Systems and methods for pixel-level dark current compensation in image sensors

An imaging system may include processing circuitry, a lens, and an array of pixels including image sensor pixels and temperature sensor pixels. The image sensor pixels may generate image pixel values in response to image light received through the lens. The temperature sensor pixels may generate thermal estimate signals based on the temperature of the pixel array. The image sensor pixels and temperature sensor pixels may generate dark current. As the temperature of the pixel array increases, the image sensor pixels and temperatures sensor pixels may generate increased dark current. Temperature sensor pixels may generate more dark current than image sensor pixels. Dark current generated by the temperature sensor pixels may be used to generate dark current compensation values that may compensate for the dark current generated by the image sensor pixels.

BACKGROUND

This relates to solid-state image sensor arrays and, more particularly, to image sensors with temperature sensor pixels for performing dark current compensation.

Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. In a typical arrangement, an electronic device is provided with an array of image pixels. The image pixels generate image signals in multiple color channels. Readout circuitry such as analog-to-digital converter circuits are commonly coupled to the image pixels for reading out image signals from the image pixels.

In an image sensor, the temperature of the image sensor substrate typically affects the image signals generated by the image pixels. In a typical pixel photodiode, there is some current (i.e., dark current) in the photodiode even when no light is incident upon the photodiode (due to the inherent movement of electrons across the corresponding semiconductor junction). As the temperature of the photodiode increases, this flow of the electrons, and therefore the dark current, increases. Increased dark current in the image sensor can cause excessive and unsightly noise in the final image signal.

Because the level of dark current generated by a photodiode is temperature-dependent, it is useful to be able to detect the temperature of the image sensor array so that dark current contributions to the final image signal can be compensated for. In some conventional image sensors, a junction sensor is placed on the image sensor substrate separated from the pixel array. However, the junction sensor can only measure the temperature of the image sensor array substrate at the location at which the junction sensor is placed, resulting in a single temperature reading that is representative of one location on the image sensor. The image sensor then uses this temperature reading to generate a correction value for correcting for temperature-based dark current in the captured image signals. However, in this scenario, the correction value will inaccurately estimate the temperature and corresponding dark current signal contribution for pixels at positions on the substrate that are far from the junction sensor.

It would therefore be desirable to provide imaging devices with improved systems and methods for detecting array temperatures and compensating for dark current in the array.

DETAILED DESCRIPTION

Electronic devices such as digital cameras, computers, cellular telephones, and other electronic devices include image sensors that gather incoming light to capture an image. The image sensors may include arrays of image sensor pixels (sometimes referred to herein as image pixels). The image pixels in the image sensors may include photosensitive elements such as photodiodes that convert the incoming light into image signals. Image sensors may have any number of pixels (e.g., hundreds, thousands, millions or more). A typical image sensor may, for example, have hundreds of thousands or millions of pixels (e.g., megapixels). Image sensors may include control circuitry such as circuitry for operating the image pixels, readout circuitry for reading out image signals corresponding to the electric charge generated by the photosensitive elements, and, if desired, other processing circuitry such as analog processing circuitry and digital processing circuitry. An image sensor may be coupled to additional processing circuitry such as circuitry on a companion chip to the image sensor, circuitry in the device that is coupled to the image sensor by one or more cables or other conductive lines, or external processing circuitry.

FIG. 1is a diagram of an illustrative imaging system that uses an image sensor to capture images. Electronic device10ofFIG. 1may be a portable electronic device such as a camera, a cellular telephone, a video camera, or other imaging device that captures digital image data. Camera module12may be used to convert incoming light into digital image data. Camera module12may include one or more lenses14and image sensor circuitry16(e.g., one or more corresponding image sensors). Image sensor circuitry16(sometimes referred to herein as image sensor16) may include an array of image sensor pixels18(sometimes referred to herein as array18or pixel array18) and pixel control and readout circuitry20(sometimes referred to herein as pixel readout circuitry20or pixel readout and processing circuitry20). During image capture operations, light from a scene may be focused onto image sensor pixel array18by lens14. Image sensor16may include any number of pixel arrays18(e.g., one pixel array, two pixel arrays, ten pixel arrays, etc.). If desired, camera module12may be provided with an array of lenses14and an array of corresponding image sensors16.

Image sensor pixels in pixel array18may generate image signals in response to receiving light from a scene. For example, image sensor pixels in array18may include photosensitive elements such as photodiodes that convert incoming light into electric charge. Image pixels in pixel array18may be controlled using pixel control and readout circuitry20. Pixel control and readout circuitry20may include any desired pixel control and/or readout circuitry (e.g., row control circuitry, column read out circuitry, etc.). Pixel control and readout circuitry20may include circuitry for converting analog image signals into corresponding digital image pixel data (e.g., a respective pixel value generated by each image sensor pixel). Pixel values generated by pixel array18and pixel control and readout circuitry20may be provided to storage and processing circuitry22.

If desired, pixel array18may include temperature sensor pixels for estimating a localized temperature of a portion of array18(e.g., of a semiconductor substrate on which array18is formed). For example, temperature sensor pixels may include image sensor pixels in which a portion of an image sensor pixel has been modified. In one illustrative example, temperature sensor pixels may be image sensor pixels that have been modified such that the temperature sensor pixels are not exposed to light incident upon the pixel array and that do not convert incoming light into electric charge. Temperature sensor pixels may produce signals in response to dark current (e.g., temperature sensor pixels may not produce a photocurrent or may only produce dark current), and may be configured to generate increased levels of dark current compared to image sensor pixels in the array (e.g., pixels used for capturing image data in response to light received from an imaged scene).

Temperature sensor pixels in array18may produce thermal estimate signals that are representative of the temperature of the temperature sensor pixels at a desired location on pixel array18. The thermal estimate signals may, for example, be dark current signals. These thermal estimate signals may be converted to digital thermal estimate values (e.g., at circuitry20) that can be used to generate dark current compensation values for the image sensor pixels. If desired, pixel control and readout circuitry20may include thermal gradient processing circuitry for performing dark current compensation operations on digital image pixel signals generated by image sensor pixels in pixel array18based on the dark current compensation values calculated from the thermal estimate signals generated by the temperature sensor pixels.

Image sensor16may receive control signals from storage and processing circuitry22and may supply pixel data to storage and processing circuitry22. Storage and processing circuitry22may include one or more integrated circuits (e.g., image processing circuits, microprocessors, storage devices such as random-access memory and non-volatile memory, etc.) and may be implemented using components that are separate from camera module12and/or that form part of camera module12(e.g., circuits that form part of an integrated circuit that includes image sensors16or an integrated circuit within module12that is associated with image sensors16). Image data that has been captured by camera module12may be processed and stored using storage and processing circuitry22. Processed image data may, if desired, be provided to external equipment (e.g., a computer or other device) using wired and/or wireless communications paths coupled to storage and processing circuitry22.

As shown inFIG. 2, image sensor16may include one or more arrays of pixels such as pixel array18containing image sensor pixels24. Array18may contain, for example, hundreds or thousands of rows and columns of image sensor pixels26. Image sensor pixels24may be covered by a color filter array that includes color filter elements over some or all of image sensor pixels24. Color filter elements for image sensor pixels24may be red color filter elements (e.g., photoresistive material that passes red light while reflecting and/or absorbing other colors of light), blue color filter elements (e.g., photoresistive material that passes blue light while reflecting and/or absorbing other colors of light), green color filter elements (e.g., photoresistive material that passes green light while reflecting and/or absorbing other colors of light), clear color filter elements (e.g., transparent material that passes red, blue and green light), yellow color filter elements, or any other desired color filter elements.

Array18may include one or more pixels such as temperature sensor pixels26. For example, array18may contain temperature sensor pixels26arranged around the border (periphery) of array18. In such an arrangement, the border formed by the temperature sensor pixels may be one pixel wide, two pixels wide, ten pixels wide, or may have a width corresponding to any other suitable number of pixels. The border arrangement illustrated inFIG. 2is merely illustrative. Temperature sensor pixels26may be positioned in any suitable location on pixel array18. For example, temperature sensor pixels26may be positioned randomly throughout array18, along one side or more than one side of array18, or in the middle of array18(e.g., temperature sensor pixels26may be formed from or may replace image sensor pixels24in array18).

During operation of electronic device10, components of camera module12and image sensor16such as pixel control and readout circuitry20, storage and processing circuitry22, integrated circuits, other electronic components, or any other components of electronic device10may generate heat. Such components may be mounted directly on or near pixel array18, such as on a pixel array substrate on which pixel array18is formed. Heat that is generated by circuitry in electronic device10, such as heat generated by components such as pixel control and readout circuitry20and other electronic components of image sensor16that are mounted directly on or near pixel array18, may change the temperature of pixel array18and cause a temperature gradient such as temperature gradient28(sometimes referred to herein as thermal gradient28) to form on pixel array18. As shown in the illustrative example ofFIG. 2, temperature gradient28may be formed along the horizontal direction across pixel array18. In such an example, temperature gradient28may include areas of pixel array18having higher temperatures such as a high temperature region30at temperature TH(sometimes referred to herein is a THregion) and a low temperature region32at temperature TL(sometimes referred to herein as a TLregion). In some instances, high temperature region30may form on image pixel array18near control circuitry of electronic device10or image sensor16, such as pixel control and readout circuitry20, whereas low temperature regions32may form in an areas of image pixel array18that are farther from control circuitry of electronic device10or image sensor16. This, however, is merely illustrative. High temperature regions30and low temperature regions32may form anywhere on image pixel array18and may or may not be near control circuitry or other electronic components of electronic device10or image sensor16.

In one situation that is sometimes discussed herein as an example, as the temperature of array18or portions of array18increases, the temperatures of pixels in array18such as image sensor pixels24increase. For example, the temperatures of image sensor pixels24in a high temperature region30may be higher than the temperatures of image sensor pixels24in a low temperature region32. The properties of image sensor pixels24in a high temperature region30may be different than the properties of image sensor pixels24in a low temperature region32. For example, image sensor pixels24in a high temperature region30may generate increased levels of dark current when compared to image sensor pixels24in a low temperature region32.

As the temperature of array18increases, the temperatures of temperature sensor pixels26may increase. For example, the temperatures of temperature sensor pixels26in a high temperature region30may be higher than the temperatures of temperature sensor pixels26in a low temperature region32. Temperature sensor pixels26that have a higher temperature may generate increased dark current when compared to temperature sensor pixels26that have a lower temperature. Temperature sensor pixels26may be configured to have different properties than image sensor pixels24, such that temperature sensor pixels26may be more sensitive to changes in temperature along temperature gradient28, and may experience greater fluctuations in dark current generation between high temperature regions30and low temperature regions32than image sensor pixels24. For example, temperature sensor pixels26that have a higher temperature may generate more dark current than image sensor pixels24having the same temperature.

FIG. 3Ais a cross-sectional side view of a portion of light gathering image sensor pixel24(e.g., a pixel for generating image data in response to light from a scene). Image sensor pixel24may be formed on substrate101in array18. Image sensor pixel24may include a photodiode such as photodiode100. Photodiode100may be formed by p+ type doped layer107and n-type doped layer108at the front surface (side) of substrate101. P+ type doped layer107reduces dark current generated by photodiode100. The front (upper) surface of epitaxial p-type doped layer115is covered by oxide layer109that isolates the doped poly-silicon charge transfer (TX) gate (not shown) from substrate101. Charge generated by impinging photons90is accumulated at n-type doped layer108. This example is merely illustrative and, if desired, photons90may be received through the front surface. Pixels such as image sensor pixels24may be isolated from each other by p+ type doped regions105and106that extend through epitaxial p-type doped layer115down to p+ type doped layer102. The pixel is covered by inter-level (IL) oxide layers112(only one inter-level oxide layer is shown) that are used for the pixel metal wiring and interconnect isolation. The active pixel circuit components are connected to the wiring by metal vias deposited through contact holes113.

In a photodiode such as photodiode100of image light sensing pixel24, it may be desirable to decrease the level of dark current generated by the photodiode so that the image signal output from image sensor pixel24is representative of photocurrent generated by photons such as photons90incident upon the photodiode100(e.g., with a minimized dark current contribution). Using a “buried” region such as n-type doped layer108(buried, for example, by p+ type doped layer107), the amount of dark current generated by photodiode100and thus the dark current contribution to the overall signal produced by photodiode100may be limited. This can provide photodiode100with greater sensitivity and allow image sensor pixel24to output a signal with a lower dark current contribution. However, the use of a buried region such as n-type doped layer108does not eliminate all dark current generated by photodiode100. Furthermore, the amount of dark current generated by photodiode100may change as the temperatures of image sensor pixel24and photodiode100in array18increase due to heat generated from electronic components of electronic device10. It is therefore desirable to be able to compensate for the dark current that is produced by photodiode100, and in particular, to compensate for dark current generated due to temperature changes on array18or temperature gradients such as thermal gradient28.

FIG. 3Bis a cross-sectional side view of a portion of an illustrative temperature sensor pixel such as temperature sensor pixel26ofFIG. 2. Temperature sensor pixel26may include a corresponding photodiode200. Photodiode200may be formed on substrate201of array18(e.g., corresponding to substrate101ofFIG. 3B). In contrast to photodiode100of light sensing image sensor pixel24described in connection withFIG. 3A, a p+ type doped layer such as p+ type doped layer107of pixel24is not formed in photodiode200of temperature sensor pixel26. As such, photodiode200is not provided with the same dark-current reducing qualities afforded to photodiode100due to p+ type doped layer107. Photodiode200of temperature sensor pixel26is formed by heavily-doped n+ type layer214at the front surface of substrate201. The front (upper) surface of epitaxial p-type doped layer215(corresponding to layer115ofFIG. 3A) is covered by oxide layer209(corresponding to layer109). Temperature sensor pixels26may be isolated from each other (and from image sensor pixels24) by p+ type doped regions205and206that may extend through epitaxial p-type doped layer215down to p+ type doped layer202. The pixel is covered by inter-level (IL) oxide layers212(only one inter-level oxide layer is shown) that are used for the pixel metal wiring and interconnect isolation. The active pixel circuit components are connected to the wiring by metal vias deposited through contact holes213.

Photodiode200may be configured to generate increased dark current relative to photodiode100of image sensor pixels24. For example, by removing p+ type doped layer107, the dark current-preventing properties afforded to photodiode100may not be provided in photodiode200. Therefore, dark current generated by photodiode200is not suppressed, and an increased dark current signal may be generated by photodiode200compared to the dark current generated by photodiode100in a similar environment. Additionally, heavily-doped n+ type layer214may allow increased dark current to be generated by photodiode200relative to image sensor pixel24. Because the temperature of temperature sensor pixels26increase as the temperature of array18increases, temperature sensor pixels26may generate increased dark current when the temperature of photodiode200increases. Photodiode200may be more sensitive to temperature increases than photodiode100due to the presence of highly-doped n+ type layer214and the absence of p+ type doped region107, and may demonstrate greater changes in dark current generation in response to temperature changes in array18than photodiode100ofFIG. 3A.

If desired, temperature sensor pixels26may be provided with light-blocking material216that covers photodiode200and prevents photons90from impinging upon photodiode200. Light-blocking material216may include metal, plastic, ceramic, or any other suitable material for blocking light and preventing photons90from impinging upon photodiode200. In one illustrative example, light-blocking material216includes aluminum. Photodiode200may therefore be configured to receive no light and may generate no photocurrent due to impinging photons90. Because photodiode200does not receive light, photodiode200may generate no photocurrent (e.g., the only current produced by photodiode200of temperature sensor pixel26may be dark current). This example is merely illustrative. In examples where array18is front-side illuminated, layer216may be formed adjacent to the front side of substrate201.

Because photodiode200may be configured to generate signals in response only to dark current and may have increased sensitivity to dark current generation in response to temperature changes in array18, temperature sensor pixels26may be well-equipped to provide temperature data on array18based on the dark current generated by pixels26. This temperature data may be used to determine the temperature at various positions on array18, and may be used to estimate the temperatures of image sensor pixels24in various regions of array18. Because the level of dark current produced by the image sensor pixels24is temperature-dependent, dark current generated by the pixels in those regions of array18may be calculated based on the dark temperature data generated by temperature sensor pixels26(by, for example, interpolation of the temperature sensor measurements determined from the dark current signals generated by temperature sensor pixels26). If the temperature at a given location of the array can be determined, the dark current generated by pixels in that area can be calculated based on the data from the temperature sensors, and the dark current contribution to the overall signal (photocurrent signal plus dark current signal, for example) generated by photodiodes100of image sensor pixels24may compensated for. This may result in greater sensitivity of image sensor pixels24.

FIG. 4is an illustrative diagram showing how pixel readout and processing circuitry in an imaging system may be used to perform dark current subtraction on digital image pixel values. Image sensor pixels24in pixel array18may generate analog image signals in response to light that is transmitted to pixel array18by lens14(FIG. 1). In some illustrative examples, each image sensor pixel24in pixel array18may generate an analog image signal in response to light that is incident upon the image sensor pixel and photodiode100contained therein. Temperature sensor pixels26in pixel array18may generate thermal estimate signals in response to the temperature of temperature sensor pixels26in image pixel array18. For example, each temperature sensor pixel26may generate a thermal estimate value that corresponds to the temperature of the individual temperature sensor pixel26that generates the thermal estimate signal. The thermal estimate signals generated by temperatures sensor pixels26may be, for example, produced entirely by dark current signal contributions. In one illustrative embodiment, temperature sensor pixels26may be arranged around the edge of image pixel array18(as shown inFIG. 2), and therefore may generate thermal estimate signals that are estimates of the temperature of pixel array18in the border region of pixel array18. By forming the border of array18from temperature pixels26, a thermal estimate signal may be generated for each row and each column of array18. This example is merely illustrative. In general, temperature sensor pixels26may be any type of pixel or suitable temperature sensor, and may be positioned in any desired location in or relative to pixel array18.

Image sensor pixels24and temperature sensor pixels26may transmit respective analog image signals and thermal estimate signals to pixel control and readout circuitry20, which may include analog-to-digital converter (ADC) converter circuitry34, thermal gradient processing circuitry36, and dark current subtraction circuitry38. Pixel control and readout circuitry20may include storage circuitry40for storing digital image pixel values from image sensor pixels24.

If desired, ADC circuitry34may include one or more analog-to-digital converter circuits. ADC circuitry34may perform analog-to-digital conversion operations on the analog image signals received from image sensor pixels24in pixel array18to generate digital image data (e.g., digital image pixel values). Each analog image signal corresponding to one of image sensor pixels24may be converted into a digital image pixel value corresponding to the image sensor pixel24. ADC circuitry34may perform analog-to-digital conversion operations on the thermal estimate signals received from temperature sensor pixels26in pixel array18to generate digital thermal estimate values. Each temperature sensor pixel26may have an associated digital thermal estimate value generated form a corresponding thermal estimate signal. ADC circuitry34may include any desired type of ADC architecture (e.g., direct conversion ADC architecture, integrating ADC architecture, successive-approximation ADC architecture, ramp-compare ADC architecture, etc.). ADC circuitry34may transmit the digital thermal estimate values to thermal gradient processing circuitry36. In one illustrative example, ADC circuitry34may transmit the digital image pixel values to dark current subtraction circuitry38(sometimes referred to herein as dark current compensation circuitry38). ADC circuitry34may transmit the digital image pixel values to storage circuitry40for temporary or permanent storage of the digital image pixel values. When ready for use by dark current subtraction circuitry38, digital image pixel values may be transmitted from storage circuitry40to dark current subtraction circuitry38for processing.

Thermal gradient processing circuitry36may generate dark current compensation values based on digital thermal estimate values received from ADC circuitry34. In one illustrative example, thermal gradient processing circuitry36may generate dark current compensation values for the image sensor pixels24in pixel array18based on the thermal estimate signals (e.g., dark current signals) from the temperature sensor pixels26located around the border of pixel array18. In an image pixel array such as pixel array18, it may be desirable to use temperature data such as digital thermal estimate values to determine the compensation appropriate for each individual image sensor pixel24in pixel array18, as the temperatures of, and therefore the dark current generated by, each image sensor pixel24in pixel array18may be different. In one illustrative example, thermal gradient processing circuitry36uses digital thermal estimate values received from ADC circuitry34to generate a digital thermal gradient for each pixel24in pixel array18.

The digital thermal gradient generated by thermal gradient processing circuitry36may, for example, be representative of thermal gradient28that develops on pixel array18. Thermal gradient processing circuitry36may generate dark current compensation values for each image sensor pixel24in array18. These dark current compensation values may be based on the digital thermal gradient generated by thermal processing circuitry36, or may be generated directly from the digital thermal estimate values provided to thermal gradient processing circuitry36. In one illustrative example, thermal gradient processing circuitry36generates a dark current compensation value for each image sensor pixel24in pixel array18based on digital thermal estimate values corresponding to temperature sensor pixels26. The dark current compensation value may represent the estimated dark current produced by the individual image sensor pixel24based on the digital thermal estimate values generated from the thermal estimate signals obtained by the temperatures sensor pixels26in the pixel array18. That is, each dark current compensation value may be used to compensate for the dark current produced by the image sensor pixel24based on the estimated temperature of the image sensor pixel24, as determined from the digital thermal estimate values. Dark current compensation values may be generated for each pixel24(e.g., each pixel location on array18) by interpolating digital thermal estimate values generated by ADC circuitry34based on thermal estimate signals generated by temperatures sensor pixels26. For example, linear combinations or weighted sums of one or more (e.g., all) of the thermal estimate values may be generated by temperature sensor pixels26and/or thermal gradient processing circuitry36for a given frame of image data.

Dark current compensation circuitry38may receive dark current compensation values from thermal gradient processing circuitry36. In one illustrative embodiment, dark current compensation circuitry38receives a separate dark current compensation value for each image sensor pixel24in pixel array18. This, however, is merely illustrative. Dark current compensation circuitry may receive dark current compensation values that correspond to one pixel, two pixels, more than two pixels, ten pixels, or any other suitable number of pixels in pixel array18. Alternatively, dark current compensation circuitry may receive dark current compensation values corresponding to a cluster of image sensor pixels24, a row of image sensor pixels24, a column of image sensor pixels24, or any other suitable grouping of image sensor pixels24in pixel array18.

Dark current compensation circuitry38may receive digital image pixel values from ADC circuitry34. In one illustrative embodiment, dark current subtraction circuitry38may receive digital image pixel values from storage circuitry40, such as when image pixel values are transmitted from ADC circuitry34to storage circuitry40for temporary or permanent storage prior to being transmitted to dark current subtraction circuitry38. Dark current subtraction circuitry38may receive a digital image pixel value for each image sensor pixel24in pixel array18.

The digital image pixel values received at dark current subtraction circuitry38may be generated from captured analog image signals having a photocurrent contribution and a dark current contribution. Therefore, the digital image pixel values received at dark current subtraction circuitry38may be representative of the combined dark current signal and photocurrent signal generated by photodiode100of image sensor pixel24in pixel array18. In order to provide pixel data that accurately represents the photocurrent generated by photodiode100and contains a minimum dark current contribution, it may be desirable to compensate the digital image pixel values for their dark current contributions. Dark current subtraction circuitry38may accomplish this by using dark current compensation values from thermal gradient processing circuitry36to compensate for the dark current contributions of digital image pixel values from ADC circuitry34or storage circuitry40. In one illustrative example in which each image sensor pixel24has a corresponding digital image pixel value and a corresponding dark current compensation value, a dark-current corrected pixel value may be generated for the image sensor pixel24by subtracting the dark current compensation value associated with image sensor pixel24from the digital image pixel value associated with image sensor pixel24. Dark current subtraction circuitry may therefore produce a dark current-corrected pixel value for image sensor pixel24that has a reduced dark current component. In one illustrative example, dark current-corrected pixel values may be calculated for every image sensor pixel24in array18in this manner. Dark current-corrected pixel values for each image sensor pixel24in pixel array18may then be transmitted to additional image processing circuitry such as storage and processing circuitry22for further storage or processing.

As discussed in connection withFIG. 4above, it may be desirable to compensate for the dark current generated by each image sensor pixel24based on the estimated temperature of each image sensor pixel24. In one illustrative example, this may be accomplished by generating a dark current compensation value for each image sensor pixel24in pixel array18.

FIG. 5Ashows one illustrative example in which a dark current compensation value for an image sensor pixel24such as image sensor pixel24C having a digital image pixel value P is to be generated. In the illustrative example ofFIG. 5A, this may be accomplished using the digital thermal estimate values from the temperature sensor pixels26located around the border of image pixel array18. For example, each temperature sensor pixel26located around the border of pixel array18may have a corresponding digital thermal estimate value. Digital thermal estimate values corresponding to multiple temperature sensor pixels26may be used to generate a dark current compensation value for a given image sensor pixel24such as image sensor pixel24C. In one illustrative example, a first temperature sensor pixel26T may be located in a column along the top edge of the border of pixel array18, and a second temperature sensor pixel26B (as shown inFIG. 2) may be located in the same column along the bottom edge of image pixel array18. Similarly, a third temperature sensor pixel26L may be located in a row along the left edge of the border of pixel array18, and a fourth temperature sensor pixel26R (as shown inFIG. 2) may be located in the same row along the right edge of image pixel array18. Each of temperature sensor pixels26T,26B,26L, and26R may be operating at a temperature that may be dependent on the location of each pixel on image pixel array18and may each generate a respective thermal estimate signal that is representative of the temperature of the respective temperature sensor pixel.

The thermal estimate signals generated by temperature sensor pixels26T,26B,26L, and26R may be dark current signals. Each of these thermal estimate signals may be used to generate a digital thermal estimate value for each of temperature sensor pixels26T,26B,26L, and26R. The digital thermal estimate values for one or more of temperature sensor pixels26T,26B,26L, and26R may then be used to generate a dark current compensation value for an image sensor pixel24in image pixel array18, such as image sensor pixel24C. In such an example, image sensor pixel24C is the image sensor pixel24that is located at the intersection of the column in which temperatures sensor pixels24T and24B are located and the row in which temperature sensor pixels24L and24R are located. This example in which the compensation value is generated for a given pixel location based on temperature estimate values from the same row and column of that pixel in the array is merely illustrative. In general, a dark current compensation value for any image sensor pixel24in pixel array18may be generated based on any one or combination of temperatures sensor pixels26in array18.

FIG. 5Bshows image sensor pixel24C for which a dark current compensation value DC has been generated. Dark current compensation value DC may be generated based on the digital thermal estimate values of temperature sensor pixels24T,24B,24L, and24R, as described in connection withFIG. 5A. InFIG. 5B, dark current compensation value DC may be interpolated based on the digital thermal estimate values of temperature sensor pixels24T,24B,24L, and24R. Such an interpolation may be performed by weighting the digital thermal estimate values for each of the temperature sensor pixels24T,24B,24L, and24R relative to their location with respect to image sensor pixel24C. In one illustrative embodiment, bilinear interpolation using the digital thermal estimate values for each of the temperature sensor pixels24T,24B,24L, and24R may be used to generate a dark current compensation value for image sensor pixel24C. For example, digital thermal estimate values of the temperature sensor pixels24T,24B,24L, and24R may be used to generate a digital thermal estimate value for image sensor pixel24C, which may then be converted to a dark current compensation value. These examples, however, are merely illustrative. Digital thermal estimate values for each of the temperature sensor pixels24T,24B,24L, and24R may be used to generate a corresponding value for image sensor pixel24C that may undergo any processing necessary to render a suitable dark current compensation value DC for image sensor pixel24C.

In one illustrative example, an averaged digital thermal estimate value for each of temperature sensor pixels26T,26B,26L, and26R may be generated to reduce noise in the digital thermal estimate values due to steep temperature gradients on image pixel array18. In such an embodiment, the digital thermal estimate values of temperature sensor pixels surrounding each of temperature sensor pixels26T,26B,26L, and26R may be used to generate an averaged digital thermal estimate value. For example, the digital thermal estimate value of temperature sensor pixel26T may be averaged with the digital thermal estimate value of a temperature sensor pixel on either side of temperatures sensor pixel26T (e.g., a temperature sensor pixel on the left of temperature sensor pixel26T and a temperature sensor pixel on the right of temperature sensor pixel26T). Similar averaged digital temperature sensor values for each of temperature sensor pixels26B,26L, and26R may be generated using a similar method as described for temperature sensor pixel26T. These averaged temperature sensor values for temperature sensor26T,26B,26L, and26R may then be used to generate a dark current compensation value DC for image sensor pixel24C as described above in connection withFIG. 5B.

FIG. 5Cshows image sensor pixel24C having a dark current-corrected digital image pixel value C. Dark current-corrected digital image pixel value C may be generated based on digital image pixel value P of image sensor pixel24C and dark current compensation value DC that was generated for image sensor pixel24C based on the digital thermal estimate values or averaged digital thermal estimate values of temperature sensor pixels26T,26B,26L, and26R. In one illustrative example, dark current compensation value DC is subtracted from digital image pixel value P to generate dark current-corrected digital image pixel value C for digital image sensor pixel24C. This, however is merely illustrative. Dark current-corrected digital image pixel value C may be generated using any suitable processing of digital image pixel values and dark current-correction values of image sensor pixels24C in pixel array18. Dark current-corrected digital image pixel value C may subsequently be transmitted to additional image processing circuitry such as storage and processing circuitry22for further storage or processing.

In one suitable arrangement, analog image signals and thermal estimate signals for each of image sensor pixels24and temperature sensor pixels26may be generated by scanning pixel array18using pixel control and readout circuitry20or other suitable circuitry in image sensor16. In such an example, scanning of image sensor pixels24and temperature sensor pixels26may occur when an image frame is being captured by camera module12. Analog image signals and thermal estimate signals may be converted to digital image pixel values and digital thermal estimate values, and the digital thermal estimate values may be used to generate a digital thermal gradient representative of a thermal gradient such as thermal gradient28on pixel array18. The digital thermal gradient may then be used to generate dark current compensation values for image pixels24. In another suitable arrangement, no digital thermal gradient may be generated, and dark current compensation values for each of image sensor pixels24may be generated directly based on the digital thermal estimate values. In one suitable arrangement, digital image pixel values may be stored in storage circuitry40while a digital thermal gradient and/or dark current compensation values are being generated for the frame for which the digital image pixel values were generated. Once the dark current compensation values have been generated for the digital image pixels, both the dark current compensation values and the digital image pixel values may be received at dark current compensation circuitry38and used to generate dark current-corrected digital image pixel values. In such an arrangement, digital thermal estimate values may be representative of the temperature of temperature sensor pixels26at the time of scanning of pixel array18to generate the specific frame for which the digital image pixel values of image sensor pixels24correspond. The dark current compensation values applied to each of digital image pixels24may therefore be based on the estimated temperature and dark current contributions of digital image pixels24at the time of scanning to generate the image frame.

Because the thermal profile of pixel array18may remain relatively constant over time, it may not be necessary to apply dark current compensation values that were generated at the time of image capture to image sensor pixels24. For example, dark current generated by image sensor pixels24may be compensated based on a previously generated digital thermal gradient or previously generated dark current compensation values. In such a scenario, both image sensor pixels24and temperature sensor pixels26may be scanned at the time of image capture by camera module12, and digital image pixel values and digital thermal estimate values may both be generated. However, digital image pixel values may be sent directly to dark current subtraction circuitry38(that is, digital image pixel values may not be sent to storage circuitry40). At dark current subtraction circuitry38, dark current correction values generated for a previous frame (for example, the frame captured immediately before the current frame) may be applied to the digital image pixel values of the current frame to generate dark current-corrected digital image pixel values that provide accurate dark current compensation. In other words, dark current compensation values and digital thermal gradients may be stored and applied to digital image pixel values other than the digital image pixel values corresponding to the frame for which the dark current compensation values were generated. In the meantime, digital thermal estimate values generated for the current frame may be processed at thermal gradient processing circuitry to generate dark current compensation values or a digital thermal profile to be applied to a subsequent frame. Performing thermal gradient processing and dark current subtraction in this manner may result in faster processing of image pixel data. This, however, is merely illustrative. Thermal estimate signals and digital thermal estimate values may be used to generated dark current correction values and digital thermal gradients to be applied to digital image pixel values that correspond to any image frame captured from image pixel array18using camera module12or any other suitable circuitry in electronic device10.

FIG. 6is a flow chart of illustrative steps that may be performed by processing circuitry such as pixel control and readout circuitry20ofFIG. 1for generating dark current-compensated digital image pixel values.

At step60, image sensor pixels24and temperature sensor pixels26may capture analog image signals and thermal estimate signals. For example, light that is incident upon image sensor pixels24may generate photocurrent that is used to generate an analog image signal, and a thermal estimate signal may be generated by temperature sensor pixel26based on dark current that flows in temperature sensor pixel26. Analog image signals and thermal estimate signals may be transmitted to ADC circuitry34.

At step62, ADC circuitry34may convert the analog image signals and thermal estimate signals to digital image pixel values and digital thermal estimate values. ADC circuitry34may transmit digital image pixel values to dark current compensation circuitry38, or to storage circuitry40. ADC circuitry34may transmit the digital thermal estimate values to thermal gradient processing circuitry36.

At step64, thermal gradient processing circuitry36may generate dark current compensation values based on the digital thermal estimate values received from ADC circuitry34. If desired, thermal gradient processing circuitry36may generate a digital thermal gradient based on the digital thermal estimate values. The digital thermal gradient may be used to generate dark current compensation values, or dark current compensation values may be generated based on digital thermal estimate values without generating a digital thermal gradient. In some examples, dark current compensation values are generated for each image sensor pixel24in pixel array18. Dark current compensation values may be generated by using weighted or averaged values of digital thermal estimate values from temperature sensor pixels26. Dark current compensation values may be transmitted to dark current subtraction circuitry38.

At step66, dark current subtraction circuitry38may use digital image pixel values from ADC circuitry34or storage circuitry40and dark current compensation values from thermal gradient processing circuitry36to generate dark current-corrected digital image pixel values for digital image pixels24in pixel array18. If desired, dark current-corrected digital image pixel values may be generated by simply subtracting a dark current compensation value from a digital image pixel value. Dark current-corrected digital image pixel values may then be transmitted to additional circuitry such as storage and processing circuitry22for further processing or storage.

FIG. 7shows in simplified form a typical processor system700, such as a digital camera, which includes an imaging device702(e.g., an imaging device702such as image sensor16and storage and processing circuitry22ofFIGS. 1-6employing temperature sensor pixels, circuitry for compensating for dark current in image sensor pixels, and the techniques for operations described above). Processor system700is exemplary of a system having digital circuits that may include imaging device702. Without being limiting, such a system may include a computer system, still or video camera system, scanner, machine vision, vehicle navigation, video phone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system, and other systems employing an imaging device.

Processor system700, which may be a digital still or video camera system, may include a lens such as lens796(which may include lenses14) for focusing an image onto a pixel array such as pixel array18when shutter release button797is pressed. Processor system700may include a central processing unit such as central processing unit (CPU)795. CPU795may be a microprocessor that controls camera functions and one or more image flow functions and communicates with one or more input/output (I/O) devices791over a bus such as bus793. Imaging device702may communicate with CPU795over bus793. Processor system700may include random access memory (RAM)792and removable memory794. Removable memory794may include flash memory that communicates with CPU795over bus793. Imaging device702may be combined with CPU795, with or without memory storage, on a single integrated circuit or on a different chip. Although bus793is illustrated as a single bus, it may be one or more buses or bridges or other communication paths used to interconnect the system components.

A method for operating an imaging system having image sensor pixels and temperatures sensor pixels arranged in a pixel array may include generating image signals with the image sensor pixels and generating thermal estimate signals with the temperature sensor pixels. The method may include converting the image signals into digital image pixel values and converting the thermal estimate signals into digital thermal estimate values with converter circuitry. The method may include generating dark current compensation values based on the digital thermal estimate values with thermal gradient processing circuitry. The method may include generating dark current-corrected digital image pixel values by subtracting the generated dark current compensation values from the digital image pixel values with dark current subtraction circuitry.

If desired, the temperature sensor pixels may be formed along a periphery of the pixel array having first and second opposing edges and third and fourth opposing edges that extend between the first and second opposing edges. The image sensor pixels and the temperature sensor pixels may be arranged in rows and columns in the pixel array. A first temperature sensor pixel may be formed on the first edge and a second temperature sensor pixel may be formed on the second edge and in a common row of the pixel array. A third temperature sensor pixel may be formed on the third edge and a fourth temperature sensor pixel may be formed on the fourth edge of the border in a common column of the pixel array. Each of the first, second, third, and fourth temperatures sensor pixels may generate a corresponding digital thermal estimate value. Generating the dark current compensation value for the image sensor pixel may include interpolating the corresponding digital thermal estimate values generated by the first, second, third and fourth temperature sensor pixels.

If desired, generating the dark current compensation value for a selected image sensor pixel may include processing digital thermal estimate values from a plurality of temperature sensor pixels. The image signals generated by the selected image sensor pixel may include a photocurrent component and a dark current component. The thermal estimate signals may include a dark current component without photocurrent components. Generating the dark current compensation value for the selected image sensor pixel may include generating the dark current compensation value based on the dark current component of the thermal estimate signals.

If desired, each image sensor pixel in the array may generate a respective digital image pixel value. Generating the dark current compensation values may include generating a respective dark current compensation value for each image sensor pixel in the array. Subtracting the dark current compensation values from the digital image pixel values to produce the dark current-corrected digital image pixel values may include subtracting the respective dark current compensation value for each image sensor pixel in the array from the respective digital image signal generated by that image sensor pixel.

If desired, the digital image pixel values and the dark current compensation values may be generated for a current image frame that is captured by the imaging system, and the dark current-corrected digital image pixel values may be generated using the digital image pixel values and the dark current compensation values generated for the current image frame that is captured by the imaging system.

If desired, the digital image pixel values may be generated for a first image frame that is captured by the imaging system, and the dark current compensation values may be generated for a second image frame that is captured by the imaging system prior to capturing the first image frame. The dark current-corrected digital image pixel values may be generated using the digital image pixel values generated for the first image frame and the dark current compensation values generated for the second image frame.

An image pixel array arranged in rows and columns and having a border may include image sensor pixels configured to generate image data in response to image light. The image sensor pixels may be formed within the border of the image pixel array. The image pixel array may include temperature sensor pixels configured to generate temperature estimate values for mitigating dark current contributions in the image data generated by the image sensor pixels. The temperature sensor pixels may be formed along the border of the image pixel array in a plurality of the rows and a plurality of the columns of the image pixel array.

If desired, the image pixel array may be formed on a semiconductor substrate. The temperature sensor pixels may include a light-blocking member formed over the semiconductor substrate that is configured to prevent the temperature sensor pixels from receiving the image light. The light-blocking member may include aluminum.

If desired, the image sensor pixels may include at least a first photodiode that produces a first amount of dark current, and the temperature sensor pixels may include at least a second photodiode that produces a second amount of dark current. The second amount of dark current may be greater than the first amount of dark current. The at least first photodiode may include a buried n-type doped layer that produces the first amount of dark current. The second photodiode may include a heavily-doped n+ type layer that produces the second amount of dark current such that the temperature sensor pixels are more sensitive to temperature fluctuations across the array than the image sensor pixels.

A system may include a central processing unit, memory, input-output circuitry, and an imaging device. The imaging device may include a pixel array of image sensor pixels and temperature sensor pixels, a lens that focuses an image on the pixel array, analog-to-digital converter circuitry, thermal gradient processing circuitry, and dark current subtraction circuitry. The image sensor pixels may generate image signals in response to the image. The temperature sensor pixels may generate thermal estimate signals in response to at least one temperature of the pixel array. The analog-to-digital converter circuitry may generate digital image pixel values based on the image signals and may generate digital thermal estimate values based on the thermal estimate signals. The thermal gradient processing circuitry may generate dark current compensation values based on the digital thermal estimate values. The dark current subtraction circuitry may subtract the dark current compensation values from the digital image pixel values.

If desired, the temperature sensor pixels may include first photodiodes that generate a first amount of dark current. The image sensor pixels may include second photodiodes that generate a second amount of dark current that is less than the first amount of dark current. The dark current compensation values may compensate for the second amount of dark current produced by the image sensor pixels.