Abstract:
A pixel sensor having a main photodetector and a parasitic photodiode and a method for reading out that pixel sensor are disclosed. The pixel sensor is read by reading a first potential on a floating diffusion node in the pixel sensor while the floating diffusion node is isolated from the main photodiode. The pixel sensor is then exposed to light such that the floating diffusion node and the photodetector are both exposed to the light. A second potential on the floating diffusion node is then readout while the floating diffusion node is isolated from the main photodiode. After the first and second potentials are readout, a third potential on the floating diffusion node is readout. The main photodiode is then connected to the floating diffusion node, and a fourth potential on the floating diffusion node is readout. First and second light intensities are determined from the readout potentials.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    CMOS imaging sensors are widely used in cameras and other imaging applications. The imaging sensors typically include a two-dimensional array of pixel sensors. Each pixel sensor includes a photodiode that measures the image intensity at a corresponding point in the image. The dynamic range of the image sensor is the ratio of the minimum amount of light that can be measured to the maximum amount. An image is formed by first emptying the photodiodes of any accumulated charge and then exposing the photodiodes to the image. Each photodiode accumulates charge at a rate determined by the light intensity emitted by the image at the corresponding point in the image, referred to as a pixel. In general, the amount of charge that can be accumulated in a photodiode has a maximum value, referred to as the maximum well capacity. Once this capacity is reached, the excess charge is removed from the pixel through a special gate that shunts the excess charge to ground to prevent artifacts in the image. The minimum charge that can be detected is determined by noise. 
         [0002]    In principle, the maximum well capacity can be increased by utilizing larger photodiodes; however, this solution increases the cost of the imaging array and requires processing electronics that can deal with the larger dynamic range of the signals generated by the pixels. Another prior art solution for increasing the dynamic range of the imaging array involves using two different photodiodes for each pixel. In this solution, a large area photodiode is used to measure low light levels and a smaller photodiode is used to measure intensities at the brighter locations in the image. If the pixel is exposed to a high brightness location, the smaller photodiode is used. At dim locations in the image, the larger photodiode is used. This solution requires two different sets of photodiodes and the increased silicon area associated with the additional photodiodes that measure the high brightness locations in the image. 
         [0003]    A second prior art solution uses multiple exposures to provide the increased dynamic range. In this solution, basically two pictures are taken of each scene. A first picture uses a very short exposure time which captures the intensities of the high brightness points in the image. Pixels at low intensity points in the image are underexposed. The second picture uses a much longer exposure period. In the second picture, the pixels at the high intensity points are overexposed, while the pixels at the low intensity points are now adequately exposed and provide the intensity values at the low intensity points. The two pictures are then combined to provide an image with increased dynamic range. This approach, however, leads to artifacts in the image, as the two pictures are separated in time by an amount that can be a problem if the scene is rapidly changing. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention includes a pixel sensor having a main photodiode and a parasitic photodiode and a method for reading out that pixel sensor. In the method for reading the pixel sensor, a first potential on a floating diffusion node in the pixel sensor is read while the floating diffusion node is isolated from the main photodiode. The pixel sensor is then exposed to light such that the floating diffusion node and the main photodetector are both exposed to the light. A second potential on the floating diffusion node is then readout while the floating diffusion node is isolated from the main photodiode. A first light intensity is determined from the first and second potentials. After the first and second potentials are readout, a third potential on the floating diffusion node is readout. The main photodiode is then connected to the floating diffusion node, and a fourth potential on the floating diffusion node is readout. A second light intensity is determined from the third and fourth potentials. 
         [0005]    In one aspect of the invention, the floating diffusion node is connected to a reset bus having a first reset potential and then disconnected from the reset bus prior to reading the first potential. In another aspect of the invention, the floating diffusion node is connected to the reset bus and then disconnected from the reset bus after the second potential is readout and prior to reading the third potential. 
         [0006]    In another aspect of the present invention, photoelectrons are shunted from the main photodiode to ground if a potential on the photodiode exceeds an overflow potential. 
         [0007]    A pixel sensor according to the present invention includes a main photodiode characterized by a first light conversion efficiency, a floating diffusion node, a gate that selectively connects the main photodiode to the floating diffusion node, a readout amplifier that generates a signal indicative of a potential on the floating diffusion node, the signal being coupled to a first conductor in response to a select signal received by the pixel sensor; and a reset gate that selectively connects the floating diffusion node to a reset bus at a reset potential. The floating diffusion node includes a parasitic photodiode characterized by a second light conversion efficiency, the first light conversion efficiency is greater than or equal to 30 times the second light conversion efficiency. 
         [0008]    In one aspect of the present invention, the pixel sensor includes an overflow gate connected to the main photodiode. The overflow gate removes charge from the photodiode if a potential on the photodiode exceeds an overflow potential. 
         [0009]    In another aspect of the invention, the floating diffusion node includes an n-type implanted region in a p-type semiconductor substrate. The main photodiode also includes an n-type implanted region in the p-type substrate. A portion of the n-type implanted region of the floating diffusion node is in direct contact with the p-type substrate, the size of that portion being adjusted to provide the second light conversion efficiency. 
         [0010]    The present invention also includes imaging arrays constructed from the pixel sensors of the present invention or readout using the method of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic drawing of a typical prior art pixel sensor in one column of pixel sensors in an imaging array. 
           [0012]      FIG. 2  illustrates a pixel sensor in which the parasitic photodiode is utilized in an image measurement. 
           [0013]      FIG. 3  is a cross-sectional view through a portion of a typical prior art pixel sensor. 
           [0014]      FIG. 4  is a cross-sectional view of a portion of a pixel sensor according to one embodiment of the present invention. 
           [0015]      FIG. 5  illustrates a two-dimensional imaging array constructed from pixel sensors according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The manner in which the present invention provides its advantages can be more easily understood with reference to  FIG. 1 , which is a schematic drawing of a typical prior art pixel sensor in one column of pixel sensors in an imaging array. Pixel sensor  21  includes a photodiode  22  that measures the light intensity at a corresponding pixel in the image. Initially, photodiode  22  is reset by placing gate  25  in a conducting state and connecting floating diffusion node  23  to a reset voltage, Vr. Gate  25  is then closed and photodiode  22  is allowed to accumulate photoelectrons. A potential on gate  27  sets the maximum amount of charge that can be accumulated on photodiode  22 . If more charge is accumulated than allowed by the potential on gate  27 , the excess charge is shunted to ground through gate  27 . 
         [0017]    After photodiode  22  has been exposed, the charge accumulated in photodiode  22  is then measured using a correlated double sampling algorithm. In this algorithm, floating diffusion node  23  is first reset to Vr using reset gate  24 . The potential on floating diffusion node  23  is then measured by connecting source follower  26  to readout line  31  using the gate connected to line  28 . This reset potential is stored in column amplifier  32 . Next, gate  25  is placed in a conducting state and the charge accumulated in photodiode  22  is transferred to floating diffusion node  23 . It should be noted that floating diffusion node  23  is effectively a capacitor that has been charged to Vr. Hence, the charge leaving photodiode  22  lowers the voltage on floating diffusion node  23  by an amount that depends on the capacitance of floating diffusion node  23  and the amount of charge that is transferred. The voltage on floating diffusion node  23  is again measured after the transfer. The difference in voltage is then used to compute the amount of charge that accumulated during the exposure. 
         [0018]    The present invention is based on the observation that a pixel of the type discussed above can be modified to include a second parasitic photodiode that is part of the floating diffusion node and has a significant photodiode detection efficiency. To distinguish the parasitic photodiode from photodiode  22 , photodiode  22  and photodiodes serving analogous functions will be referred to as the “conventional photodiode”. Refer now to  FIG. 2 , which illustrates a pixel sensor in which the parasitic photodiode is utilized in an image measurement. To simplify the following discussion, those elements of pixel sensor  41  that serve functions analogous to those discussed above with respect to  FIG. 1  have been given the same numeric designations and will not be discussed further unless such discussion is necessary to illustrate a new manner in which those elements are utilized. In general, parasitic photodiode  42  has a detection efficiency that is significantly less than that of main photodiode  22 . The manner in which the ratio of the photodiode detection efficiencies of the two photodiodes is adjusted will be discussed in more detail below. 
         [0019]    The manner in which pixel sensor  41  is utilized to measure the intensity of a pixel will now be explained in more detail. The process may be more easily understood starting from the resetting of the pixel after the last image readout operation has been completed. Initially, main photodiode  22  is reset to Vr and gate  25  is closed. This also leaves floating diffusion node  43  reset to Yr. This voltage is measured at the start of the exposure by connecting floating diffusion node  43  to column amplifier  52 . During the image exposure, parasitic photodiode  42  generates photoelectrons that are stored on floating diffusion node  43 . These photoelectrons lower the potential on floating diffusion node  43 . At the end of the exposure, the voltage on floating diffusion node  43  is again measured by connecting the output of source follower  26  to column amplifier  52 , and the amount of charge generated by parasitic photodiode  42  is determined to provide a first pixel intensity value. Next, floating diffusion node  43  is again reset to Vr and the potential on floating diffusion node  43  is measured by connecting the output of source follower  26  to column amplifier  52 . Gate  25  is then placed in the conducting state and the photoelectrons accumulated by main photodiode  22  are transferred to floating diffusion node  43 . The voltage on floating diffusion node  43  is then measured again and used by column amplifier  52  to compute a second pixel intensity value. 
         [0020]    If the light intensity on the corresponding pixel was high, main photodiode  22  will have overflowed; however, parasitic photodiode  42 , which has a much lower conversion efficiency, will have a value that is within the desired range. On the other hand, if the light intensity was low, there will be insufficient photoelectrons accumulated on parasitic photodiode  42  to provide a reliable estimate, and the measurement from main photodiode  22  will be utilized. 
         [0021]    The above-described readout strategy assumes that the ratio of the light detection efficiency of main photodiode  22  and parasitic photodiode  42  is adjusted such that the values obtained from parasitic photodiode  42  are complementary to those obtained from main photodiode  22 . If parasitic photodiode  42  has too low a light detection efficiency, the intensity values obtained from parasitic photodiode  42  when main photodiode  22  has become saturated will be too low or too noisy to be useful. Similarly, if the light detection efficiency of parasitic photodiode  42  is too high, a useful extension of the dynamic range of the pixel sensor will not be achieved. 
         [0022]    Refer now to  FIG. 3 , which is a cross-sectional view through a portion of a typical prior art pixel sensor. To simplify the drawing, a number of implants that are not central to the present invention have been omitted. Pixel sensor  60  is fabricated in a p-type substrate  61 . Main photodiode  62  is a pinned photodiode and is constructed from a p-implant  63  at the surface of p-type substrate  61  and an n-implant  64 . The p-implant reduces the dark current. The photodiode junction is the junction between n-implant  64  and p-type substrate  61 . The gate electrodes corresponding to gates  25  and  27  are shown at  65  and  67 , respectively. Floating diffusion node  68  includes an n+implant  66  that is surrounded by a p-implanted region. The difference in doping between p-type substrate  61  and region  68  provides a potential barrier that reduces the light collection efficiency of the parasitic photodiode formed by implant  66  and p-type substrate  61 . This difference inhibits electrons and holes generated in p-type substrate  61  from being collected by the parasitic photodiode. Prior art pixel sensors are designed to reduce or eliminate the parasitic photodiode action. The present invention provides its advantages by increasing this parasitic photodiode effect so as to provide a usable second photodiode in each pixel sensor without substantially increasing the size of the pixel sensors. 
         [0023]    Refer now to  FIG. 4 , which is a cross-sectional view of a portion of a pixel sensor according to one embodiment of the present invention. To simplify the following discussion, those elements of pixel sensor  70  that serve functions analogous to those served by elements in  FIG. 3  have been given the same numeric designations and will not be discussed in detail again. In pixel sensor  70 , the p-implant  78  associated with implant region  76  that forms the floating diffusion node has been removed from at least a portion of implant region  76 . Hence, the parasitic photodiode formed by implant region  76  in p-type substrate  61  has a depletion region that extends into p-type substrate  61 . Accordingly, the photoelectron collection efficiency of the parasitic photodiode is substantially increased. It should be noted that this increase in efficiency is obtained without substantially altering the size of implant region  76 , and hence, the second main photodiode is provided without substantially increasing the size of the pixel sensor. 
         [0024]    The light conversion efficiency of the parasitic photodiode can be adjusted by adjusting the area of the interface between n+implant region  76  and p-type substrate  61 . The light conversion efficiency can be reduced by leaving a p- implant that partially covers the bottom surface of implant region  76 . The light conversion efficiency can be increased by increasing the size of implant region  76  or by increasing the reset voltage on n+implant region  76  during the exposure. 
         [0025]    Increasing the size of the n+implant region is not, however, preferred. The charge-to-voltage conversion gain of the source pixel sensor depends on the capacitance associated with the floating diffusion node. If the capacitance is increased, the charge-to-voltage conversion gain is reduced. In general, a high charge-to-voltage conversion gain is desired; hence, increasing the size of the floating diffusion node is not preferred. The size of the depletion region associated with the parasitic photodiode can be increased by increasing the voltage on implant region  76  during the exposure. However, there is a maximum voltage that is set by the semiconductor process and the complexities of providing different reset voltages for the main photodiode and the parasitic photodiode. 
         [0026]    However, using one or more of the above described factors, a useful range for the ratio of the light conversion efficiencies of the main photodiode and parasitic photodiode can be obtained. In general, there is a light intensity-exposure time at which the main photodiode saturates. For exposures that are greater than this exposure, the additional photoelectrons are removed from the photodiode, and hence, the output of the main photodiode does not change with additional exposure. This exposure will be referred to as the main photodiode saturation exposure in the following discussion. The light collection efficiency of the parasitic photodiode is preferably set such that sufficient photoelectrons are generated in the parasitic photodiode at the main photodiode saturation exposure to provide a good measurement of the light intensity for exposures at or greater than the main photodiode saturation exposure up to some upper exposure limit, and hence, the resulting pixel sensor will have an improved dynamic range compared to a pixel sensor that does not utilize such a parasitic photodiode. 
         [0027]    The optimum ratio between the light conversion efficiencies of the main photodiode and the parasitic photodiode will depend on the noise levels in the parasitic photodiode. In general, the noise level in the parasitic photodiode will be greater than that in the main photodiode, since the parasitic photodiode is not a pinned photodiode. Hence, a greater exposure will be needed to provide a pixel intensity measurement with some predetermined signal-to-noise ratio. In addition, the light conversion efficiency of the parasitic photodiode must be less than that at which the capacity of the parasitic photodiode to hold the charge is exceeded to prevent image artifacts such as blooming when a few pixels have very high exposures. In one aspect of the invention, the light conversion efficiency of the parasitic photodiode is less than 1/30th of the light conversion efficiency of the main photodiode. 
         [0028]    A pixel sensor according to the present invention can be utilized in a number of different camera or light sensing operations. Refer now to  FIG. 5 , which illustrates a two-dimensional imaging array constructed from pixel sensors according to the present invention. Imaging array  80  includes a two dimensional array of pixel sensors  81 . Each pixel sensor has a main photodiode  86  and a parasitic photodiode  91  and operates in a manner analogous to that discussed above. The reset circuitry and amplification circuitry in each pixel is shown at  87 . The pixel sensors are arranged as a plurality of rows and columns. Each pixel sensor in a column is connected to a readout line  83  that is shared by all of the pixel sensors in that column. The readout lines are analogous to readout line  31  shown in  FIG. 2 . Each pixel sensor in a row is connected to a row select line  82  which determines which pixel sensor in that row is connected to the corresponding readout line. For example, the row select line can be connected to line  28  shown in  FIG. 2 . 
         [0029]    The operation of imaging array  80  is controlled by a controller  92  that receives a pixel address to be readout. Controller  92  generates a row select address that used by row decoder  85  to enable the readout of the pixel sensors on a corresponding row in imaging array  80 . The column amplifiers are included in an array of column amplifiers  84  which execute the readout algorithm discussed above. It should be noted that if readout time is not critical, a single readout amplifier could be shared between the various readout lines, the currently active readout line being determined by a column decoder or multiplexer. 
         [0030]    The imaging array shown in  FIG. 5  is an exemplary imaging array. It is to be understood, that other imaging array configurations that utilize the pixel sensors of the present invention could also be constructed. For example, a one-dimensional imaging array that includes a single column of pixels could be constructed for use in a scanner in which the image is swept past the column of pixel sensors in a direction perpendicular to that of the columns of pixel sensors. 
         [0031]    The above-described embodiments of the present invention have been provided to illustrate various aspects of the invention. However, it is to be understood that different aspects of the present invention that are shown in different specific embodiments can be combined to provide other embodiments of the present invention. In addition, various modifications to the present invention will become apparent from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.