Multi-cell pixel array for high dynamic range image sensors

A pixel includes an array of a plurality of photodiodes. The array of photodiodes includes a plurality of rows of photodiodes and a plurality of columns of photodiodes. The plurality of photodiodes includes a set of first photodiodes that has a first surface area and at least one second photodiode that has a second surface area that is smaller than the first surface area. The first photodiodes are arranged to be symmetric with respect to the at least one second photodiode. Output circuitry is electrically coupled to each of the first photodiodes in the set of first photodiodes. A switch is selectively, operably closed to electrically couple the output circuitry to the second photodiode.

BACKGROUND INFORMATION

Field of Disclosure

The present invention is generally related to image sensors, and more specifically, is directed to high dynamic range image sensors.

Background

Standard image sensors have a limited dynamic range of approximately 60 to 70 dB. However, the luminance dynamic range of the real world is much larger. For instance, natural scenes often span a range of 90 dB and greater. To capture details in bright lights and dim shadows simultaneously, high dynamic range (HDR) technologies have been used in image sensors to increase the captured dynamic range. The most common technique to increase dynamic range is to merge multiple exposures captured with different exposure settings using standard (low dynamic range) image sensors into a single linear HDR image, which results in a much larger dynamic range image than a single exposure image.

Another HDR technique incorporates different exposure integration times or different light sensitivities (for example by inserting neutral density filters) into a single image sensor. The single image sensor could have in effect 2, 3, 4, or even more different exposures in the single image sensor. Thus, multiple exposure images are available in a single shot using this HDR image sensor. However, the overall image resolution is decreased using this HDR sensor compared to a normal full resolution image sensor. For example, for an HDR sensor that combines 4 different exposures into one image sensor, each HDR image would be only a quarter resolution of the full resolution image.

Other approaches to implement HDR image sensors present many other challenges. These other approaches are not space efficient and are difficult to miniaturize to a smaller pitch to achieve higher resolutions. In addition, due to the asymmetric layouts of many of these HDR image sensors, reducing the size and pitch of the pixels to realize high resolution image sensors result in crosstalk and other unwanted side effects, such as diagonal flare that can occur in these image sensors as the pitches are reduced. Furthermore, many HDR image sensors require structures with very large full well capacities (FWC) to accommodate the large dynamic ranges. However, the large FWC requirements cause lag, white pixels (WP), dark current (DC), and other unwanted problems. Thus, these other HDR imaging approaches are also not suitable for high resolutions because of the high FWC requirements that are difficult to scale.

DETAILED DESCRIPTION

Methods and apparatuses directed to a pixel array with multiple photodiodes, including a plurality of large photodiodes that are arranged symmetrically around at least one small photodiode, are disclosed. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.

Reference throughout this specification to “one example” or “one embodiment” means that a particular feature, structure, or characteristic described in connection with the example is included in at least one example of the present invention. Thus, the appearances of the phrases “in one example” or “in one embodiment” in various places throughout this specification are not necessarily all referring to the same example. Furthermore, the particular features, structures, or characteristics may be combined in any suit able manner in one or more examples.

FIG. 1illustrates an exemplary pixel100that includes multiple photodiodes. A plurality of first photodiodes102collectively form a large photodiode103. The plurality of first photodiodes102may be arranged around a second photodiode, that forms a small photodiode104, in accordance with the teachings of the present invention. Each of the individual first photodiodes102may be substantially the same size (e.g., have substantially the same surface area) as the small photodiode104. The multiple first photodiodes102and the small photodiode104may be arranged along the surface106of an imaging sensor and may be positioned to receive light that is incident on the imaging sensor surface106. The first photodiodes102and the small photodiode104may be n-type pinned photodiodes. Each first photodiode102may be adapted to photogenerate image charge (e.g., a charge of e−) in response to incident light. Such image charge may generate a corresponding electrical signal in which the strength of the electrical signal may be based upon the amount of image charge generated as a result of the light incident on the large photodiode104. Likewise, the small photodiode104may photogenerate image charge as a result of the light that is incident on the small photodiode104. Such image charge may result in an electrical signal being generated by the small photodiode104in which the strength of the electrical signal may be based upon the amount of image charge generated as a result of the light incident on the small photodiode104. The electrical signals generated by the first photodiodes102and the small photodiode104may be used to generate image data. The surface106of the imaging sensor may include a plurality of exemplary pixels100. The collection of image data generated from such a plurality of pixels100arranged along the surface106of the imaging sensor may be used to render an image.

The surface area of the first photodiodes102that collectively form the large photodiode103arranged along the surface106of the imaging sensor to receive incident light may be larger than the corresponding surface area of the small photodiode104that is arranged along the surface106of the imaging sensor. In some instances, for example, the pitch of each first photodiode102may be approximately 1 μm. In some implementations, the pitch of one or more of the first photodiodes102may be approximately 0.9 μm and/or 0.7 μm. In some implementations, the pitch of each of the first photodiodes102in the pixel100may be the same. The pitch of the small photodiode104may be the same as, or substantially the same as, the pitches of each of the corresponding first photodiodes102in the pixel100.

The amount of the image charge generated from a photodiode, such as the large photodiode103and/or the small photodiode104, may be based, at least in part, on the surface area of the photodiode that is arranged to receive incident light. For example, the strength of the electrical signal may be proportional to and thereby increase as the amount of surface area that is positioned to receive incident light increases. In such an implementation, the first photodiodes102that collectively form the large photodiode103may therefore generate a relatively stronger electrical signal collectively in comparison to the electrical signal generated by the small photodiode104that receives that same intensity of incident light. As such, the large photodiode103may be used to sense dimmer low to medium intensity light conditions, whereas the relatively smaller small photodiode104may be used to sense bright or high intensity light conditions. In some implementations, the small photodiode104may also be used to capture flickers from light emitting diodes (e.g., LED flickers) that may otherwise be missed by the large photodiode103. For instance, in one example, the exposure time of the small photodiode104to the incident light may be increased relative to the exposure time of the large photodiode103to capture LED flickers (e.g., ˜11 ms interval). The capture of such LED flickers may be useful in various situations, such as, for example, when the exemplary pixel100is used as part of image sensor that is used in a vehicle to render images of and detect objects in the surrounding environment. The array of first photodiodes102and small photodiode104may be used to provide the pixel100with an image output having a high dynamic range (HDR). The amount of image charge generated from a photodiode may also depend on other factors, including, for example, the effective length of the photodiode. Photodiodes having a relatively deeper effective length may generate more image charge than a similar photodiode with a relatively shallower effective length.

The multiple first photodiodes102that form large photodiode103and the small photodiode104may collectively form an array108of photodiodes as part of the pixel100. The array108of photodiodes may include a plurality of rows110and/or a plurality of columns112of photodiodes. Each of the rows110and/or columns112may include a plurality of photodiodes. As shown inFIG. 1, for example, each row110includes a set of three photodiodes, and each column112also includes a set of three photodiodes. The first photodiodes102may be arranged to be symmetric with respect to the small photodiode104with respect to one or more axes of symmetry. For example, as shown inFIG. 1, the first photodiodes102may be arranged around the small photodiode104to be symmetric with respect to a first axis of symmetry114and with respect to a second axis of symmetry116. Such a symmetric arrangement of the first photodiodes102with respect to the small photodiode104may be used to reduce the glare that may result from light that is incident at an angle (e.g., 45°) on the surface106of the imaging sensor. In some implementations, the pixel100may include a plurality of small photodiodes104and a plurality of first photodiodes102. In such an implementation, the plurality of first photodiodes102may be arranged to be symmetric with respect to one or more of the plurality of small photodiodes104.

FIG. 2Aillustrates a schematic of an exemplary pixel200A that includes multiple first photodiodes202that collectively form a large photodiode203and at least one small photodiode204. Each of the first photodiodes202may be selectively, electrically coupled to output circuitry206using one or more first transfer switches208. Such first transfer switches208may be implemented using, for example, NMOS transistors. In some implementations, one or more of the first transfer switches208may be controlled to selectively move between an open state and a closed state using control circuitry (see, e.g.,FIG. 3). One or more of the first transfer switches208may be closed, for example, to provide an electrical pathway for the image charges generated at each of the corresponding first photodiodes202in response to incident light on the first photodiodes202. Such an electrical pathway may extend between one or more of the first photodiodes202and the output circuitry206, for example.

A second transfer switch210(one shown inFIG. 2A) may be used to selectively, electrically couple corresponding small photodiode204(one shown inFIG. 2A) to the output circuitry206. Second transfer switch210may be implemented using, for example, an NMOS transistor. The second transfer switch210may be controlled to selectively move between an open state and a closed state using control circuitry (see, e.g.,FIG. 3). The second transfer switch210may be closed, for example, to provide an electrical pathway for the image charges generated at the small photodiode204in response to incident light on small photodiode204. Such an electrical pathway may extend between the small photodiode204and the output circuitry206, for example.

Output circuitry206may be selectively electrically coupled to the first photodiodes202as well as to the small photodiodes204. As shown in the example illustrated inFIG. 2A, the output circuitry206may include a source follower transistor214, a voltage source216, and an output node218. In the depicted example, a floating diffusion node211is an input node of the output circuitry206, which is coupled to the gate terminal of the source follower transistor214. The output node218of the output circuitry206is coupled to the source terminal of the source follower transistor214. In the example, the floating diffusion node211is coupled to receive the image charge from the first photodiodes202and/or the small photodiodes204through the first transfer switches208and/or the second transfer switches210, respectively. In such a situation, the output signal generated at the output node218by the source follower transistor214is responsive to the charge present in the floating diffusion node211that is coupled to the gate terminal of the source follower transistor214.

For instance, during operation of the depicted example, the source follower transistor214serves as a buffer or amplifier responsive to the charge in the floating diffusion node211, and the voltage of the output signal generated at the source terminal of the source follower transistor214follows the voltage at the floating diffusion node211that is coupled to the gate terminal of the source follower transistor214, minus the gate source voltage between the gate terminal and the source terminal of the source follower transistor214. Thus, the resulting voltage at the output node218may be used to generate image data that is associated with the exemplary pixel200A.

In some instances, the set of first transfer switches208may be controlled to act together, so that all of the first transfer switches208transition together between an open state and a closed state. Controlling first transfer switches208collectively may result in the exemplary pixel200A having lower resolution because the image charge generated at each of the first photodiodes202will be transmitted to the floating diffusion node211that is coupled to the gate terminal of the source follower transistor214. The resolution of the exemplary pixel200A may be increased by selectively closing each of the first transfer switches208independently of each other. The control circuitry may control the first transfer switches208to limit the amount of image charge generated at the first photodiodes202that may then travel to the output circuitry206. For example, the control circuit may control the first transfer switches208to perform a Very Short Exposure (“VS Exposure”) in certain conditions, such as when an intense light is incident on the exemplary pixel200A. Limiting the amount of exposure time may be used to increase the dynamic range of the exemplary pixel200A.

In the depicted example, pixel200A also includes dual conversion gain circuitry220and reset circuitry228. The dual conversion gain circuitry220may include a dual floating diffusion (DFD) switch222and a capacitor224. The dual conversion gain circuitry220may be used by pixel200A to realize high dynamic range (HDR) imaging. In operation, in bright outdoor lighting conditions, the DFD switch222may be closed to couple capacitor224to the floating diffusion node211to increase the floating well capacity (FWC), which provides additional dynamic range capabilities to pixel200A. As such, low conversion gain (LCG) is realized by coupling capacitor224to the floating diffusion node211to accommodate the amount of charge generated during bright light conditions. In an example with dim lighting conditions, the DFD switch may be opened, which decouples the capacitor224from the floating diffusion node211. As such, high conversion gain (HCG) is realized by decoupling the capacitor224as the FWC of the floating diffusion node211is sufficiently large without adding the capacitor224. In one example, a voltage source226may be electrically coupled to the side of the capacitor224opposite the DFD switch222. The voltage source226may thereby be used to enable the storage of at least some of image charge generated by the large photodiodes202when the DFD switch222is in a closed state.

The reset circuitry228may include a reset switch230and a voltage source232. The reset switch230may be selectively closed along with DFD switch222by the control circuitry (not shown) to create an electrical path to the voltage source232. In a reset operation, all of the switches in the exemplary pixel200A—including each of the first transfer switches208, second transfer switch210, DFD switch222, and reset switch230—may be set into the closed state to provide an electrical pathway between each of the first photodiodes202, the small photodiode204, and the floating diffusion node211to the voltage source232in the reset circuitry228. In this reset operation, the image charge generated by each of the first photodiodes202, the small photodiode204, and the image charge present in the floating diffusion node211may be swept to the voltage source232to reset the exemplary pixel200A.

FIG. 2Billustrates a schematic of an exemplary pixel200B that includes multiple first photodiodes202that may collectively form a large photodiode203, at least one small photodiode204, output circuitry206, dual conversion gain circuitry220, and reset circuitry228like exemplary pixel200A. Exemplary pixel200B further includes LOFIC circuitry234that may comprise a lateral overflow integration capacitor236and a voltage source238. The LOFIC circuitry234may be electrically coupled to the small photodiode204when the second transfer switch210is in a closed state. In instances in which the exemplary pixel200B has multiple small photodiodes204, each of the small photodiodes204may have an associated LOFIC circuitry234to which the small photodiode204may be electrically coupled. In other instances, multiple small photodiodes204may be selectively electrically coupled to a single LOFIC circuitry234.

The voltage source238may be electrically coupled to the side of the lateral overflow integration capacitor236that is opposite the side of the lateral overflow integration capacitor236that may be electrically coupled to the small photodiode204. As such, the voltage source238may be used to apply an electric potential to the lateral overflow integration capacitor236to thereby attract at least some of the image charge photogenerated by the small photodiode204when the lateral overflow integration capacitor236is electrically coupled to the small photodiode204. The lateral overflow integration capacitor236may be used, for example, to increase the amount of image charge that may be generated at the small photodiode204. As such, the lateral overflow integration capacitor236may be used to extend the dynamic range of the exemplary pixel200B by reducing the amount of image charge that is transferred to the output circuitry206. In some instances, the lateral overflow integration capacitor236may be comprised of a large in-metal capacitor. In some instances, the lateral overflow integration capacitor236may be comprised of a metal stacked capacitor. In some instances, the lateral overflow integration capacitor236may be comprised of a metal-to-metal capacitor, such as one or more of a metal-oxide-metal capacitor and/or a metal-insulator-metal capacitor.

The exemplary pixel200B may include a small photodiode enable switch240that may be positioned between the small photodiode204and the output circuitry206. The small photodiode enable switch240may be used to selectively electrically couple the small photodiode204with the output circuitry206and/or the reset circuitry228. When the exemplary pixel200B is being used to capture image charge to generate image data, for example, the small photodiode enable switch240may be selectively closed such that the small photodiode204may be electrically coupled with the output circuitry206, thereby allowing image charges to flow from the small photodiode204to the floating diffusion node211coupled to the output circuitry206. The small photodiode enable switch240may also be selectively closed, along with DFD switch222and reset switch230, during a reset operation so that the small photodiode204and the LOFIC circuitry234may be electrically coupled to the voltage source232. In this situation, the image charges present at the small photodiode204and/or the LOFIC circuitry234may be swept up by the voltage source232. In such implementations, the small photodiode enable switch240may be controlled by the control circuitry (not shown).

FIG. 2Cillustrates a schematic of an exemplary pixel200C that includes multiple first photodiodes202that collectively form a large photodiode203, at least one small photodiode204, output circuitry206, dual conversion gain circuitry220, and reset circuitry228like exemplary pixel200A. The exemplary pixel200C also includes a filter242that may be electrically coupled to the small photodiode204when the second transfer switch210is in a closed state. The filter242may selectively filter image charge generated by light of a desired wavelength or within a desired range of wavelengths that is incident on the small photodiode204. As such, the filter242may, for example, operate as band-stop filter that selectively filters light within a given range of wavelengths.

The filter242may thereby be used to increase the dynamic range of the exemplary pixel200C. Further, the range of wavelengths that the filter242filters may have an impact on the dynamic range of the exemplary pixel200C. For example, in some implementations, the filter242may filter image charge photogenerated at the small photodiode204by light having a wavelength that falls within a wavelength range associated with a certain color. Such selective filtering by the filter242may be used to increase the dynamic range of the exemplary pixel200C. For example, the filter242may selectively filter image charge photogenerated at the small photodiode204by the portion of the incident light having a wavelength that falls within the wavelength range of green light (e.g., about 520-560 nm). This type of filter that filters image charge associated with green light may increase the dynamic range of the exemplary circuitry200C because the visual perception of the human eye is particularly sensitive to green light. The filter242may filter image charge generated at the small photodiode204by light at wavelengths other than green light.

In some implementations, the exemplary pixel200C may include an optical filter244instead of or in addition to filter242. The optical filter244may be a filter located proximate the small photodiode204and positioned between the small photodiode204and the outside environment. As such, light that is incident on the small photodiode204may travel along an optical path such that it passes through the optical filter244. In this situation, the optical filter244may filter a portion of the light incident on the small photodiode that has a wavelength falling within a range of wavelengths. For example, in some implementations, the optical filter244may filter light having a wavelength that falls within a wavelength range associated with a certain color. Such selective filtering by the optical filter244may be used to increase the dynamic range of the exemplary pixel200C. For example, the optical filter244may selectively filter the portion of the incident light having a wavelength that falls within the wavelength range of green light (e.g., about 520-560 nm). The optical filter244may thereby increase the dynamic range of the exemplary circuitry200C because the visual perception of the human eye is particularly sensitive to green light. The optical filter244may filter light incident on the small photodiode at wavelengths other than green light. In one example, one or more optical filters244may also be located proximate to one or more of the first photodiodes202and positioned between the first photodiodes202and the outside environment. As such, light that is incident on the first photodiodes202may travel along an optical path such that it passes through the optical filter244to the first photodiodes202. In this situation, the optical filters244may filter a portion of the light incident on the first photodiodes that has a wavelength falling within a range of wavelengths. In some instances, the range of wavelengths filtered by the optical filters244located proximate to the first photodiodes202may be identical to the range of wavelengths filtered by the optical filter244located proximate to the small photodiode204. In other instances, the range of wavelengths filtered by the optical filters244located proximate to the first photodiodes202may be different than the range of wavelengths filtered by the optical filter244located proximate to the small photodiode204.

FIG. 3shows an imaging system300that includes a pixel array302, control circuitry304, and readout circuitry306. The readout circuitry306may be communicatively coupled to function logic308.

The illustrated embodiment of pixel array302is a two-dimensional (“2D”) array of imaging sensors or pixel cells310(e.g., pixel cells P1, P2, . . . , Pn). In one example, each pixel cell310includes one or more first photodiodes along with one or more small photodiodes that can be used for HDR imaging in accordance with the teachings of the present invention. As illustrated, the array of pixel cells310is arranged into a plurality of rows (e.g., rows R1to Ry) and a plurality of columns (e.g., columns C1to Cx) to acquire image data of a person, place, or object, etc., which can then be used to render an image of the person, place, or object, etc. As will be described in greater detail below, each pixel cell310(e.g., pixel cells P1, P2, . . . , Pn) may include one or more first photodiodes along with one or more small photodiodes to provide HDR imaging in accordance with the teachings of the present invention.

In one example, after each pixel cell310has acquired its image data or image charge, the image data is read out by readout circuitry306through readout column bit lines312and then transferred to function logic308. In various examples, readout circuitry306may include amplification circuitry (not illustrated), a column readout circuit that includes analog-to-digital conversion (ADC) circuitry, or otherwise. Function logic308may simply store the image data or even manipulate the image data by applying post image effects (e.g., crop, rotate, remove red eye, adjust brightness, adjust contrast, or otherwise). In one example, readout circuitry306may read out a row of image data at a time along readout column lines (illustrated) or may read out the image data using a variety of other techniques (not illustrated), such as a serial read out or a full parallel read out of all pixels simultaneously.

In one example, control circuitry306is coupled to pixel array302to control operational characteristics of the pixels310(e.g., pixel cells P1, P2, . . . , Pn) in the pixel array302. For instance, the control circuitry308may generate the transfer gate signals and other control signals to control the transfer and readout of image data from the first photodiode(s) and/or small photodiode(s) of each pixel cell310of pixel array302. In addition, control circuitry308may generate a shutter signal for controlling image acquisition. In one example, the shutter signal is a global shutter signal for simultaneously enabling all pixels310within pixel array302to simultaneously capture their respective image data during a single acquisition window. In another example, the shutter signal is a rolling shutter signal such that each row, column, or group of pixels310is sequentially enabled during consecutive acquisition windows. The shutter signal may also establish an integration time, which is the length of time that the photodiodes of each pixel cell310photogenerate image charge in response to incident light. In one implementation, the exposure time is set to be the same for each of the frames.

FIG. 4shows a method400of operating an exemplary pixel that has a plurality of photodiodes, including one or more first photodiodes and at least one small photodiode, as well as output circuitry that may be selectively, electrically coupled to either or both of the first photodiode(s) and/or the at least one small photodiode. Method400begins at402at which the operation of the exemplary pixel may be initiated. Such initiation may occur, for example, when the exemplary pixel is powered up. Such initiation may occur after the exemplary pixel undergoes an image acquisition operation. Method400then transitions from402to404.

At404, the control circuitry transmits one or more signals to cause the exemplary pixel to perform a reset. As such, the control circuitry may transmit one or more signals to cause each of the transfer switches in the exemplary pixel to enter into a closed state to thereby electrically couple the first photodiode(s), the small photodiode, and floating diffusion node to reset circuitry. The reset circuitry may include a voltage source that will be used to sweep image charge from the first photodiode(s), the small photodiode(s), and floating diffusion node and thereby perform a reset operation. Such a reset operation may be used to ready the photodiodes in the exemplary pixel to photogenerate image charge and thereby generate image data for an image acquisition operation. The control circuitry may then transmit one or more signals to cause each of the transfer switches in the exemplary pixel to enter into an open state.

At406, the output circuitry may receive image charge that may be generated at one or more of the first photodiodes. Such image charge may be received upon causing one or more of the transfer switches to selectively electrically couple one or more of the first photodiodes to the output circuitry. Such image charge may be generated when light is incident on one or more of the first photodiodes. In such an implementation, an electrical path may be present between the first photodiode(s) and the floating diffusion coupled to the output circuitry. In some instances, the image charge that is then present at the floating diffusion coupled to the output circuitry may be used to generate image data for the exemplary pixel. The first photodiode(s) collectively have a relatively large surface area, as compared to the small photodiode, that may be used to photogenerate image charge. In addition, because the exemplary pixel includes more photodiodes that comprise the first photodiodes as compared to conventional pixels, the first photodiode(s) in the exemplary pixel may be used to generate image data during low light situations. The use of additional first photodiodes to comprise the large photodiode, as compared to a conventional pixels, may also provide for improved full well capacity of the exemplary pixel. Such an increased full well capacity may reduce the instances of white pixel noise in which the exemplary pixel enters a saturated condition. In this situation, the DFD switch may remain in an open position so that the exemplary pixel operates in a high conversion gain mode.

At408, the output circuitry may generate an output signal based on the received image charges at the floating diffusion node. For example, the output circuitry may include an output amplifier or buffer that has a source follower transistor that generates an output signal in response to the image charge in the floating diffusion node received from one or more of the first photodiode(s) and/or the small photodiode.

The method400ends at410. At that point, the exemplary pixel may be used for a subsequent image acquisition operation in which, for example, example400may be repeated in whole or in part.

FIG. 5shows a method500of operating an exemplary pixel that has a plurality of photodiodes, including one or more first photodiodes and at least one small photodiode, output circuitry, and a floating diffusion node that may be selectively, electrically coupled to either or both of the one or more first photodiodes and/or the at least one small photodiode. The exemplary pixel may also include a lateral overflow integration capacitor (LOFIC) and that may operate in accordance with the teachings of the present invention. Method500begins at502at which the operation of the exemplary pixel may be initiated. Such initiation may occur, for example, when the exemplary pixel is powered up. Such initiation may occur after the exemplary pixel undergoes an image acquisition operation. Method500then transitions from502to504.

At504, the control circuitry transmits one or more signals to cause the exemplary pixel to perform a reset. As such, the control circuitry may transmit one or more signals to cause each of the transfer switches in the exemplary pixel to enter into a closed state to thereby electrically couple the first photodiode(s), the small photodiode, and floating diffusion node to reset circuitry. The reset circuitry may include a voltage source that will be used to sweep image charge from the first photodiode(s), the small photodiode(s), and floating diffusion node and thereby perform a reset operation. Such a reset operation may be used to ready the photodiodes in the exemplary pixel to photogenerate image charge and thereby generate image data for an image acquisition operation. The control circuitry may then transmit one or more signals to cause each of the transfer switches in the exemplary pixel to enter into an open state.

At506, the floating diffusion node coupled to the output circuitry may receive image charge that may be generated at one or more of the first photodiodes. Such image charge may be generated when light is incident on one or more of the first photodiodes. In such an implementation, an electrical path may be present between one or more of the first photodiodes and the floating diffusion node coupled to the output circuitry. In some instances, the image charge that is then present at the floating diffusion node coupled to the output circuitry may be used to generate image data for the exemplary pixel. The first photodiode(s) have collectively a relatively large surface area, as compared to the small photodiode, that may be used to photogenerate image charge. In addition, because the exemplary pixel includes more first photodiodes that comprise the large photodiode, the first photodiode(s) in the exemplary pixel may be used collectively to generate image data during low light situations. The use of additional first photodiodes to comprise the large photodiode may also provide for improved full well capacity in the first photodiodes of the exemplary pixel.

At508, control circuitry may send one or more signals that cause an electrical switch located between the small photodiode and the floating diffusion node coupled to the output circuitry to transition to a closed state, thereby establishing an electrical path between the small photodiode and the floating diffusion node coupled to the output circuitry. As a result, the small photodiode and the floating diffusion node coupled to the output circuitry will be electrically coupled such that image charge photogenerated at the small photodiode by incident light may travel from the small photodiode to the floating diffusion node output circuitry.

At510, the LOFIC circuitry may be selectively electrically coupled to the small photodiode to capture at least some of the image charge generated at the small photodiode. Such LOFIC circuitry may include a capacitor that is electrically coupled on one side to the small photodiode and on the other side to a voltage source. By energizing the voltage source, the LOFIC circuitry may be used to capture at least some of the image charge generated by the small photodiode. Such image charge that is collected by the LOFIC circuitry may not be further transmitted to other components of the exemplary pixel while the LOFIC voltage source is energized. Accordingly, by reducing the image charge that is received at the output circuitry, the LOFIC circuitry may be used to increase the dynamic range of the exemplary pixel.

At512, the floating diffusion node coupled to the output circuitry receives image charge photogenerated at the small photodiode. Because the small photodiode has a relatively small surface area for receiving incident light, as compared to the first photodiode(s) collectively, the exposure time of the small photodiode may be increased to provide image data for bright flashes of light such as LED flickers that are incident on the exemplary pixel and prevent the occurrence of a white pixel condition.

At514, the output circuitry may generate an output signal based on the received image charges. For example, the output circuitry may include an output amplifier or buffer that has a source follower transistor that generates an output signal in response to the image charge received from one or more of the first photodiode(s) and/or the small photodiode.

The method500ends at516. At that point, the exemplary pixel may be used for a subsequent image acquisition operation in which, for example, example500may be repeated in whole or in part.

FIG. 6shows a method600of operating an exemplary pixel that has a plurality of photodiodes, including one or more first photodiodes and at least one small photodiode, as well as output circuitry that may be selectively, electrically coupled to either or both of the first photodiode(s) and/or the at least one small photodiode. Method600begins at602at which the operation of the exemplary pixel may be initiated. Such initiation may occur, for example, when the exemplary pixel is powered up. Such initiation may occur after the exemplary pixel undergoes an image acquisition operation. Method600then transitions from602to604.

At604, the control circuitry transmits one or more signals to cause the exemplary pixel to perform a reset. As such, the control circuitry may transmit one or more signals to cause each of the transfer switches in the exemplary pixel to enter into a closed state to thereby electrically couple the first photodiode(s), the small photodiode, and floating diffusion node to reset circuitry. The reset circuitry may include a voltage source that will be used to sweep image charge from the first photodiode(s), the small photodiode(s), and floating diffusion node and thereby perform a reset operation. Such a reset operation may be used to ready the photodiodes in the exemplary pixel to photogenerate image charge and thereby generate image data for an image acquisition operation. The control circuitry may then transmit one or more signals to cause each of the transfer switches in the exemplary pixel to enter into an open state.

At606, the control circuitry transmits one more signals to cause the transfer switches associated with the first photodiodes to close and to cause a DFD switch to close to thereby electrically couple the first photodiodes with a low conversion gain circuitry, such as at least one dual conversion gain capacitor, which increases the FWC of the floating diffusion node. By closing the DFD switch, the low conversion gain circuitry may be selectively, electrically coupled to receive image charge photogenerated at the first photodiodes. As such, the exemplary pixel may be in a low conversion gain mode when the DFD switch is closed. The switches that are closed as part of606may further form an electrical pathway between the first photodiodes, the dual conversion gain capacitor, and output circuitry.

At608, image charge may be received at the floating diffusion node coupled to the output circuitry. Such image charge may be generated when light is incident on one or more of the first photodiodes. In such an implementation, an electrical path may be present between the first photodiode(s) and the floating diffusion node coupled to the output circuitry. In some instances, the image charge that is then present at the floating diffusion node coupled to the output circuitry may be used to generate image data for the exemplary pixel. The first photodiode(s) collectively have a relatively large surface area, as compared to the small photodiode, that may be used to photogenerate image charge. In addition, because the exemplary pixel includes more first photodiodes that comprise the large photodiode, the first photodiodes in the exemplary pixel may be used to generate image data during low light situations. The use of additional first photodiodes to comprise the large photodiode, may also provide for improved full well capacity of the exemplary pixel. Such an increased full well capacity may reduce the instances of white pixel noise in which the exemplary pixel enters a saturated condition.

At610, the output circuitry may generate an output signal based on the received image charges at the floating diffusion node. For example, the output circuitry may include an output amplifier or buffer that has a source follower transistor that generates an output signal in response to the image charge received at the floating diffusion node from one or more of the first photodiode(s) and/or the small photodiode.

The method600ends at612. At that point, the exemplary pixel may be used for a subsequent image acquisition operation in which, for example, example600may be repeated in whole or in part.

FIG. 7is a signal-to-noise ratio (SNR) diagram700illustrating the dynamic range of an exemplary pixel in that operates in accordance with the teachings of the present invention. The exemplary pixel includes a 3×3 layout of photodiodes in which a small photodiode is positioned in the middle of the array and is surrounded by 8 first photodiodes that collectively may form a large photodiode. The exemplary pixel also includes LOFIC circuitry that is selectively electrically coupled to the small photodiode.

The x-axis702of SNR diagram700represents the illumination on the exemplary pixel and is provided in lux. The y-axis704of the SNR diagram700represents the signal-to-noise ratio in decibels. The first plot706shows the SNR response of the large photodiode having high conversion gain as the intensity of the pixel illumination from the incident light increases moving rightward along the x-axis. As seen in SNR diagram700, the large photodiode having high conversion gain provide an output signal until the lux exceeds about 1.0E+0.0with a maximum signal-to-noise ratio of about 28 dB. The second plot708shows the SNR response of the large photodiode having low conversion gain as the intensity of the pixel illumination from the incident light increases moving rightward along the x-axis. The large photodiode having low conversion gain provides an output signal until just before 1.0E+02 with a maximum signal-to-noise ratio of about 42 dB. The third plot710shows the SNR response of the small photodiode that is electrically coupled to the LOFIC circuitry. Such a small photodiode provides an output signal until the lux for the pixel illumination exceeds about 1.0E+04 with a maximum signal-to-noise ratio of about 40 dB. The total dynamic range of the exemplary pixel may be based on the dual conversion gain of the large photodiode, the output of the small photodiode, and the output when the exemplary pixel performs a VS Exposure. Accordingly, the exemplary node that provides the output graphed in SNR diagram700may have a dynamic range of about 130.6 dB.