PATENT DOCUMENT

Publication Number: US-11877071-B1
Application Number: US-202217951060-A
Country: US
Kind Code: B1

Title: Flicker and proximity detection in image sensors with embedded low power readout circuitry

Abstract:
Disclosed herein are cameras and image sensors, and electronic devices containing them, having pixel arrays operable both for obtaining images and detecting flicker in ambient light. For flicker detection, such as prior to image capture, light-generated current from a set of pixels of the pixel array is received at a transimpedance amplifier (TIA) that is formed in a common semiconductor substrate with the pixel array. An output signal of the TIA is digitized and signal processed to detect the flicker in the ambient light. Also disclosed are image sensors having pixel arrays with an embedded modulated light source. The modulated light source may be used for proximity detection, either by time-of-flight or intensity variation of reflected light.

Claims:
What is claimed is: 
     
       1. An image sensor, comprising:
 a pixel array, each pixel of the pixel array containing a respective photodiode (PD) electrically connected to respective readout circuit elements of a pixel; and 
 flicker current detection circuitry (FCDC); 
 wherein:
 the pixel array and the FCDC are formed on a common semiconductor substrate; 
 a set of pixels of the pixel array is electrically connected to the FCDC; and 
 the image sensor is operable to detect a flicker in an ambient light by:
 collecting light-generated charge carriers of the respective PDs of the set of pixels; 
 combining, by the FCDC, the light-generated charge carriers of the respective PDs of the set of pixels at the FCDC to form a total light-generated current; and 
 analyzing signal patterns related to the total light-generated current to characterize the flicker in the ambient light. 
 
 
 
     
     
       2. The image sensor of  claim 1 , wherein the FCDC includes a transimpedance amplifier. 
     
     
       3. The image sensor of  claim 1 , wherein the respective light-generated charge carriers of the respective PDs of the set of pixels are received at the FCDC from at least one of respective in-pixel anode or cathode connections of the set of pixels. 
     
     
       4. The image sensor of  claim 3 , wherein the respective in-pixel anode or cathode connections of the set of pixels are located opposite to a light-receiving side of the pixel array. 
     
     
       5. The image sensor of  claim 3 , wherein the respective in-pixel anode or cathode connections of the set of pixels are located on a light-receiving side of the pixel array. 
     
     
       6. The image sensor of  claim 3 , further comprising deep trench isolation walls separating pixels of the pixel array. 
     
     
       7. The image sensor of  claim 6 , wherein:
 the respective PDs of the set of pixels have a section of increasing positive doping gradient adjacent at least to one of the deep trench isolation walls, the section of increasing positive doping gradient extending toward a light-receiving side of the pixel array; and 
 at least one pixel ground connection is positioned on the light-receiving side adjacent to the section of increasing positive doping gradient and is electrically connected to the FCDC. 
 
     
     
       8. The image sensor of  claim 6 , wherein:
 the deep trench isolation walls include conductive material; and 
 the respective in-pixel anode or cathode connections of the set of pixels contact the deep trench isolation walls and are located opposite to a light-receiving side of the pixel array. 
 
     
     
       9. The image sensor of  claim 1 , wherein the total light-generated current is received through reset transistors of the set of pixels. 
     
     
       10. An electronic device comprising:
 a housing having an aperture; and 
 a camera configured to receive light through the aperture; 
 wherein:
 the camera includes a pixel array positioned to receive ambient light through the aperture on a light-receiving side; 
 the pixel array includes flicker current detection circuitry (FCDC) formed with pixels of the pixel array on a common semiconductor substrate; 
 a set of pixels of the pixel array is electrically connected with FCDC; and 
 the electronic device is operable to:
 detect a flicker in the ambient light using a combination of light-generated currents from the set of pixels of the pixel array during a first time interval; and 
 record an image with the camera during a subsequent time interval using compensation for the flicker detected in the ambient light. 
 
 
 
     
     
       11. The electronic device of  claim 10 , wherein the FCDC includes a transimpedance amplifier that is operable to receive the combination of light-generated currents. 
     
     
       12. The electronic device of  claim 11 , wherein the camera further includes:
 an analog-to-digital converter that is operable to receive an output signal of the transimpedance amplifier and produce a digital output signal based on the output signal of the transimpedance amplifier; and 
 a digital signal processor operable to determine frequency components of the flicker in the ambient light based on the digital output signal. 
 
     
     
       13. The electronic device of  claim 12 , wherein the analog-to-digital converter is formed on the common semiconductor substrate. 
     
     
       14. The electronic device of  claim 11 , wherein the FCDC is further operable to:
 detect a fault in a subset of the set of pixels; and 
 electrically exclude the combination of light-generated currents of the subset of the set of pixels of the set of pixels from the combination of light-generated currents during the first time interval. 
 
     
     
       15. The electronic device of  claim 14 , wherein the fault is a current level exceeding a tolerance level. 
     
     
       16. The electronic device of  claim 10 , wherein:
 the set of pixels of the pixel array each is connected to a grid of metallic conductors positioned on the light-receiving side of the pixel array; and 
 pixels of the set of pixels are electrically separated from other pixels of the pixel array by deep trench isolation walls. 
 
     
     
       17. An electronic device comprising:
 a housing having an aperture; and 
 a camera positioned to receive ambient light through the aperture; wherein, the camera includes a pixel array and a light source; 
 the camera is operable to direct the received ambient light onto the pixel array; 
 the light source is operable to emit light from the electronic device toward an exterior object; and 
 the electronic device is operable to detect a proximity to the exterior object based on reflections of the emitted light received on the pixel array through the aperture. 
 
     
     
       18. The electronic device of  claim 17 , wherein:
 the light source is a light-emitting diode; 
 the emitted light is amplitude modulated by a low frequency square wave having successive high and low amplitude periods; and 
 the proximity to the exterior object is based on differences between electrical signals generated at the pixel array during high amplitude periods and low amplitude periods. 
 
     
     
       19. The electronic device of  claim 17 , wherein:
 the light source is a light-emitting diode; 
 the emitted light is amplitude modulated by a high frequency sinusoid wave; and 
 the proximity to the exterior object is based on a phase shift between the high frequency sinusoid wave and a signal generated by the reflections of the emitted light received on the pixel array. 
 
     
     
       20. The electronic device of  claim 19 , wherein:
 the pixel array includes a set of pixels connected to a metallic grid formed on the pixel array; 
 the set of pixels of the pixel array is operable to generate respective currents on the metallic grid from the reflections of the emitted light received on the pixel array; 
 the metallic grid is connected to a transimpedance amplifier formed on the pixel array; and 
 the transimpedance amplifier is operable to receive a combination of the generated respective currents.

Description:
FIELD 
     The present disclosure generally relates to electronic devices with cameras or image sensors that include pixel arrays used for image capture. As described herein, a pixel array may be used to detect a presence of ‘flicker’ in the ambient lighting of the electronic device (e.g., flicker caused by florescent lights). 
     BACKGROUND 
     Electronic devices may include cameras or other image sensors, which cameras or image sensors may include pixel arrays. Examples of such electronic devices include cell phones, tablet or laptop computers, personal digital assistants, and the like. 
     Such cameras or image sensors may obtain images of scenes in which there is ambient lighting that has a ‘flicker’—i.e., a periodic variation in intensity. Flicker may be more common in images of indoor scenes. In some image capture processes of such electronic devices (e.g., a row shutter operation of a pixel array), the flicker may produce horizontal or vertical striations in an acquired image. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description section. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     Disclosed herein are electronic devices, image sensors, and cameras that include a pixel array configured for both image capture and detection of flicker in ambient lighting. Also disclosed are methods of operation of such devices, image sensors, and cameras. 
     More specifically, described herein is an image sensor that includes a pixel array, and flicker current detection circuitry (FCDC) formed on a common semiconductor substrate with the pixel array. Pixels of the pixel array include respective photodiodes (PDs) electrically connected to readout circuit elements of the pixel. A set of pixels of the pixel array is electrically connected to the FCDC. Flicker is detected by electrically isolating the respective PDs of the set of pixels from their respective readout circuit elements, combining, by the FCDC, respective light-generated currents of the respective PDs of the set of pixels at the FCDC to form a total light-generated current, and analyzing signal patterns related to the total light-generated current to characterize the flicker in the ambient light. The FCDC may include a transimpedance amplifier that receives the total light-generated current as input. 
     Also described are electronic devices that include a housing with an aperture, and a camera configured to receive light through the aperture. The camera includes a pixel array positioned to receive ambient light through the aperture on a light receiving side thereof. The pixel array includes flicker current detection circuitry formed with pixels of the pixel array on a common semiconductor substrate. A set of pixels of the pixel array is electrically connected with the FCDC. The electronic device is operable to detect a flicker in an ambient light using a combination of light-generated currents from the set of pixels of the pixel array during a first time interval in which the set of pixels is electrically isolated from respective image readout components, and record an image with the camera during a subsequent time interval using compensation for the detected flicker in the ambient light. The FCDC may include a transimpedance amplifier that is operable to receive the combination of light-generated currents. 
     The present disclosure also describes an electronic device that includes a housing with an aperture and a camera positioned to receive ambient light through the aperture. The camera includes a pixel array and an associated light source integrated into a common system. The camera is operable to receive the ambient light through the aperture and direct the received ambient light onto the pixel array. In some embodiments, the associated light source is operable to emit light through the aperture, whereas in other embodiments, the light source is operable to emit light through an additional aperture of the electronic device. The electronic device is operable to detect a proximity to an exterior object based on reflections of the emitted light received on the pixel array through the aperture. The light source may be a light-emitting diode. The light source may be modulated by a low frequency square wave, and proximity to the exterior object may be detected based on differences between electrical signals generated at the pixel array during the high amplitude periods and the low amplitude periods of the square wave. Alternatively, the light source may be modulated by a high frequency sinusoidal wave, and the proximity to the exterior object is based on a phase shift between the high frequency sinusoid wave and a signal generated by reflections of the emitted light received on the pixel array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements. 
         FIGS.  1 A and  1 B  show an example of an electronic device. 
         FIG.  2    shows an example block diagram of an electronic device. 
         FIG.  3 A  illustrates an example layout of a pixel array. 
         FIG.  3 B  shows example circuit elements of a pixel. 
         FIG.  4 A  shows a flow chart of a method of flicker detection, according to an embodiment. 
         FIG.  4 B  shows a timing diagram of a method for flicker detection, according to an embodiment. 
         FIG.  5 A  shows a circuit diagram and current flow during a flicker detection operation, according to an embodiment. 
         FIG.  5 B  shows a circuit diagram and current flow during a flicker detection operation, according to an embodiment. 
         FIG.  5 C  illustrates connections of a pixel array for flicker detection, according to an embodiment. 
         FIGS.  6 A and  6 B  show cross-sectional views of photodiode sections within pixels of a pixel array, according to an embodiment. 
         FIG.  6 C  shows a cross-sectional view of a photodiode section within a pixel of a pixel array and an electrical connection grid, according to an embodiment. 
         FIGS.  7 A and  7 B  show cross-sectional views of photodiode sections within pixels of a pixel array and electrical connections for flicker detection, according to an embodiment. 
         FIG.  7 C  shows a plan view of a pixel array with connections and circuitry for flicker detection, according to an embodiment. 
         FIG.  8 A  shows a cross-sectional view of a photodiode section within a pixel of a pixel array with electrical connections at deep trench isolation walls, according to an embodiment. 
         FIG.  8 B  shows a circuit diagram for certain components of a pixel of a pixel array, according to an embodiment. 
         FIG.  9 A  shows a cross-sectional view of components of a pixel of a pixel array, according to an embodiment. 
         FIG.  9 B  shows a conduction band profile of the pixel of  FIG.  9 A , according to an embodiment. 
         FIG.  10 A  shows a configuration of an image sensor of an electronic device for proximity sensing, according to an embodiment. 
         FIG.  10 B  shows a configuration of an image sensor of an electronic device for proximity sensing, according to an embodiment. 
     
    
    
     The use of cross-hatching or shading in the accompanying figures is generally provided to clarify the boundaries between adjacent elements and also to facilitate legibility of the figures. Accordingly, neither the presence nor the absence of cross-hatching or shading conveys or indicates any preference or requirement for particular materials, material properties, element proportions, element dimensions, commonalities of similarly illustrated elements, or any other characteristic, attribute, or property for any element illustrated in the accompanying figures. 
     Additionally, it should be understood that the proportions and dimensions (either relative or absolute) of the various features and elements (and collections and groupings thereof) and the boundaries, separations, and positional relationships presented therebetween, are provided in the accompanying figures merely to facilitate an understanding of the various embodiments described herein and, accordingly, may not necessarily be presented or illustrated to scale, and are not intended to indicate any preference or requirement for an illustrated embodiment to the exclusion of embodiments described with reference thereto. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims. 
     The embodiments described herein are directed to image sensors, cameras, and electronic devices that contain such image sensors and cameras. The image sensors or cameras may contain an array of light-gathering pixels, hereafter a ‘pixel array.’ For simplicity of discussion, hereinafter, ‘image sensor’ will also refer to cameras or other electronic devices having a pixel array as a component. 
     During an image or picture capture operation, received light is directed by the image sensor onto the pixel array. A pixel of the pixel array may contain a photodiode (PD) that generates charges (holes and electrons) in response to impinging photons in the received light. Additional electrical components within the pixel, such as reset, gating, and other transistors, may control the flow of the light-generated charges onto pixel output lines during an image read operation, in which the amount of light-generated charge in the photodiode is processed to infer a part of the image to be obtained in the image capture operation. The photodiodes and transistors of the pixels of the pixel array may be fabricated in various semiconductor technologies, such as any of the insulated gate technologies NMOS, PMOS, or CMOS. Various embodiments may use other semiconductor technologies. 
     Ambient light in the environment of the electronic device may have a time-varying intensity called ‘flicker.’ Sources of such flicker include fluorescent lighting, which may have a flicker frequency of 120 Hz, twice the power line frequency. Other sources of flicker may also be present in the ambient light. Flicker may cause artifacts in an image obtained by a pixel array of an image sensor. For example, an image sensor may use a ‘rolling shutter’ image capture operation, in which only a few rows of pixels of a pixel array at a time capture light-generated charges, to allow for concurrent row readout operations of light-generated charges in previously exposed rows of pixels. This may produce horizontal (or vertical) streaking in the total captured image. Other artifacts of flicker in the ambient light may also occur, and may occur in other types of image capture operations. If the flicker signal, such as its frequency components, amplitude, and/or phase, are known before an image capture operation, appropriate compensation steps may be used in the image capture operation. 
     In some electronic devices, flicker detection may be accomplished through use of an auxiliary sensor. However, this may add extra components, area, and complexity and require additional power consumption. 
     Instead, certain embodiments disclosed herein implement flicker detection by using the pixel array of the image sensor itself. The pixel arrays in such image sensors have embedded flicker current detection circuitry (FCDC) formed with the pixels of the pixel array on a common semiconductor substrate. A certain subset of the pixels of the pixel array (hereafter ‘set of pixels of the pixel array,’ or just ‘set of pixels’) is electrically connected to the FCDC. In some embodiments, the set of pixels of the pixel array may include all pixels of the pixel array. 
     During a flicker detection operation prior to an image capture operation, the set of pixels of the pixel array may be configured to operate as a photocurrent source whose current is used as an input to the FCDC. More specifically, in some embodiments certain readout circuit elements of the pixels in the set of pixels may be disabled so that little or no light-generated response is output to the image sensor&#39;s readout circuit lines. During the flicker detection operation, light-generated charges may flow into in-pixel connections and onto certain circuit lines on the pixel array, which may be dedicated for flicker detection. The light-generated charges of the set of pixels may be combined into a single current, termed herein the ‘flicker current.’ 
     The FCDC may include a transimpedance amplifier (TIA) that receives the flicker current and generates an output signal which may drive further circuit elements. The FCDC may be positioned to detect the flicker current either at a junction proximate to the high supply voltage of the pixel array, V DD , or at a junction proximate to the ground supply of the pixel array. 
     In various embodiments, the pixels of the pixel array may be configured for better detection of the flicker current. For example, deep trench isolation (DTI) walls may be formed around each, or some, pixel of the pixel array, though in some embodiments shallow trench isolation may be used. Also, the photodiodes of the pixels may be doped to provide an in-pixel ground connection. 
     The output signal of the TIA may be received by an analog-to-digital converter (ADC) that produces quantized samples of the output signal at discrete time steps. The quantized samples may then be used by a digital signal processor to infer, such by using a fast Fourier transform, the frequency components of the flicker current, which are related to the frequencies of the flicker in the received ambient light. The ADC and/or the digital signal processor may also be formed on the same semiconductor substrate as the pixel array, or may be formed on a separate chip or substrate, and may be part of other components or processors of an electronic device containing the image sensor. 
     Additionally and/or alternatively, some embodiments of image sensors may include a light source associated with the pixel array. The light source may be a light-emitting diode (LED), such as a laser LED. In some embodiments, the light source may be embedded in the pixel array, a camera or image sensor may use a single aperture, such as with a lens, both for emission of light from the embedded light source and for receiving ambient light for an image capture operation. In other embodiments, the light source may be a component separate from the pixel array, and may either emit its light through the single aperture, or emit its light through an additional aperture. The light source may be separate from the image sensor and camera, and have its own lens and/or optical system. 
     The light source may be modulated to provide methods for detection of proximity to an object in the environment of the electronic device. The light source may be modulated with a square wave in which the intensity of its emitted light alternates between a high and low value. The low value need not be a zero intensity, but may be. Reflections of the emitted light may be received on the pixel array, and proximity detection may make use of signals of the FCDC within the pixel array. Either the whole pixel array itself or a set of pixels of the pixel array may be used to detect changes in intensity of received reflections of the emitted light. The detected changes may then be used to infer a proximity to the object. 
     In other embodiments, the light source may have the intensity of its emitted light modulated with a sinusoid wave. Changes in phase, or of other parameters, between the emitted light and received reflections thereof on the pixel array may be used to infer a time-of-flight, from which proximity to the object may be inferred. 
     These and other embodiments are discussed below with reference to  FIGS.  1 A- 10 B . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. 
     Other examples and implementations are within the scope and spirit of the disclosure and appended claims. For example, features implementing functions may also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. Also, as used herein, including in the claims, “or” as used in a list of items prefaced by “at least one of” indicates a disjunctive list such that, for example, a list of “at least one of A, B, or C” means A or B or C or AB or AC or BC or ABC (i.e., A and B and C). Further, the term “exemplary” does not mean that the described example is preferred or better than other examples. 
     Directional terminology, such as “top”, “bottom”, “upper”, “lower”, “front”, “back”, “over”, “under”, “above”, “vertical”, “horizontal”, “below”, “left”, “right”, etc. is used with reference to the orientation of some of the components in some of the figures described herein. Because components in various embodiments can be positioned in a number of different orientations, directional terminology is used for purposes of illustration and is not always limiting. Directional terminology is intended to be construed broadly, and therefore should not be interpreted to preclude components being oriented in different ways. Also, as used herein, the phrase “at least one of” preceding a series of items, with the term “and” or “or” to separate any of the items, modifies the list as a whole, rather than each member of the list. The phrase “at least one of” does not require selection of at least one of each item listed; rather, the phrase allows a meaning that includes at a minimum one of any of the items, and/or one of any combination of the items, and/or one of each of the items. By way of example, the phrases “at least one of A, B, and C” or “at least one of A, B, or C” each refer to only A, only B, or only C; any combination of A, B, and C; and/or one or more of each of A, B, and C. Similarly, it may be appreciated that an order of elements presented for a conjunctive or disjunctive list provided herein should not be construed as limiting the disclosure to only that order provided. 
       FIGS.  1 A and  1 B  show an example of a device  100  that may include an illumination projector. The device&#39;s dimensions and form factor, including the ratio of the length of its long sides to the length of its short sides, suggest that the device  100  is a mobile phone (e.g., a smartphone). However, the device&#39;s dimensions and form factor are arbitrarily chosen, and the device  100  could alternatively be any portable electronic device including, for example, a mobile phone, tablet computer, portable computer, audible device, portable music player, wearable device (e.g., an electronic watch, health monitoring device, or fitness tracking device), augmented reality (AR) device, virtual reality (VR) device, mixed reality (MR) device, gaming device, portable terminal, digital single-lens reflex (DSLR) camera, video camera, vehicle navigation system, robot navigation system, or other portable or mobile device. The device  100  could also be a device that is semi-permanently located (or installed) at a single location.  FIG.  1 A  shows a front isometric view of the device  100 , and  FIG.  1 B  shows a back isometric view of the device  100 . The device  100  may include a housing  102  that at least partially surrounds a display  104 . The housing  102  may include or support a front cover  106  that defines a front surface of the device  100 , and/or a back cover  108  that defines a back surface of the device  100  (with the back surface opposite the front surface). More generically, the device  100  may include one or more “covers.” The front cover  106  may be positioned over the display  104 , and may provide a window through which the display  104  may be viewed. In some embodiments, the display  104  may be attached to (or abut) the housing  102  and/or the front cover  106 . In alternative embodiments of the device  100 , the display  104  may not be included and/or the housing  102  may have an alternative configuration. 
     The display  104  may include one or more light-emitting elements, and in some cases may be a light-emitting diode (LED) display, an organic LED (OLED) display, a liquid crystal display (LCD), an electroluminescent (EL) display, a thin film transistor (TFT) display, or another type of display. In some embodiments, the display  104  may include, or be associated with, one or more touch and/or force sensors that are configured to detect a touch and/or a force applied to a surface of the front cover  106 . 
     The various components of the housing  102  may be formed from the same or different materials. For example, a sidewall  118  of the housing  102  may be formed using one or more metals (e.g., stainless steel), polymers (e.g., plastics), ceramics, or composites (e.g., carbon fiber). In some cases, the sidewall  118  may be a multi-segment sidewall including a set of antennas. The antennas may form structural components of the sidewall  118 . The antennas may be structurally coupled (to one another or to other components) and electrically isolated (from each other or from other components) by one or more non-conductive segments of the sidewall  118 . The front cover  106  may be formed, for example, using one or more of glass, a crystal (e.g., sapphire), or a transparent polymer (e.g., plastic) that enables a user to view the display  104  through the front cover  106 . In some cases, a portion of the front cover  106  (e.g., a perimeter portion of the front cover  106 ) may be coated with an opaque ink to obscure components included within the housing  102 . The back cover  108  may be formed using the same material(s) that are used to form the sidewall  118  or the front cover  106 . In some cases, the back cover  108  may be part of a monolithic element that also forms the sidewall  118  (or in cases where the sidewall  118  is a multi-segment sidewall, those portions of the sidewall  118  that are conductive or non-conductive). In still other embodiments, all of the exterior components of the housing  102  may be formed from a transparent material, and components within the device  100  may or may not be obscured by an opaque ink or opaque structure within the housing  102 . 
     The front cover  106  may be mounted to the sidewall  118  to cover an opening defined by the sidewall  118  (i.e., an opening into an interior volume, in which various electronic components of the device  100 , including the display  104 , may be positioned). The front cover  106  may be mounted to the sidewall  118  using fasteners, adhesives, seals, gaskets, or other components. 
     A display stack or device stack (hereafter referred to as a “stack”) including the display  104  may be attached (or abutted) to an interior surface of the front cover  106  and extend into the interior volume of the device  100 . In some cases, the stack may include a touch sensor (e.g., a grid of capacitive, resistive, strain-based, ultrasonic, or other type of touch sensing elements), or other layers of optical, mechanical, electrical, or other types of components. In some cases, the touch sensor (or part of a touch sensor system) may be configured to detect a touch applied to an outer surface of the front cover  106  (e.g., to a display surface of the device  100 ). 
     In some cases, a force sensor (or part of a force sensor system) may be positioned within the interior volume above, below, and/or to the side of the display  104  (and in some cases within the device stack). The force sensor (or force sensor system) may be triggered in response to the touch sensor detecting one or more touches on the front cover  106  (or a location or locations of one or more touches on the front cover  106 ), and may determine an amount of force associated with each touch, or an amount of force associated with a collection of touches as a whole. In some embodiments, the force sensor (or force sensor system) may be used to determine a location of a touch, or a location of a touch in combination with an amount of force of the touch. In these latter embodiments, the device  100  may not include a separate touch sensor. 
     As shown primarily in  FIG.  1 A , the device  100  may include various other components. For example, the front of the device  100  may include one or more front-facing cameras  110 , speakers  112 , microphones, or other components  114  (e.g., audio, imaging, and/or sensing components) that are configured to transmit or receive signals to/from the device  100 . In some cases, a front-facing camera  110 , alone or in combination with other sensors, may be configured to operate as a bio-authentication or facial recognition sensor. The device  100  may also include various input devices, including a mechanical or virtual button  116 , which may be accessible from the front surface (or display surface) of the device  100 . In some embodiments, a virtual button  116  may be displayed on the display  104  and, in some cases, a fingerprint sensor may be positioned under the button  116  and configured to image a fingerprint through the display  104 . In some embodiments, the fingerprint sensor or another form of imaging device may span a greater portion, or all, of the display area. 
     The device  100  may also include buttons or other input devices positioned along the sidewall  118  and/or on a back surface of the device  100 . For example, a volume button or multipurpose button  120  may be positioned along the sidewall  118 , and in some cases may extend through an aperture in the sidewall  118 . In other embodiments, the button  120  may take the form of a designated and possibly raised portion of the sidewall  118 , but the button  120  may not extend through an aperture in the sidewall  118 . The sidewall  118  may include one or more ports  122  that allow air, but not liquids, to flow into and out of the device  100 . In some embodiments, one or more sensors may be positioned in or near the port(s)  122 . For example, an ambient pressure sensor, ambient temperature sensor, internal/external differential pressure sensor, gas sensor, particulate matter concentration sensor, or air quality sensor may be positioned in or near a port  122 . 
     In some embodiments, the back surface of the device  100  may include a primary rear-facing camera  124  that includes one or more image sensors (see  FIG.  1 B ). In some cases, the device  100  may have a second imaging sensor  126 , which may be an autofocus camera, a telephoto camera, a second camera used in conjunction with the rear-facing camera  124 —such as to provide depth or 3D imaging—or another optical sensor. The device  100  may also have a flash or light source that may be positioned on the back of the device  100  (e.g., near the rear-facing camera). In some cases, the back surface of the device  100  may include multiple rear-facing cameras. Either or both the primary rear-facing camera  124  and the second imaging sensor may include a pixel array with embedded flicker current detection circuitry, as described below. 
       FIG.  2    shows an example block diagram of an electronic device  200 , which in some cases may be the electronic device described with reference to  FIGS.  1 A and  1 B , or another type of electronic device including one or more of the image sensors having one or more pixel arrays as described herein. The electronic device  200  may include an electronic display  202  (e.g., a light-emitting display), a processor  204 , a power source  206 , a memory  208  or storage device, a sensor system  210 , an input/output (I/O) mechanism  212  (e.g., an input/output device, input/output port, or haptic input/output interface), and/or an illumination projector  214 . The processor  204  may control some or all of the operations of the electronic device  200 . The processor  204  may communicate, either directly or indirectly, with some or all of the other components of the electronic device  200 . For example, a system bus, other bus(es), or other communication mechanism  216  can provide communication between the electronic display  202 , the processor  204 , the power source  206 , the memory  208 , the sensor system  210 , the I/O mechanism  212 , and the illumination projector  214 . 
     The processor  204  may be implemented as any electronic device capable of processing, receiving, or transmitting data or instructions, whether such data or instructions is in the form of software or firmware or otherwise encoded. For example, the processor  204  may include a microprocessor, a central processing unit (CPU), an application-specific integrated circuit (ASIC), a digital signal processor (DSP), a controller, or a combination of such devices. As described herein, the term “processor” is meant to encompass a single processor or processing unit, multiple processors, multiple processing units, or other suitably configured computing element or elements. In some cases, the processor  204  may provide part or all of the processing system or processor described herein. 
     It should be noted that the components of the electronic device  200  can be controlled by multiple processors. For example, select components of the electronic device  200  (e.g., the sensor system  210 ) may be controlled by a first processor and other components of the electronic device  200  (e.g., the electronic display  202 ) may be controlled by a second processor, where the first and second processors may or may not be in communication with each other. 
     The power source  206  can be implemented with any device capable of providing energy to the electronic device  200 . For example, the power source  206  may include one or more batteries or rechargeable batteries. Additionally or alternatively, the power source  206  may include a power connector or power cord that connects the electronic device  200  to another power source, such as a wall outlet. 
     The memory  208  may store electronic data that can be used by the electronic device  200 . For example, the memory  208  may store electrical data or content such as, for example, audio and video files, documents and applications, device settings and user preferences, timing signals, control signals, instructions, and/or data structures or databases. The memory  208  may include any type of memory. By way of example only, the memory  208  may include random access memory, read-only memory, Flash memory, removable memory, other types of storage elements, or combinations of such memory types. 
     The electronic device  200  may also include one or more sensor systems  210  positioned almost anywhere on the electronic device  200 . The sensor system(s)  210  may be configured to sense one or more types of parameters, such as but not limited to, vibration; light; touch; force; heat; movement; relative motion; biometric data (e.g., biological parameters) of a user; air quality; proximity; position; connectedness; surface quality; and so on. By way of example, the sensor system(s)  210  may include a heat sensor, a position sensor, a light or optical sensor, a self-mixing interferometry (SMI) sensor, an image sensor (e.g., one or more of the image sensors or cameras described herein), an accelerometer, a pressure transducer, a gyroscope, a magnetometer, a health monitoring sensor, an air quality sensor, and so on. Additionally, the one or more sensor systems  210  may utilize any suitable sensing technology, including, but not limited to, interferometric, magnetic, capacitive, ultrasonic, resistive, optical, acoustic, piezoelectric, or thermal technologies. 
     In particular, the sensor system(s)  210  of the electronic device  200  may include one or more cameras, or other types of image sensors or active optical sensors, that include pixel arrays having embedded flicker current detection circuitry or components as described herein, and which may be operated or controlled, such as by the processor  204 . 
     The I/O mechanism  212  may transmit or receive data from a user or another electronic device. The I/O mechanism  212  may include the electronic display  202 , a touch sensing input surface, a crown, one or more buttons (e.g., a graphical user interface “home” button), one or more microphones or speakers, one or more ports such as a microphone port, and/or a keyboard. Additionally or alternatively, the I/O mechanism  212  may transmit electronic signals via a communications interface, such as a wireless, wired, and/or optical communications interface. Examples of wireless and wired communications interfaces include, but are not limited to, cellular and Wi-Fi communications interfaces. 
     The illumination projector  214  may be configured as described with reference to  FIGS.  1 A and  1 B  and elsewhere herein, and in some cases may be integrated or used in conjunction with one or more of the sensor system(s)  210 . For example, the illumination projector  214  may illuminate an object or scene, and light that reflects or scatters from the object or scene may be sensed by a light or optical sensor, an SMI sensor, or an image sensor (e.g., one or more of the image sensors or cameras described herein). In some embodiments, an illumination projector  214  may be part of a sensor system  210 . 
       FIG.  3 A  shows a plan view (e.g., a top view) of a section of a pixel array  300  that may be a component of an optical sensor, such as the camera  124  or the imaging sensor  126 . The pixel array  300  may be configured as a rectangular array, with a first row containing the individual pixels  302   a - d . A second row of the pixel array  300  includes the pixel  304   a  in the same column as pixel  302   a  of the first row, and pixel  304   b  in the same column as pixel  302   b  of the first row. The rows and columns of the pixel array  300  may extend to form an M×N array of M rows and N columns, for M and N large integers. In some embodiments, M and N may be on the order of 10 3  or more. One skilled will recognize that, in other embodiments, alternate geometric configurations for the pixels of pixel array  300  are possible, such as a hexagonal array of pixels arranged in an area-filling configuration with shifted rows. The pixel array  300  may include embedded flicker current detection circuitry or components, as described herein. The pixel array  300 , and the pixel arrays as described herein, may be formed or configured either to be as backside illuminated or frontside illuminated. However, as these terms refer how the semiconductor chip containing the pixel array is fabricated, herein the terms ‘on’ or ‘opposite’ to a ‘light-receiving side’ will be used. 
       FIG.  3 B  shows an example equivalent circuit of certain electrical components and configurations of a pixel  310 , such as may be part of the pixel array  300 . One skilled in the art will recognize that a pixel of the embodiments of pixel arrays described herein may have alternate configurations. Also, one skilled in the art will recognize that the various transistors of the pixels shown and described below may be implemented in any of various NMOS, PMOS, or CMOS semiconductor technologies. For simplicity of description, the following descriptions will be for pixels and pixel arrays implemented in CMOS technologies. 
     The pixel  310  includes a semiconductor photodiode (PD)  312 . The pixel array  300  containing the pixel  310  may be configured within a camera or image sensor of an electronic device, such as camera  124  or image sensor  126  of electronic device  100  described above, so that light from the environment, such as during an image capture operation, is received on the pixel array  300 , and on the photodiode  312  in particular. The photons of the light received on the PD  312  induce charge carriers, which may be either electrons or holes, to accumulate within the PD  312  during an exposure period of the image capture operation. 
     The transfer gate (TG) transistor  314  may be configured (by the appropriate voltage applied at its gate) as ‘off’ or ‘open circuit’ during the exposure period. At the end of the exposure period, the TG transistor  314  may be triggered as ‘on’ or ‘closed circuit’ (another corresponding voltage applied to its gate), to allow the accumulated charge carriers to flow to the floating diffusion node (FD)  316 . The TG transistor  314  may then be returned to the ‘open circuit’ state, such as at the conclusion of the exposure period. A reset (RST) transistor  318 , when closed by the appropriate voltage applied to its gate, may provide a path for light-generated charges to be cleared into the supply voltage line V DD    319 . 
     The pixel  310  includes at least two readout circuit elements: the source follow (SF) transistor  320  and the row select (RS) transistor  322  connected in a cascode structure, with the source of the RS transistor connected to the current source  324 . During a readout operation of the pixel array, the RS transistor  322  may be triggered so that the light-generated charge at the FD node  316  may cause a corresponding signal in the output of the SF transistor  320 . The output signal of pixels for the image capture operation may then be taken from the source of the RS transistor  322 . 
       FIG.  4 A  is a flow chart of an exemplary operation  400  for detecting flicker in an ambient light of the environment of an image sensor that contains a pixel array, such as a pixel array with pixels as described in relation to  FIGS.  3 A and  3 B . The operation for detecting flicker may take place prior to an image capture operation so that appropriate compensation techniques may be applied during the image capture operation. 
     At stage  402  of the flicker detection operation  400 , output elements of a set of pixels of the pixel array is disabled so that little or no light-generated charge carriers flow to image signal output connections of the pixels in the set of pixels. The set of pixels of the pixel array may be either all the pixels of the pixel array, or alternatively, a proper subset of all the pixels of the pixel array. For example, for the case of the pixel  310 , in some embodiments the disabled output elements include the RS transistor  322  and the SF transistor  320 , but the TG transistor  314  is closed to allow light-generated charge carriers to flow through the RST transistor  318  and be detected, as described below. In this way, the light-generated charge carriers of the set of pixels of the pixel array used for flicker detection may operate as a single current source of the flicker detection operation  400 . Depending on which of the various embodiments described below is used to detect the current, the detected light-generated charge carriers may be either holes (h + ) or electrons (e − ). Using multiple pixels in the set of pixels as a single photocurrent source may improve the signal-to-noise ratio by averaging the variation in charge carrier generation of each pixel, as well as noise due to thermal charge carrier generation and other noise sources. 
     At stage  404 , a transimpedance amplifier (TIA) receives the combined currents from the set of pixels of the pixel array of the image sensor. In various embodiments, the TIA may be fabricated or formed on the same semiconductor wafer or chip as the pixel array. This may allow the pixel array to be used, either concurrently or non-concurrently, both for flicker detection as well as image capture. Also, having the TIA on the same semiconductor chip as the pixel array may allow for an electronic device with the image sensor to avoid the need for a separate light sensor dedicated to flicker detection. 
     The TIA may function to produce an output signal related to the received combined currents of the set of pixels. The output signal may be a voltage, current, or other parameter signal, and have sufficient strength to drive subsequent components that provide analysis of the signal or signals within the combined currents. 
     At stage  406 , a high dynamic range analog-to-digital converter (ADC) receives an output signal of the TIA and produces digital output values at discrete time intervals. The sampling time period, T S , is taken to satisfy the Nyquist criterion of being at least twice the expected highest frequency component in the output signal of the TIA. As an example, fluorescent lights flicker at approximately 120 Hz, so the sampling frequency should be at least 240 Hz, or higher, to include harmonics or for better resolution. If flicker in the ambient light is expected to be from sources with a higher flicker frequency, the sampling frequency of the ADC may be chosen accordingly. In some embodiments, the sampling frequency of the ADC may be adjustable. The ADC may be on the same semiconductor chip as the pixel array and TIA, or may be a separate component, or part of another system, such as a processor. 
     At stage  408 , the number of bits used for the digital output values of the ADC may be chosen based on the expected strength of the flicker signal and also the ambient light. The number of bits may be adjustable in operation. The digital output signal of the ADC may then be used, such as by a digital signal processing (DSP) unit, to determine the presence or strength of a flicker signal in the ambient light. The DSP unit may be on the same semiconductor chip as the pixel array and FCDC, a component of the image sensor, or a separate component. The DSP may be able to perform various signal processing functions, such as fast Fourier transforms and filtering, which may be performed in hardware sections, or by received programs, such as in a field programmable gate array section. The DSP may a component of a processing unit of the electronic device that controls multiple aspects of image capture and user interface operations. 
       FIG.  4 B  shows a time sequence  410  of enabling signals with respect to the time axis  411  that show a flicker detection operation, such as the flicker detection operation  400 , used in conjunction with an image capture operation. 
     In the first time interval  413  from time T 0  to T 1 , the set of pixels of the pixel array and/or the FCDC is enabled by the flicker sampling enable signal  412  to perform flicker current detection. During the first time interval  413 , other operations may be disabled. 
     In the second time interval  415  from time T 1  to T 2 , a flicker processing signal  414  enables digital signal processing, such as an FFT and determination of exposure parameters of the camera, to occur on the quantized and sampled output of the ADC. The results, such as flicker frequencies or other parameters, may be used during the start of the image sensor power signal  416  that prepares the image sensor for an image capture operation. The image sensor power signal  416  may begin after the end of the flicker processing signal  414  at time T 1  and before the time T 2 , and continues until the end of the image capture operation at time T 4 . 
     In the third time interval  417  from time T 2  to T 3 , a frame capture signal  418  enables the pixel array to obtain an image frame. The captured image frame may be a frame in a video sequence. The image capture operation may use a rolling shutter or global shutter operation applied to the pixel array. 
     In the fourth time interval  419  from time T 3  to T 4 , a frame readout enabling signal  420  enables readout circuitry of a pixel array to obtain receive readouts, such as of light-generated charges, from the pixels of the pixel array, that may be used to produce the total image. 
       FIG.  5 A  shows an exemplary equivalent circuit diagram  500  for either a single pixel in which there are four PDs, PD1-4,  502   a - d , or for four pixels with a common connection  505 . In the former case, there may be a common reset transistor (RST)  508  with the drain connected to the supply voltage V DD    509 , a common charge storage capacitor  506 , which may be a common FD node, connected to the gate of a common source follower transistor (SF), and a common row-select (RS) transistor with the drain connected to the SF transistor  510 . In the latter case, for each pixel, there may be an individual respective RST transistor, FD node, SF transistor, and RS transistor for each pixel. Each of PD1-4,  502   a - d , has its cathode connected to a respective transfer gate (TG) transistor  504   a - d , which control the flow of charges onto the storage capacitor(s)  506 . 
     During a readout stage of an image capture operation, light-generated charges captured in PD1-4,  502   a - d , and transferred over the connection line to the charge storage capacitor(s)  506  can induce a related output current from the source of the SF transistor  510  through the connection line  511   a  into the drain of the RS transistor  512  and onto the output signal line  511   b . The related output current may be buffered or amplified by the system readout circuitry  514 . 
     In the bias configuration shown in  FIG.  5 A , the RS transistor  512  has its gate biased at a low voltage so that little or no current flows onto the output signal line  511   b . Instead, the gate of the RST transistor  508  and the TG transistors  504   a - d  are set low. In this bias configuration, light-generated charge carriers collected from the PDs  502   a - d  are holes, and flow to respective in-pixel anode or cathode connections, which may be at a ground, as described in relation to  FIGS.  6 A-C  and  FIGS.  7 A-B . This hole current flow  511  is to the receiving connection V SS_PIX    516 . In the bias configuration of  FIG.  5 A , the pixel array may also be used for concurrent capture of an image and for flicker detection. 
       FIG.  5 B  shows an exemplary equivalent circuit diagram  520  for either a single pixel in which there are four PDs, PD1-4,  502   a - d , or for four pixels with a common connection  505 . Like numbered components are as described in  FIG.  5 A . In the bias configuration shown in FIG.  5 B, the RS transistor  512  has its gate still biased at a low voltage so that little or no current flows onto the output signal line  511   b . Instead, in this alternate bias configuration, the gate of the RST transistor  508  and the TG transistors  504   a - d  are set to a high voltage V H . In this bias configuration, the light-generated charge carriers in the PDs  502   a - d  are electrons, and the resulting electron current flow is to the drain of the RST transistor  508  and then to the supply voltage V DD    509 . 
       FIG.  5 C  shows a circuit diagram  530  for an exemplary connection of the set of pixels of a pixel array to FCDC. For discussion purposes, four rows of pixels  532   a - d  are indicated, though embodiments may contain more, and individual connections  533   a - d  connect to four pixels of the row  532   a . Similar considerations apply to rows  532   b - d . During a flicker current detection operation, the pixels of the row  532   a  are enabled to produce currents from light-generated charges in the PDs, which may be either holes as described in relation to  FIG.  5 A , or electrons as described in relation to  FIG.  5 B . Each of rows  532   a - d  may connect to respective complementary pairs of control transistors  535   a - d  and  538   a - d , which may function to allow currents from the pixels of the respective rows to flow either to the FCDC or be shunted to ground. An FCDC enabling transistor  534  may disable current flow to the FCDC except during a flicker detection operation. 
     In the situation of  FIG.  5 C , a fault or other problem has occurred in one or more pixels of the row  532   c , leading to currents from one or more pixels in row  532   c  to exceed a fault indication threshold value. From design considerations or initial testing, a range of expected values for the light-generated current from the pixels in a row may be known, so values outside the range may indicate a fault, such as a short circuit. Light-generated current values outside the expected range may mask variations in the light-generated current due to flicker in the ambient light. In such a situation the gate of the control transistor  535   c  may be turned off to prevent current flow to the FCDC during a flicker detection operation. An image sensor or electronic device may test a pixel array for faults on a periodic basis. 
     During a flicker detection operation, light-generated currents from the fault-free rows are combined at the common connection point  537  into a single current termed herein the ‘flicker current.’ The FCDC enabling transistor  534  is then disabled to allow the flicker current to be transmitted to the FCDC. 
       FIGS.  6 A-C  show cross-sections of respective embodiments of PDs  600 ,  620 , and  630 , of pixels that may be included in pixel arrays operable for flicker detection operations described herein. 
       FIG.  6 A  shows a cross-section of a PD  600  of a pixel, such as may be a pixel of a pixel array with FCDC. Other components of the pixel, such as the various in-pixel transistors, are not shown for simplicity of explanation. The PD  600  is formed in a lightly p-type doped substrate  602 , forming the cathode region of the PD  600 . An n-type doped section  603  may be formed in the substrate  602 , such as by deposition, to form the anode of the PD  600 . The pixel with PD  600  is electrically isolated from other pixels in the pixel array by deep trench isolation (DTI) walls  610   a - b . One skilled in the art will recognize that the DTI walls  610   a - b  may be connected and surround the pixel with PD  600 . 
     Additionally, for flicker current detection based on holes as described in relation to  FIG.  5 A , an additional heavily doped p-type section  604  may be formed in the substrate  602 . The heavily doped p-type section  604  is connected to ground contact  606 . Charge carriers in the PD  600  generated by received light  612 , which are holes in the bias configuration of  FIG.  5 A , are collected at the heavily doped p-type section  604  and flow to and then through the ground contact  606 . The hole current  608  is then received by the FCDC. 
       FIG.  6 B  shows a cross-section of a PD  620  of a pixel with an alternative embodiment of the PD  600  of  FIG.  6 A . The PD  620  instead is formed with shallow trench isolation (STI) walls  622   a  and  622   b . The STI walls  622   a - b  may be connected and surround the components of the pixel containing the PD  620  to provide electrical isolation from other pixels of the pixel array. The other indicated regions and components of the PD  620  shown in  FIG.  6 B  are as described above in relation to  FIG.  6 A . 
       FIG.  6 C  shows a cross-section of a PD  630  with an alternative embodiment of the PD  600  of  FIG.  6 A . The elements of the PD  630  with like numbers to those of  FIG.  6 A  are as described for the PD  600  of  FIG.  6 A . The PD  630  is formed with a doping gradient  632  of p-type doping increasing toward the light receiving side. The doping gradient  632  may provide passivation and may improve charge transfer. 
     The pixel that includes the PD  630  is connected to a metallic grid, with cross-sections of grid lines  634   a  and  634   b  shown, on the light-receiving side of the pixel array. The metal of the metallic grid may be tungsten in some embodiments. The metallic grid may serve to reduce cross-talk in pixel arrays configured for red-green-blue (RGB) color reception. An interlayer dielectric (not shown for clarity) may separate the grid lines  634   a  and  634   b  from the substrate  602  of the pixel array. A via  635  provides electrical contact between the grid line  634   b  and the region of the PD  630  of highest p-type doping. During a flicker current detection operation, the grid line  634   b  may receive some of the light-generated holes  636 , but hole current  608  may still be collected at the heavily p-type doped section  604 . 
       FIGS.  7 A and  7 B  show two configurations  700  and  720  for connection of a PD section of a pixel to components of a FCDC. The two configurations each allow for the pixel to be operated either for image capture and for flicker detection. 
     The configuration  700  of  FIG.  7 A  shows a cross-section of a PD region of a pixel that is included in a pixel array having embedded FCDC. The configuration  700  is based on the embodiment described in relation to  FIG.  6 A . The pixel is formed in a p-type doped substrate  702 , which may be part of a common semiconductor substrate of the pixel array, and that also forms the cathode section of the PD region of the pixel. An n-type doped region  703  forms the anode region of the PD, and is connected to output circuit elements of the pixel, such as a TG transistor (not shown in the cross-section view of  FIG.  7 A ). The pixel has an additional heavily p-type doped region  704 . The heavily p-type doped region  704  is connected to ground contact  706 . Charge carriers are generated in the PD section of the pixel when illuminated by received light  712 . In the bias configuration of  FIG.  5 A , holes are collected at the heavily doped p-type section  704 , and flow to and then through the ground contact  706 . There are DTI walls  710   a  and  710   b  which may be connected and surround the pixel to provide electrical isolation from other pixels of the pixel array. 
     In the configuration  700 , the pixel array is also connected to a metallic grid with grid lines  718   a  and  718   b . The metallic grid lines  718   a  and  718   b  may be as described in relation to the metallic grid lines  634   a - b  of  FIG.  6 C , and may be separated from the substrate  702  by an interlayer dielectric material (not shown for clarity). The metallic grid line  718   b  connects to the substrate  702  through the via  719 . 
     During an image capture operation, a flicker disable transistor  714  allows the current  708  to flow to ground. But during a flicker current detection operation, the flicker disable transistor  714  is gated open so that light-generated current  708  is then received at the transimpedance amplifier  716 . The current  708  may be combined with currents from a set of pixels of the pixel array to form the net flicker current for the net input to the transimpedance amplifier  716 . The flicker disable transistor  714 , the transimpedance amplifier  716 , and other elements of the FCDC may be embedded in the common semiconductor substrate  702  of the pixels of the pixel array. 
     The configuration  720  of  FIG.  7 B  shows a cross-section of a PD region of a pixel that is included in a pixel array having embedded FCDC. The configuration  720  is a variation of the embodiment described in relation to  FIG.  7 A  with like numbered elements as described in relation to  FIG.  7 A . But in the configuration  720  of  FIG.  7 B , the PD region has a p-type doping gradient as described in relation to  FIG.  6 C . 
     In the configuration  720 , during a flicker current detection operation, the FCDC receives the flicker current  709  induced in the PD region by received light  712  through the metallic grid, with the metallic grid lines  718   a  and  718   b , positioned on the light-receiving side of the pixel array. In the configuration  720 , the flicker disable transistor  714  functions as described in relation to  FIG.  7 A  either to shunt the flicker current  709  from the input of the transimpedance amplifier  716 , or to allow it to be joined with other flicker currents of a set of pixels of the pixel array as an input to the transimpedance amplifier  716 . 
       FIG.  7 C  shows a configuration  730  in which a certain set of pixels of a pixel array is connected to act as a common current source for input to the transimpedance amplifier  716  of the FCDC. The complementary pairs of control transistors  738   a - b  and  739   a - b , as well as flicker disable transistor  714  control the current input to the transimpedance amplifier  716 . In the configuration  730 , only a proper subset of the pixels of the pixel array is connected to provide flicker current to the transimpedance amplifier  716 . 
     In the configuration  730 , there are four 4-by-4 blocks of pixels: block  732   a  in the upper left, block  732   b  in the lower left, block  734   a  in the upper right, and block  734   b  in the lower right. In the configuration  730 , in each of the 4-by-4 blocks  732   a - b  and  734   a - b , only the pixels in the upper right most position, pixels  733   a ,  733   b ,  735   a , and  735   b  respectively, are connected as inputs to the transimpedance amplifier  716  and other FCDC components. 
     In a modification of the configuration  730 , to provide greater separation between pixels connected to the FCDC, pixels  737   a  and  737   b  may be connected in place of pixels  735   a  and  735   b . Other selection patterns for the set of pixels of the pixel array to be connected to the FCDC may be used. 
       FIG.  8 A  shows a cross-section of a PD  800 , such as may be included in a pixel of a pixel array with embedded FCDC. The PD  800  is a variation of the PD  700  of  FIG.  7 A , except that in place of the additional region of heavily doped p-type in-pixel ground connection  704 , the DTI walls  802   a - b  are formed of a conductive material to function as a ground connection. As with the PD  700 , the DTI walls  802   a - b  may be connected and surround some or all of the pixels containing the PD  800 , which may provide electrical isolation from other pixels of the pixel array. The PD  800  is formed in weakly doped p-type semiconductor substrate  702  as the cathode and includes an n-type doped region  703  forming the anode, as described above with respect to the PD  700  of  FIG.  7 A . The capacitively coupled ground connection  806  joins directly with the DTI walls  802   a - b.    
     Photons of the received light  712  generate both electrons  804   a  and holes  804   b . The former flow under the applied bias to the n-type doped region  703 . The latter flow to the DTI walls  802   a - b  and then through the capacitively coupled ground connection  806 . The current through the capacitively coupled ground connection  806  may then become part of the flicker current and be received at the FCDC, as described above. 
       FIG.  8 B  shows an equivalent circuit of elements of a pixel  810  that may be included in a pixel array with embedded FCDC, according to an embodiment. The pixel  810  includes a PD  812  in which charge carriers are generated by received photons. The pixel  810  also includes the TG  814  which controls flow of the charge carriers to a capacitive junction represented by the capacitor C PIX    821 . The pixel  810  includes a RST transistor  816 , a SF transistor  818 , and a RS transistor  820 , which may be as described previously. As described above, in an image capture operation, the light-generated charge carriers produce a current on the column output connection, which may be amplified or buffered by the system readout circuitry  514 . 
     However, the pixel  810  is configured so that the anode of its PD  812  does not connect directly to a ground connection of the pixel array as in previously described pixels, but instead connects both through a flicker disable transistor  815  to a ground connection of the pixel array, and to a flicker enable transistor  817 . The flicker enable transistor  817  connects to the pixel array ground connections through resistive transistor  819  configured as resistor. 
     During a flicker current detection operation, a voltage applied to the gate of the flicker disable transistor  815  makes it an open circuit connection, and the corresponding opposite voltage applied to the gate of the flicker enable transistor  817  causes it to function as a closed circuit connection, so that charge carriers of the PD  812  may accumulate in the capacitive region C PIX    821 . Flicker current from the pixel  810  is then an input to the transimpedance amplifier  716 , as described above. 
       FIG.  9 A  shows a cross-section of a pixel  900 , such as may be a pixel of a pixel array with embedded FCDC. The pixel  900  is formed in a weakly doped p-type semiconductor substrate  902  and has a PD section that includes the strongly doped p-type cathode region  904  and the n-type anode region  903 . The TG transistor  905  includes a p-type channel region  906  formed in the substrate  902 . A voltage applied to the gate of the TG transistor  905  allows light-generated charge carriers to flow through the p-type channel region  906  to the n-type FD node region  908 . The RST transistor  909  controls flow of charge carriers through the p-type channel region  910  to the RST transistor drain region  912 . The configuration shown allows for flow of electrons through the RST transistor  909  and collection of electrons at a V DD  node, and for FCDC positioned opposite to the light receiving side of the pixel  900 . 
       FIG.  9 B  shows a graph  920  of corresponding voltage levels of the conduction band of the semiconductor regions of the pixel  900  of  FIG.  9 A . The doping levels of the respective regions of the pixel  900  are adjusted to produce the increasing (from left to right) threshold voltage profile shown, under applied voltages for conduction. The PD region is at voltage level  922   a , the TG transistor channel region  906  is at voltage level  922   b , the FD node  908  is at voltage level  922   c , the p-type channel region of the RST transistor  909  is at voltage level  922   d , and the drain region  912  of the RST transistor  909  is at voltage level  922   e . The voltage levels  922   a - e  increase when the pixel is biased for electron current flow to the V DD  node. 
       FIG.  10 A  illustrates the configuration  1000  in which an electronic device  1002  is able to detect the proximity to an exterior object  1003  in its environment. The electronic device  1002  includes the image sensor  1004 , which may be a component of a camera. The image sensor  1004  includes a pixel array  1008 , such as any of the pixel arrays described above. In some embodiments, the pixel array  1008  in turn may include an embedded light source  1006 , which may be formed in a common semiconductor substrate with the pixels of the pixel array  1008 . In alternative embodiments, the light source  1006  may be separate from the pixel array. For simplicity of discussion, the pixel array  1008  and the light source  1006  are shown side-by-side in  FIG.  10 A . The light source  1006  may be a light emitting diode (LED); in particular, it may be a laser light emitting diode. In the configuration  1000 , the electronic device  1002  has a lens in an aperture  1007 . The image sensor  1004  of the electronic device  1002  is configured so that light from the environment is received through the aperture  1007  and focused onto the pixel array  1008  of the image sensor  1004 . Emitted light from the light source  1006  may be emitted either through the aperture  1007 , or through a separate aperture (not shown). The image sensor  1004  in the configuration  1000  may be operable both for image capture and proximity detection. 
     The light source  1006  emits a light  1010 , which may, though not necessarily, be in the infrared, visible, or ultraviolet wavelengths, and which may have a predominantly single wavelength, such as from a laser LED. The light source  1006  emits light that is amplitude modulated, with a modulation frequency that may be on the order of 10 MHz. 
     Reflections  1012  of the emitted light  1010  may be received on the pixel array  1008  of the image sensor  1004 . A phase shift between the modulated emitted light  1010  and the reflections  1012  may be used to calculate a time-of-flight value between emissions and received reflections. In turn, the time-of-flight value allows for estimation of the proximity of the electronic device  1002  and the exterior object  1003 . 
       FIG.  10 B  illustrates an alternative configuration  1020  for proximity detection of an object  1023  by an electronic device  1022  that includes the image sensor  1024 . The image sensor  1024  may include a pixel array  1028  that in turn may include an embedded light source  1026 . Alternatively, the light source  1026  may be a component separate from the pixel array. In the case of an embedded light source  1026 , the embedded light source  1026  may be formed in a common semiconductor substrate with the pixels of the pixel array  1028 . For simplicity of discussion, the pixel array  1028  and the light source  1026 , whether embedded or separate, are shown side-by-side in  FIG.  10 B . The light source  1026  may be a light emitting diode (LED); in particular, it may be a laser light emitting diode. In the configuration  1020 , the electronic device  1022  has a lens in an aperture  1027 . The image sensor  1024  of the electronic device  1022  is configured so that light from the environment is received through the aperture  1027  and focused onto the pixel array  1028  of the image sensor  1024 . The emitted light from the light source  1026  may either be emitted through the aperture  1027  or through a separate aperture. The image sensor  1024  in the configuration  1020  may be operable both for image capture and proximity detection. 
     The light source  1026  emits a light  1030 , which may, though not necessarily, be in the infrared, visible, or ultraviolet wavelengths, and which may have a predominantly single wavelength, such as from a laser LED. The light source  1026  emits light that is amplitude modulated by a square wave having successive high and low amplitudes. The modulation frequency may be on the order of 1 kHz. 
     Reflections  1032  of the emitted light  1030  may vary in intensity due to the amplitude modulation of the emitted light  1030 . The reflections  1032  of the emitted light  1030  may be focused onto a region  1035  of a section  1034  of the pixel array  1028 . The image sensor  1024  may have current detection circuitry analogous to the FCDC described above, which is separate from the image capture readout circuitry. Such current detection circuitry may be embedded in the pixel array  1028  as part of a common semiconductor substrate. 
     During a proximity detection operation by the electronic device  1022 , output transistors of pixels of the pixel array  1028 , such as the SF transistors and RS transistors, are disabled during a proximity detection operation. Then light-generated current can be measured by the current detection circuitry during each of the successive periods of high and low amplitudes in the modulated emitted light  1030 . 
     Variations in the intensity of the reflected light  1032  between periods of high and low amplitude may be used to estimate the proximity of the electronic device  1022  to the object  1023 . The intensity of the reflected light  1032  inferred from the measured current during a low amplitude period may provide a baseline value to account for properties of the object  1023 , such as color and reflectivity, among other properties. An increased intensity of the reflected light  1032  inferred from the measured current during a high amplitude period may then be correlated with the distance to the object  1023 . The average intensity is often proportional as the inverse square of the distance to the object  1023 . 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not targeted to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.

Metadata:
Filing Date: 20220922
Publication Date: 20240116
Grant Date: 20240116
Priority Date: 20220922
Inventors: ORLOWSKI, JOHN L.
SINGH, RITU RAJ
CELLEK, Oray O.
Assignee: APPLE INC
CPC Classifications: [{"code": "H04N23/745", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04N25/62", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N25/77", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N25/78", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N23/745", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04N25/62", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N25/77", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N25/78", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N25/709", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04N25/78", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N23/745", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 89511289