Abstract:
A pixel circuit, and a method for operating a high-low sensitivity (HLS) pixel circuit, to provide increased dynamic range in an imager. The pixel circuit combines a four transistor (“4T”) and a three-transistor plus capacitor (“3TC”) configuration in one pixel, where the 4T portion of the pixel is coupled to a high sensitivity buried photodiode region, and the 3TC portion of the pixel is coupled to a low sensitivity buried photodiode region. The pixel circuit first reads out charge from the high sensitivity photodiode region and compares it to a reset voltage, then reads out charge from the low sensitivity photodiode region. Under an alternate embodiment, multiple HLS pixels are coupled through a common floating diffusion node.

Description:
[0001]     The present invention relates to a pixel circuit and related method of operating a pixel circuit to increase intrascene dynamic range while reducing fixed pattern noise.  
       BACKGROUND OF THE INVENTION  
       [0002]     Intrascene dynamic range refers to the range of incident light that can be accommodated by an image sensor in a single frame of pixel data. Examples of scenes that generate high dynamic range incident signals include an indoor room with outdoor window, an outdoor scene with mixed shadows and bright sunshine, night-time scenes combining artificial lighting and shadows and, in an automotive context, an auto entering or about to leave a tunnel or shadowed area on a bright day.  
         [0003]     Dynamic range is measured as the ratio of the maximum signal that can be meaningfully imaged by a pixel to its noise level in the absence of light. Typical CMOS active pixel sensors (and charge coupled device (CCD) sensors) have a dynamic range from 60 dB to 75 dB. This corresponds to light intensity ratios of about 1000:1 to about 5000:1. Noise in image sensors, including CMOS active pixel image sensors, is typically between 10 e-rms and 50 e-rms. The maximum signal accommodated is approximately 30,000 to 60,000 electrons. The maximum signal is often determined by the charge-handling capacity of the pixel or readout signal chain. Smaller pixels typically have smaller charge handling capacity.  
         [0004]     Typical scenes imaged by cameras have lighting levels that generate signals on the order of 10 to 1,000 electrons under low light (i.e., 1 to 100 lux),  1000  to  10 , 000  electrons under indoor light conditions (i.e., 100 to 1000 lux), and 10,000 to &gt;1,000,000 electrons (i.e., 1000 to 100,000 lux) under outdoor conditions. To accommodate lighting changes from scene to scene, i.e., the interscene dynamic range, an electronic shutter is used to change the integration time of all pixels in the arrays from frame to frame.  
         [0005]     To cover a single scene that might involve indoor lighting (100 lux) and outdoor lighting (50,000 lux), the required intrascene dynamic range is on the order of 5,000:1 (assuming 10 lux of equivalent noise), corresponding to 74 dB. In digital bits, this requires 13 to 14 bits of resolution. However, most CMOS active pixel sensors have only 10 bits of output and 8 bits of resolution that are typically delivered to the user in most image formats such as JPEG. Companding of the data is often used to go from 10-12 bits to 8 bits. One type of companding is gamma correction, where roughly the square root of the signal is generated.  
         [0006]     In order to accommodate high intrascene dynamic range, several different approaches have been proposed in the past. A common denominator of most approaches is performing signal companding within the pixel by having either a total conversion to a log scale (known as a logarithmic pixel) or a mixed linear and logarithmic response in the pixel.  
         [0007]     These prior approaches have several major drawbacks. First, the “knee point” in a linear-to-log transition is difficult to control, leading to fixed pattern noise in the output image. Second, under low light, the log portion of the circuit is slow to respond, leading to lag. Third, a logarithmic representation of the signal in the voltage domain (or charge domain) means that small variations in signal due to fixed pattern noise will lead to large variations in the represented signal.  
         [0008]     Linear approaches have also been described where the integration time is varied during a frame to generate several different signals. This approach has architectural problems if the pixel is read out at different points in time since data must be stored in an on-board memory before the signals can be fused together. Another approach is to integrate two different signals in the pixel, one with low gain and one with high gain. However, the low gain portion of the pixel often presents color separation processing problems.  
         [0009]     Furthermore, the idea of including capacitors in the pixel area has not been effectively developed, due to the limited area available on the pixel. Since the pixel area is primarily used for light detection and readout circuitry, capacitors have not been effectively implemented in the pixel structure.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     The present invention relates to increasing intrascene dynamic range for image capturing in a pixel circuit. Under one embodiment, a high-low sensitivity (HLS) pixel circuit comprises two separate pixels with a shared output diode (i.e., floating diffusion node). The output diode is coupled to a four transistor (4T) buried photodiode pixel circuit via a transfer gate, and is also connected to a three transistor plus capacitor (3TC) buried photodiode pixel circuit via a connecting gate. The 3TC circuit also includes a capacitor for storing charge from one of the buried photodiode regions. The combined pixels share common reset, source-follower and select transistors. Both pixel circuits are operated so that the pinning potential is set at a low value (e.g., less than 1 volt, or zero volts). In this manner, the pixels may be operated using lower operating voltages.  
         [0011]     Under an alternate embodiment, several high-low sensitivity (HLS) pixels circuits are coupled together using a common output diode (i.e., floating diffusion node). Along with the output diode, the coupled HLS pixels also share a reset, source-follower, and select transistor, thus improving pixel density with a reduced chip area. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     These and other features and advantages of the invention will be more clearly seen from the following detailed description of the invention which is provided in connection with the accompanying drawings, in which:  
         [0013]      FIG. 1  is a block diagram of an exemplary imaging device of the present invention;  
         [0014]      FIG. 2  is an exemplary high-low sensitivity pixel circuit in accordance with one aspect of the present invention;  
         [0015]      FIG. 3  is a timing diagram for the circuit of  FIG. 2 ;  
         [0016]      FIG. 4  illustrates the voltage v. green scene lux relationship for the pixel circuit of  FIG. 2 ;  
         [0017]      FIG. 5  illustrates the signal-to-noise ratio v. green scene lux relationship for the pixel circuit of  FIG. 2 ;  
         [0018]      FIG. 6  is another embodiment of the invention, where several high-low sensitivity pixels are coupled to a common output diode node; and  
         [0019]      FIG. 7  depicts a block diagram of a processor system employing the pixel circuits of  FIG. 2 - FIG. 6 , in accordance with yet another exemplary embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     The present invention is used in a CMOS imaging device generally illustrated in  FIG. 1  by reference numeral  10 . The imaging device  10  includes an array of pixels arranged in rows and columns (not shown) with each pixel having a pixel circuit  150 ; each pixel in the array is associated with a column line to which all pixels of a same column are connected, the pixels being selected row-by-row. The pixel circuit  150  provides a reset signal V RST  and a pixel image signal V SIG  as outputs during reset and integration periods, respectively. The reset signal V RST  and pixel image signal V SIG  are then captured by a sample and hold circuit  50  associated with that column in response to sampling signals SHS (for the image signal) and SHR (for the reset signal), respectively. The sample and hold circuit  50  passes the sampled reset signal V RST  and sampled image signal V SIG  to an amplifier  40  which in turn provides a signal representing the difference between the reset signal and pixel image signal (V RST -V SIG ) as an output. This difference signal is provided to an analog-to-digital converter  60  and, from there, to an image processor  80  that receives digitized pixel signals from all pixel circuits  150  of the pixel array and provides an image output.  
         [0021]     An exemplary pixel circuit constructed in accordance with the present invention is generally illustrated in  FIG. 2  by reference numeral  150 . The pixel circuit  150  has two pixels combined into a single operational pixel. The first pixel is referred to as a buried (or “pinned”) photodiode 4T cell, and is generally defined by the high sensitivity photodiode region (PDH)  113 , transfer transistor  104 , reset transistor  107 , source-follower transistor  108  and select transistor  109 . The second pixel is referred to as the buried (or “pinned”) photodiode 3TC pixel, and is generally defined by low sensitivity photodiode region (PDL)  112 , capacitor  102 , reset transistor  107 , source-follower transistor  108  and select transistor  109 . The sensitivity of each diode region  112 ,  113  is predetermined through the doping of each respective region or through other diode structures or configurations. Capacitor  102  is preferably a poly-insulator-poly (PIP) type capacitor. PIP capacitors use electrically-conductive polysilicon for forming lower and upper electrodes, whereby oxidation occurs at an interface between the upper/lower electrodes and a dielectric layer so as to form natural oxide therebetween.  
         [0022]     When using PIP capacitors in a small pixel (e.g., 5 μm×5 μm), the microlens (not shown) can be focused in a small footprint within the pixel (e.g., 2 μm×2 μm), where the rest of the pixel may be used for readout electronic circuitry. By using high value capacitors, smaller capacitor sizes may be implemented in the pixel circuit  150 . For example, a PIP capacitor having a capacitance of 5-10 fF/μm 2  would provide over 100,000 e-/volt within a few square microns of footprint. Thus, it would be possible to integrate one or more capacitors in a pixel without adversely affecting pixel size, especially since the pixel size is limited by optic considerations and cannot scale indefinitely. It should be understood that other types of capacitors may also be used to effect the same results.  
         [0023]     Turning back to  FIG. 2 , the pixel circuit  150  is modified so that the pinning potential of the photodiode is set at a low value (e.g., &lt;1volt, or even 0 volts) to help the pixel to operate at a lower voltage. The floating diffusion node (or “output diode” OD)  111  is operationally coupled to the 4T pixel circuit via transfer transistor  104 . Transfer transistor  104  controls the flow of charge accumulated in the photodiode (shown generally as n-type material  106  underneath a p-type layer  105 ) in the PDH region  113 . Connecting transistor  103  couples the 3TC circuit to the floating diffusion node  111 , and controls the flow of charge accumulated in the photodiode (shown generally as n-type material  100  underneath a p-type layer  101 ) in the PDL region  112 .  
         [0024]     The combined 4T and 3TC pixels share a common reset transistor  107 , source-follower transistor  108 , select transistor  109  and common column busline  110 . After an integration period, charge is accumulated in the PDL region  112  and the PDH region  113  proportional to the light flux incident on each photodiode. Because of the different sensitivities of each photodiode region  112 ,  113 , the collection area of the photodiodes may be unequal.  
         [0025]      FIG. 3  illustrates an exemplary timing diagram for the circuit of  FIG. 2 . The timing diagram illustrates the signal timing within a first  200  and second  201  frame period. At the beginning of a frame period, the  SEL  line is triggered high to activate transistor  109 . Subsequently, the floating diffusion region  111  is reset by pulsing a high  RST  signal to the gate terminal of reset transistor  107 . After being reset, the floating diffusion region  111  is read out onto the column line via transistors  108  and  109  and sampled (see time  202 ). Transfer transistor  104  is activated when signal  TX  goes high, allowing the charge accumulated in the PDH region  113  to spill over to the floating diffusion region  111 . The accumulated voltage is subsequently read out and sampled (see time  203 ), where the difference between the two voltages is proportional to the charge accumulated in the PDH region  113 .  
         [0026]     After the accumulated voltage is read out, the floating diffusion region is reset once again with a  RST  pulse, and connecting transistor  103  is activated when connecting control signal  CX  goes high. Once transistor  103  is activated, accumulated charge from the PDL region  112  spills over into the floating diffusion region  111 . The accumulated voltage at floating diffusion region  111  is read out and sampled (see time  204 ) from the column busline  110 . While connecting control signal  CX  remains high, the PDL region  112  is reset again by reset pulse RST and the resulting voltage on the floating diffusion region  111  is sampled (see time  205 ). The difference between the two sampled voltages (obtained at  204  and  205 ) is proportional to the charge accumulated in the PDL region  112 .  
         [0027]     Following the readout of the four samples ( 202 - 205 ), the PDL region  112  and the PDH region  113  may be concurrently or separately reset an additional time to further control the integration of each photodiode region. The dotted lines under times  206  and  207  illustrate a separate resetting of the PDH  113  and PDL  112  regions.  
         [0028]     It should be noted that the sizing of various components may add to the performance of the circuit of  FIG. 2 . For example, it is preferable that the capacitance of the floating diffusion region  111  is small (as low as 1fF). By keeping the capacitance of the floating diffusion region  111  low, read noise from the equivalent conversion gain of the PDH region  113  will be reduced. Thus, assuming a 1 fF capacitance, the equivalent conversion gain for the PDH region  113  would be 160 μV/e-. Since the correlated double sampling (CDS) of the floating diffusion region  111  for the PDH region  113  will suppress kTC noise, the read noise will be limited by the signal chain. Further, assuming the signal chain contribution to be approximately 150 μV rms, the read noise would be approximately 1 e- rms. If a 1 volt swing is designed for the floating diffusion region  111 , then the full well signal for the floating diffusion region  111  would be approximately 6250 e- with a concomitant noise of 79 e- rms.  
         [0029]     Furthermore, the capacitance of the PDL region  112  should be as large as possible, taking into consideration kTC noise associated with the shot noise of a full well PDH, as well as footprint size within the pixel area. The larger capacitance helps to extend the bright light limit to be as large as possible. As an example, if 5,000 e- is an effective full well for PDH region  113 , the shot noise would be approximately 70 e- rms. Further assuming a soft reset of the PDL region  112 , the read noise would be  
           kTC   q       ,       
 
 or approximately 30 fF for 70 e- rms. This would then require an area between 3-6 μm 2 . This capacitance corresponds to a full well of about 187,000 e- for a 1 volt swing. Under this example, the dynamic range would be 20 log(187,000/1), or about 105 dB. 
 
         [0030]     Still referring to  FIGS. 2-3 , the pixel data collected from the two CDS samples represents approximately 17 bits of dynamic range, using the values given above. Each of the double-samples are digitally converted (A/D) separately and the two resulting digital values are subsequently combined. For the combining process, each A/D conversion should be, preferably, approximately 10-12 bits to avoid excessive quantization during the combination of the digital signals. Mapping the data back to 8 or 10 bits for display purposes may require additional signal processing, which may be included on-chip. Additional enhancements may be made through converting a single sample at multiple gains.  
         [0031]      FIG. 4  is a graph illustrating exemplary responses of the  FIG. 2  circuit  150 . The graph shows the response in terms of voltage versus scene lux, and simultaneously shows the PHD output  300 , the PDL output  301 , signal chain noise  302  and PDL shot noise  303 . In the example of  FIG. 4 , the illustrated outputs are based on a 50% scene reflectivity, with a 2×2 μm PDH area and a 1×1 μm PDL area, both having 100% integration duty cycle, and both with 50% QE over the PD area. The PDH region (see output  300 ) is limited by photon shot noise over most of the range shown in  FIG. 4  until about 1000 lux, where the shot noise becomes limited. The PDL region is limited by kTC noise (see output  301 ).  
         [0032]      FIG. 5  illustrates the signal-to-noise ratio (SNR) of the PDL region (shown in  FIG. 5  as “SNR L”  400 ) and PDH region (shown in  FIG. 5  as “SNR H”  401 ), where the SNR is approximately set to zero for saturation. As can be seen from the exemplary illustration, the high sensitivity SNR H region has the better SNR response. For the SNR H signal  401 , the SNR increases steadily until about 1000 lux, where the SNR drops to zero. For the SNR L signal  400 , the SNR increases steadily until about 100,000 lux, where the SNR drops to zero. It should be noted that signal processing needs to be arranged so that a smooth switchover to the low sensitivity signal is achieved before the high sensitivity signal saturates.  
         [0033]      FIG. 6  illustrates a circuit  500  that combines two high-low sensitivity circuits  520 ,  521  using a common floating diffusion region (or “output diode”). Circuit  520  is substantially similar to the circuit described in  FIG. 2 . The pixel circuit  520  generally consists of two pixels combined into a single operational pixel. The first pixel is referred to as a buried (or “pinned”) photodiode 4T cell, and is generally defined by the high sensitivity photodiode region (PDH)  530 , transfer transistor  512 , reset transistor  516 , source-follower transistor  517  and select transistor  518 . The second pixel is referred to as the buried (or “pinned”) photodiode 3TC pixel, and is generally defined by low sensitivity diode region (PDL)  531 , capacitor  515 , reset transistor  516 , source-follower transistor  517  and select transistor  518 . Capacitor  515  is preferably a PIP-type capacitor.  
         [0034]     The pixel structure in  FIG. 6  is modified so that the pinning potential is set at a low value (e.g., &lt;1 volt, or even 0 volts) to help the pixel to operate at a lower voltage. The floating diffusion region  522  is operationally coupled to the 4T pixel circuit via transfer gate  512 . Transfer gate  512  controls the flow of charge accumulated in the photodiode (shown generally as n-type material  514  underneath a p-type layer  513 ) in the PDH region  530 . Connecting transistor  511  couples the 3TC circuit to the floating diffusion node  522 , and controls the flow of charge accumulated in the photodiode (shown generally as n-type material  510  underneath a p-type layer  509 ) in the PDL region  531 .  
         [0035]     The combined 4T and 3TC pixels share a common reset transistor  516 , source-follower transistor  517 , select transistor  518  and column busline  519 . The floating diffusion region  522  of circuit  520  is also coupled to the floating diffusion region  508  of circuit  521 . Circuit  521  has a transfer gate  505 , which controls the flow of charge accumulated in the high sensitivity photodiode (shown generally as n-type material  507  underneath a p-type layer  506 ) in the PDH region  532 . Connecting transistor  504  couples the 3TC circuit to the floating diffusion region  508 , and controls the flow of charge accumulated in the low sensitivity photodiode (shown generally as n-type material  501  underneath a p-type layer  502 ) in the PDL region  533 . The photodiode in the PDL region  533  is further coupled to capacitor  503 .  
         [0036]     During operation, both PDL  531 ,  533  and PDH  530 ,  532  regions are reset via reset transistor  516  by reset signal RST. The transfer transistors  505 ,  512  and the connecting transistors  504 ,  511  should preferably be held at a bias that is slightly more positive than reset transistor  516 . As each PDH region  530 ,  532  accumulates charge and saturates during an integration period, the charge will flow under transfer transistors  505 ,  512 , through the floating diffusion regions  508 ,  522  (after filling the regions), under connecting transistor  504 ,  511  and on to PDL regions  531 ,  533 .  
         [0037]     For readout, connecting transistors  504 ,  511  are turned on, and the voltage resulting from the sharing of charge between the floating diffusion  508 ,  522  and the PDL regions  531 ,  533  is read out through transistors  517 ,  518  and sampled. After pulsing the reset signal at transistor  516 , the resulting voltage on the shared floating diffusion regions  508 ,  522  is read out and sampled. Connecting transistors  504 ,  511  are then turned off, another reset pulse  RST  is applied to transistor  516 , and the voltage on the shared floating diffusion regions  508 ,  522  is read out and sampled again. Transfer transistors  505 ,  512  are then activated to allow charge to transfer from the PDH region  530 ,  532  to the shared floating diffusion region  508 ,  522 . The resulting PDH voltage is then read out and sampled.  
         [0038]     The PDH voltage being sampled will have low noise characteristics. The advantage of this readout method is that all photo-signals can be received via the PDH region. The result of this technique is that only the PDH region would require a microlens and color filter; the PDL region could be kept in the dark.  
         [0039]      FIG. 7  illustrates an exemplary processing system  2000  which utilizes a pixel circuit such as that described in connection with  FIGS. 2-6 . The processing system  2000  includes one or more processors  2001  coupled to a local bus  2004 . A memory controller  2002  and a primary bus bridge  2003  are also coupled the local bus  2004 . The processing system  2000  may include multiple memory controllers  2002  and/or multiple primary bus bridges  2003 . The memory controller  2002  and the primary bus bridge  2003  may be integrated as a single device  2006 .  
         [0040]     The memory controller  2002  is also coupled to one or more memory buses  2007 . Each memory bus accepts memory components  2008 . Any one of memory components  2008  may contain a high-low pixel circuit  150  or any other pixel circuits as described in connection with  FIGS. 1-6 .  
         [0041]     The memory components  2008  may be a memory card or a memory module. The memory components  2008  may include one or more additional devices  2009 . For example, in a SIMM or DIMM, the additional device  2009  might be a configuration memory, such as a serial presence detect (SPD) memory. The memory controller  2002  may also be coupled to a cache memory  2005 . The cache memory  2005  may be the only cache memory in the processing system. Alternatively, other devices, for example, processors  2001  may also include cache memories, which may form a cache hierarchy with cache memory  2005 . If the processing system  2000  include peripherals or controllers which are bus masters or which support direct memory access (DMA), the memory controller  2002  may implement a cache coherency protocol. If the memory controller  2002  is coupled to a plurality of memory buses  2007 , each memory bus  2007  may be operated in parallel, or different address ranges may be mapped to different memory buses  2007 .  
         [0042]     The primary bus bridge  2003  is coupled to at least one peripheral bus  2010 . Various devices, such as peripherals or additional bus bridges may be coupled to the peripheral bus  2010 . These devices may include a storage controller  2011 , a miscellaneous I/O device  2014 , a secondary bus bridge  2015 , a multimedia processor  2018 , and a legacy device interface  2020 . The primary bus bridge  2003  may also be coupled to one or more special purpose high speed ports  2022 . In a personal computer, for example, the special purpose port might be the Accelerated Graphics Port (AGP), used to couple a high performance video card to the processing system  2000 .  
         [0043]     The storage controller  2011  couples one or more storage devices  2013 , via a storage bus  2020 , to the peripheral bus  2010 . For example, the storage controller  2011  may be a SCSI controller and storage devices  2013  may be SCSI disc drives. The I/O device  2014  may be any sort of peripheral. For example, the I/O device  2014  may be an local area network interface, such as an Ethernet card. The secondary bus bridge  2015  may be used to interface additional devices via another bus  2024  to the processing system  2000 . For example, the secondary bus bridge  2015  may be an universal serial port (USB) controller used to couple USB devices  2017  via to the processing system  2000 . The multimedia processor  2018  may be a sound card, a video capture card, or any other type of media interface, which may also be coupled to one additional device such as speakers  2019 . The legacy device interface  2020  is used to couple legacy devices  2025 , for example, older styled keyboards and mice, to the processing system  2000 .  
         [0044]     The processing system  2000  illustrated in  FIG. 7  is only an exemplary processing system with which the invention may be used. While  FIG. 7  illustrates a processing architecture especially suitable for a general purpose computer, such as a personal computer or a workstation, it should be recognized that well known modifications can be made to configure the processing system  2000  to become more suitable for use in a variety of applications. For example, many electronic devices which require processing may be implemented using a simpler architecture which relies on a CPU  2001  coupled to memory components  2008  and/or memory devices  2009 . The modifications may include, for example, elimination of unnecessary components, addition of specialized devices or circuits, and/or integration of a plurality of devices.  
         [0045]     Other circuits containing the pixel circuits described in this disclosure include circuitry for use in memory modules, device drivers, power modules, communication modems, processor modules, and application-specific modules, and may include multilayer, multichip modules. Such circuitry can further be a subcomponent of a variety of electronic systems, such as a clock, a television, a cell phone, a personal computer, an automobile, an industrial control system, an aircraft, and others.  
         [0046]     While the invention has been described in detail in connection with preferred embodiments known at the time, it should be readily understood that the invention is not limited to the disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not limited by the foregoing description or drawings, but is only limited by the scope of the appended claims.