Abstract:
An electronic imager includes a plurality of pixels having photosensors for accumulating charge corresponding to individual pixel values of a sensed image. Each of the pixels includes an anti-blooming function which allows charge in excess of a predetermined amount to be drained from the photosensor thus reducing the charge from the pixel that migrates to adjacent pixels. The imager also includes circuitry which controls the anti-blooming function in response to image intensity to reduce dark current in the imager caused by the anti-blooming function.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention concerns semiconductor imaging devices and, in particular, such devices having anti-blooming features. 
         [0002]    A semiconductor imaging device typically includes an array of pixel cells for capturing an image. Each pixel cell includes a photo-sensor which collects photoelectrons generated during an exposure time when the electronic imager is capturing an image. During the exposure time, the photo-sensor accumulates electric charge, (i.e. electron-hole pairs) in response to impinging photons. 
         [0003]    In one example, an electronic imager may include an array of complimentary metal-oxide semiconductor (CMOS) active pixel sensor (APS) pixel cells. CMOS pixel cells typically consist of a photo-diode for photocurrent generation, a reset transistor for resetting accumulated charge produced by the photo-diode and a readout circuit composed of one or more of transistors for translating the accumulated electric charge into a readout voltage. It is known that photocurrent generation efficiency of an electronic imager increases when the photo-diode exposure area of the pixel cell is increased. Therefore, it is desired for the readout circuitry and other non-photocurrent generating hardware to consume as little area as possible in the pixel cell. One solution is an imager wherein multiple pixel cells share readout circuitry. 
         [0004]    Electronic imagers may also use charged coupled device (CCD) pixel cells. The CCD architecture typically consists of a photoactive region for photocurrent generation, and a serial readout shift register for reading the pixel cell values. 
         [0005]    One drawback to a typical pixel cell architecture is blooming. Blooming is a deteriorating effect on the captured image that occurs when a photo-diode or photoactive region in a pixel cell is saturated and excess electric charge spills into adjacent photo-diodes or photoactive regions, thus corrupting their ability to correctly capture an image. Blooming is typically prevented by an anti-blooming (AB) circuit that allows the excess electric charge to flow to a reference potential away from the adjacent photo-diodes. 
         [0006]    Anti-blooming circuitry for CCD arrays may include a lateral overflow drain (LOD) built into the pixel cell. AB is performed by the LOD wherein excess charge is attracted to a potential applied to the LOD rather than spilling into adjacent pixel cells. For APS pixels, the floating diffusion may function in a manner similar to the LOD. In these devices, the floating diffusion is held at a reference potential during the integration period so that excess charge from the photodiode spills into the floating diffusion rather than into adjacent pixels. 
         [0007]    Another common problem in electronic imagers is dark current. Dark current results in extraneous charge being collected by the pixel cells even when the imager is not exposed to light and, thus, erroneous pixel values being generated. One source of dark current is leakage in the charge collection region of a photodiode, which is strongly dependent on the doping implantation conditions. Dark current may also be caused by current generated from trap sites inside or near the photodiode depletion region; band-to-band tunneling induced carrier generation as a result of high fields in the depletion region; and junction leakage coming from the lateral sidewall of the photodiode. For both of these devices, an anti-blooming barrier having a potential close to that of a saturated photosensor (e.g. a CCD photogate or APS photodiode) allows excess charge to spill into the LOD or into the floating diffusion before it spills into neighboring pixels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic diagram of an exemplary APS pixel suitable for use with an embodiment of the invention. 
           [0009]      FIG. 2  is a block diagram of an exemplary imaging circuit that may include an embodiment of the invention. 
           [0010]      FIG. 3  is a schematic diagram of another exemplary APS pixel suitable for use with an embodiment of the invention. 
           [0011]      FIG. 4  is a schematic diagram of an exemplary sensor circuit suitable for use with an embodiment of the invention. 
           [0012]      FIG. 5  is a block diagram of an exemplary controller circuit suitable for use with an embodiment of the invention. 
           [0013]      FIGS. 6 ,  7  and  8  are flow-chart diagrams that are useful for describing the operation of embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    An electronic imager typically includes an array of pixel cells for capturing an image. Each pixel cell includes a photo-sensor for photocurrent generation during an integration time when the electronic imager is capturing an image. During the integration time, the photo-sensor accumulates electric charge in response to impinging photons. 
         [0015]    In one example, an electronic imager may consist of complimentary metal-oxide semiconductor (CMOS) active pixel sensor (APS) pixel cells. As described above, CMOS pixel cells typically consists of a photo-diode for photocurrent generation to generate an accumulated charge and a floating diffusion to store the charge accumulated by the photodiode. Although photo charges are generated as electron-hole pairs, it is the accumulation of the photoelectrons in the reverse-biased photodiode that produces the pixel values addressed here. In addition, a CMOS APS pixel may include a reset transistor for resetting accumulated charge produced by the photo-diode and a readout circuit having one or more transistors for translating the electric charge into a readout voltage. As described above, it is desirable for the readout circuitry and other non-photocurrent generating hardware to use as little area as possible in the pixel cell. One way to increase the relative area occupied by the photodiode is for multiple pixel cells to share readout circuitry and other hardware which is multiplexed among the shared pixels. 
         [0016]      FIG. 1  shows a four transistor (4T) architecture of a CMOS pixel cell  124  including: photo-diode  100  for photocurrent generation; transfer transistor  104  for transferring the electric charge from photo-diode  100  to floating diffusion  106  for electric charge storage; RESET/AB transistor  108  for applying a reference voltage to floating diffusion  106 ; source follower transistor  110  for translating the electric charge stored in floating diffusion  106  to a pixel output voltage and a select transistor  111  which provides the selected pixel signal to image processing circuitry. In a shared pixel configuration, the floating diffusion  106  and source follower transistor  110  may be shared among multiple pixels, where each pixel includes at least a photodiode and a transfer transistor. In this configuration, the signals applied to the transfer transistor  104  and the select transistor  111  are synchronized to provide each pixel in the share group at an appropriate time. 
         [0017]    Operation of the 4T architecture is now described in reference to  FIG. 1 . Although  FIG. 1  shows a 4T structure, the same analysis applies to pixels which share a floating diffusion and read-out circuitry (e.g. 2.5T, 1.75T and 1.5T pixels). Pixels that share a floating diffusion typically include a dedicated photodiode and transfer transistor. The floating diffusion, reset transistor and source-follower transistor are shared among the pixels in a time-division multiplex scheme so that each pixel in the shared group may be read at a respectively different time. For the sake of simplicity, the following discussion concerns a 4-T or 5-T pixel having a dedicated floating diffusion and dedicated reset, amplification and select transistors. It is contemplated, however, that the invention may be practiced using multiplexed pixels. 
         [0018]    Prior to the beginning of the integration period, the photodiode  100  and floating diffusion  106  are reset by turning on both the reset transistor  108  and the transfer transistor  104 . At the end of the reset period, the transfer gate  104  is set to an anti-blooming potential and, if the reset transistor  108  is also being used as an anti-blooming transistor, transistor  108  remains turned on. In one example pixel, the anti-blooming potential applied to the gate of the transfer transistor  104  may be one threshold voltage (V t ) greater than the maximum charge potential that is to be accumulated on the photodiode (V min , e.g. ground), where V t  is the difference between the gate potential and the source potential at which transfer transistor  104  becomes conductive. 
         [0019]    This potential represents an anti-blooming barrier. In this example, if the charge on the photodiode is greater than ground potential (i.e. not saturated), there will be no leakage of charge through the transfer transistor  104 . Leakage will occur only when the potential on the photodiode  100  is less than or equal to ground potential. For example, the anti-blooming potential may be ground+V t . Because the floating diffusion  106  is held at the reset potential (e.g. +3.5V), excess photoelectrons accumulated by the photodiode  100 , or other photodiodes (not shown) sharing the floating diffusion  106 , leak through the transfer transistor  104  to the floating diffusion, preventing the charge accumulated on the photodiode from reaching ground potential and thus, limiting pixel blooming. 
         [0020]    During the integration period, electrons accumulate on photo-diode  100 . Once the integration period has concluded, RESET/AB transistor  108  is turned off and transfer transistor  104  is turned on allowing the accumulated charge to be transferred from the photodiode  100  to the floating diffusion  106  for storage. To ensure that the charge is completely transferred from the photodiode  100  to the floating diffusion  206 , the example pixel uses a pinned photodiode as the photodiode  100 . The pinned photodiode is “pinned” to a potential less than the reset potential of the floating diffusion  106 , causing the accumulated photoelectrons to preferentially flow from the photodiode  100  to the floating diffusion  106 . 
         [0021]    The stored charge of floating diffusion  106  is then translated to a voltage signal by source follower transistor  110 . The voltage signal which is representative of the accumulated electric charge, is used along with voltage signals from other pixels to generate a signal representing the captured image. After the voltage signal provided by the source follower transistor  110  has been read, and while transfer transistor  104  is on, RESET/AB transistor  208  is turned on to reset floating diffusion  106  and photo-diode  100  as described above. 
         [0022]    In the 4T architecture, a single transistor  108  is used both to reset the charge on photo-diode  100  and floating diffusion  106  and to perform anti-blooming (AB). By adding a fifth transistor, it is possible to have a dedicated reset transistor and a dedicated AB transistor.  FIG. 3  shows a five transistor (5T) architecture of a CMOS pixel cell  224 ′ that includes all of the elements of the 4T architecture with the addition of a fifth AB transistor  312 . The operation of the pixel shown in  FIG. 3  is the same as that shown in  FIG. 1  except that, during the integration period, the reset transistor  108  and the transfer transistor  104  are turned off and the gate ABG of transistor  312  is held at (V min +V t ). Also, at the end of the integration period, the reset transistor  108  is turned on while the transfer transistor  104  is turned off to reset the floating diffusion before the accumulated charge on the photodiode is transferred to the floating diffusion  106 . 
         [0023]    A structure similar to the 5T structure may be used in configurations in which the floating diffusion  106 , reset transistor  108 , source follower transistor  110  and select transistor  111  are shared among multiple pixels. In these embodiments, however, each pixel would have an AB transistor  312  in addition to the photodiode  100  and the transfer transistor  104 . 
         [0024]    Although anti-blooming circuitry is beneficial in eliminating blooming in a captured image it also has a drawback: it may increase dark current. In traditional CMOS or CCD imager systems, AB is performed without consideration of the image being captured. AB is desired in some light conditions and while in other light conditions it is unnecessary. For example, in images with relatively low-intensity light characteristics it is unlikely that the photo-diodes will saturate and thus that blooming effects will be noticeable. Because pixel levels are relatively low, however, dark current will be noticeable. The opposite is also be true for images captured under relatively high-intensity light characteristics. Furthermore, dark current is less noticeable in images captured under high-intensity light characteristics. Therefore, a problem arises in traditional systems since AB techniques are employed without consideration of the light characteristics of the image. A technique for controlling an AB feature of an imager based on the light characteristics of the image being captured would be beneficial. 
         [0025]    As described above, dark current may have many sources and may depend not only on the voltages applied to the imager but on the processes used to form the imager. Thus, both the anti-blooming voltage applied to the gate of the transfer transistor  104  or to the gate of the anti-blooming transistor  312  and the voltage applied to the floating diffusion  106  or to the drain of the anti-blooming transistor  312  may affect the dark current in the pixel. Consequently, it may be desirable to control both the anti-blooming voltage and the reset potential in order to decrease both dark current distortion and blooming. 
         [0026]    An example embodiment of an electronic imager system with AB control is shown in  FIG. 2 . The imager system  200  includes: pixel cells  224  for photocurrent generation and translation of the electric charge into a pixel voltage; analog circuitry  222  for amplifying and digitizing the voltage signals from the pixel cells; system on chip (SOC)  220  for performing image processing functions on the digitized pixel values from the analog circuit; an optional sensor  202  for measuring a light intensity; a controller  210  for controlling an AB feature of the pixel cell; and optional analog to digital converters (ADCs)  206  and  214 . 
         [0027]    During exposure, system  200  measures the light intensity of the image being captured and controls the AB feature of the pixel cells so that AB is applied only when a relatively high light intensity is detected. One way to measure the light intensity uses at least one pixel cell  224  of the imager. Another way uses at least one dedicated sensor  202 . Sensor  202  may comprise a photo-diode, photo-transistor, photo-resistor or some other photo-current generating or modulating device that is separate from pixel cell  224 . One example is shown in  FIG. 4  as photo-diode  400  in parallel with resistor  402 . During exposure to high-intensity light, photo-current generation is performed by photo-diode  400 . During exposure to low-intensity light, accumulated charge from photo-diode  400  is dissipated through resistor  402  to ensure accurate tracking of incident illumination. Sensor  102  may be connected directly to controller  110  or through ADC  106  if controller  110  is a digital controller. 
         [0028]    Another way in which the light intensity may be measured is from the analog circuitry  222 . To ensure that images are properly digitized, the circuitry  222  may include an automatic gain control (AGC) function (not shown). The AGC controls the output signal of analog amplifiers (not shown) in the analog circuitry  222  so that the signals applied to ADCs (not shown) in the analog circuitry  222  are kept within a predetermined range. AGC circuitry typically includes a voltage-controlled amplifier (not shown) that is controlled by a Gain signal. This Gain signal may be used as a measure of overall light intensity. When the Gain signal is low, light intensity is high and vice-versa. In the example embodiment shown in  FIG. 2 , the Gain signal  216  is applied to the controller  210  either directly or through optional ADC  214 . 
         [0029]    A third possible way to measure light intensity is to use a function on the SOC  220 . SOC  220  may, for example, generate a histogram of pixel intensities for an image in order to control image processing. If an image includes more than a threshold number of bright pixels, as determined by this histogram, the SOC may provide a signal either directly to the pixel circuitry  124  or to the controller  210  to control the AB function of the imager  200 . 
         [0030]    During charge integration, the measured intensity signal may be provided to controller  210  for processing and control of the AB function. The gate of the AB transistor  312 , shown in  FIG. 3 , or of the transfer transistor  104  (shown in  FIG. 1 ) is set to an anti-blooming potential (e.g. V min +V t ) when the intensity signal exceeds the threshold and is turned off when the signal does not exceed the threshold. As described below, the controller  210  may also adjust the voltage applied to the floating diffusion  106  in  FIG. 1  or to the drain of the anti-blooming transistor  312  during the integration interval based on the relative intensity of the image. For an image having high intensity illumination, it may be desirable for this voltage to be relatively high, at or close to the reset voltage. This ensures that any excess charge can be quickly moved out of the photodiode. For lower intensity images, it may be desirable to reduce the AB voltage in order to reduce the electric fields generated in the imager and, so, reduce the dark current. In the exemplary embodiment of the invention, the controller  210  adjusts the potential applied to the gate electrode of the reset transistor  108 , shown in  FIG. 1  to adjust this voltage. It may also adjust the AB voltage applied to the drain electrode of the anti-blooming transistor  312  during the integration period to achieve a similar result. This voltage may be controlled in addition to controlling the anti-blooming voltage applied to the gate electrodes of the transfer transistor  104  in  FIG. 1  or the anti-blooming transistor  312  in  FIG. 3 . 
         [0031]    One example of controller  210  is a circuit including at least one comparator  508  as shown in  FIG. 5 . The light intensity signal is applied to the comparator  508  and possibly to other comparators  516  and  524 . The light intensity signal is compared to at a threshold  504  and possibly to other thresholds  514 , and  520 . When multiple thresholds are used, comparators  508 ,  516  and  524  provide output signals to decoder circuit  528 . Example decoder  528  is used to determine the range of the light intensity and to provide a control signal  530  which controls the anti-blooming voltage applied to the gate electrodes of the reset transistors  108  to control the voltage applied to the floating diffusion during the integration interval. When a single comparator is used, the controller  210  may control the potential applied to the gate electrodes of the transistors  104  and  312  and the anti-blooming voltage applied to the gate of the reset transistor  108  to either turn on or turn off the anti-blooming feature. 
         [0032]    In the example embodiment, some potential may be applied to the floating diffusion  106  or to the drain of the anti-blooming transistor  312  when the anti-blooming function is disabled. This potential may be determined, for example, by the configuration of the pixel or the process steps used to make it. It is a potential which produces the best dark current performance for the pixel. In a typical pixel, for example, this potential may be slightly negative, i.e. slightly less than ground potential, one exemplary range of values may be between −0.05 V and −0.1 V. 
         [0033]    When multiple comparators are used, the controller  210  may also control the gate electrode of transistor  108  to control voltage applied to the floating diffusion  106  or control the voltage applied to the drain of anti-blooming transistor  312  in  FIG. 3 , in discrete steps such that a higher voltage is applied for higher light-intensity images than for lower light-intensity images. The AB voltage applied to the drain electrode of transistor  312  may be controlled, for example, by modulating the conductivity of a field effect transistor (not shown) in series with the transistor  312 . 
         [0034]    The AB control procedure is now described with reference to  FIG. 6 . The light incident on the imager is measured either by sensor  102  or is provided by the analog circuitry  222  or the SOC  220 , at step  610 . this intensity value is then compared to threshold  1  and optionally to thresholds  2  through i at steps  612 ,  614  and  616 . When a single comparator is used, the output signal of the comparator  508  turns the anti-blooming function on or off. When multiple comparators are used, the binary output signals of the comparisons are then decoded at step  618  to find the light intensity range. The level of the voltage applied to the gate electrode of the reset transistor  108  or to the drain of the anti-blooming transistor  312  is then set at step  620  with respect to the decoded threshold range. 
         [0035]    Alternatively, Controller  110  may be a micro-controller. The micro-controller, for example, may calculate a mean value, maximum value, minimum value or some other statistical value based on the measured light intensity during a predetermined interval, for example, an image frame interval. This metric may then be compared to at least one threshold value and a decision may be made in response to the comparison and appropriate control signals may be provided to the pixel cells  124  via the control signal(s)  212 . The level of AB is controlled, as described above, corresponding to statistical voltage in a threshold range. 
         [0036]    The AB control procedure for the example micro-controller is now described in reference to  FIG. 7 . Light intensity is measured at step  710  and then converted to a digital representation at step  712 . The digital signal is then used to calculate a statistical metric, at step  714 , as described above. The statistical metric is compared to threshold  1  and, optionally to thresholds  2  through i, at steps  716 ,  718  and  720 . The binary outputs of the comparisons are then decoded at step  722  to determine the statistical threshold range. The level of AB may then be set with respect to the determined statistical threshold range. 
         [0037]    As described above, during normal pixel read-out of an image, low level pixel values corresponding to low light images may be amplified by analog circuitry  222 . In some instances, it may be desirable to control the AB voltage in inverse proportion to the amplification provided by analog circuitry  222  (i.e. to provide a relatively high AB voltage when the gain signal is low and to provide a relatively low AB voltage when the gain signal is high). In this instance, controller  210  may include a differential amplifier. Analog circuit  222  may provide a Gain signal to controller  210 . 
         [0038]    Operation of the differential amplifier controller is now described with reference to  FIG. 8 . At step  810 , the light intensity is measured and the Gain signal is then determined in the analog circuit  222 . This Gain signal may be determined, for example, in response to an average illumination level determined over an entire image or by the SOC  220 , which may perform a histogramming function on the image to control the Gain signal. The Gain signal may be applied to the negative input terminal of the differential amplifier while the max gain value is applied to the positive input terminal. The Gain signal is then subtracted from the maximum possible gain and AB is controlled with respect to the gain difference. In one example system, the gain difference signal may be appropriately scaled and applied to the gate of the reset transistor  108  during the integration interval to control blooming without producing excess dark current. 
         [0039]    System on chip (SOC)  202  is another option that can be used for AB control. SOC  202  can operate on the measured light intensity or amplification level of analog circuit  222 . SOC  202  can furthermore perform digital calculations and threshold comparisons in a similar manner to controller  210 . The digital nature of SOC  220  may require a digital to analog converter  214  in to convert the digital AB control signal into one or more analog AB control signals to be applied to the pixel cells  124 . 
         [0040]    Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.