provide a single control signal to the ring-shaped conductive layer.
9. An imaging pixel comprising:
a photodiode configured to generate charge in response to incident light;
a floating diffusion region;
a transfer transistor interposed between the photodiode and the floating diffusion region;
an isolated diffusion region overlapped by a central opening of a ring-shaped gate;
a first transistor interposed between the photodiode and the isolated diffusion region, wherein a first portion of the ring-shaped gate forms a gate for the first transistor; and
a second transistor interposed between the isolated diffusion region and the floating diffusion region, wherein a second portion of the ring-shaped gate forms a gate for the second transistor.
10. The imaging pixel defined in claim 9 , further comprising:
a storage capacitor that is coupled to the isolated diffusion region by a conductive layer.
11. The imaging pixel defined in claim 10 , wherein the photodiode is configured to generate the charge in response to incident light during an integration time and wherein the first transistor is configured to allow charge to overflow from the photodiode to the isolated diffusion region during the integration time.
12. The imaging pixel defined in claim 11 , wherein the second transistor is configured to couple the storage capacitor to the floating diffusion region.
13. The imaging pixel defined in claim 9 , wherein the isolated diffusion region is isolated from the floating diffusion region by the ring-shaped gate.
14. The imaging pixel defined in claim 9 , wherein the first transistor is an overflow transistor and the second transistor is a dual conversion gain transistor.
15. The imaging pixel defined in claim 9 , wherein the gate for the first transistor and the gate for the second transistor receive a common control signal that is provided to the ring-shaped gate by a control path that is coupled to the ring-shaped gate.
16. An imaging pixel comprising:
a semiconductor substrate;
a photodiode in the semiconductor substrate;
a floating diffusion region in the semiconductor substrate;
an enclosed diffusion region in the semiconductor substrate;
a first gate, wherein the first gate and the semiconductor substrate are overlapping and the first gate is interposed between the photodiode and the floating diffusion region;
a second gate that is ring-shaped, wherein the second gate and the semiconductor substrate are overlapping, wherein a portion of the second gate is interposed between the enclosed diffusion region and the floating diffusion region, and wherein the enclosed diffusion region is overlapped by a central opening of the second gate;
a potential barrier in the semiconductor substrate interposed between the photodiode and the enclosed diffusion region; and
a capacitor that is electrically connected to the enclosed diffusion region.
17. The imaging pixel defined in claim 16 , wherein the photodiode is configured to generate charge during an integration time and wherein the potential barrier is configured to allow some of the generated charge to overflow from the photodiode to the enclosed diffusion region.
18. The imaging pixel defined in claim 16 , wherein a gap is present between the photodiode and the second gate.
19. The imaging pixel defined in claim 16 , wherein the first gate is a transfer gate that is formed separately from the second gate.