Abstract:
A pixel having an electronic shutter suitable for use in a pixel array of an imaging device includes a pinned diode and a shutter transistor. The pinned diode is utilized as a storage device while the shutter transistor controls charge transfer from the electronic shutter. The use of a pinned diode as a charge storage device for the electronic shutter permits greater charge transfer efficiency, has lower leakage (or “dark” current), and permits the resulting pixel to have a greater fill factor than pixels utilizing conventional electronic shutter circuits.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates generally to semiconductor imagers. More specifically, the present invention relates to a pixel architecture supporting an electronic shutter. 
       BACKGROUND OF THE INVENTION 
       [0002]      FIG. 1  illustrates a conventional four transistor (4T) imager pixel  100  coupled via interconnect  125  to a conventional pixel reading circuit  150 . The pixel  100  includes a photodiode  101 , reset, source follower, row select, and transfer transistors  110 - 114 , and nodes A, B, E, and P. Control signals RESET, TX, and ROW are respectively applied to the gates of the reset transistor  110 , the transfer transistor  114 , and the row select transistor  112 . Node A is connected to a supply voltage source (VAAPIX) for the pixel  100 . Node E is a charge storage node. Node P is a charge accumulation node of the photodiode  101 . The outputs produced by the pixel  100  are made available at node B. These outputs include a reset output voltage Vrst and a pixel image signal output voltage Vsig. A bias circuit  113  biases a column line  125  between the pixel and a sample and hold circuit  152 . The pixel reading circuit  150  includes a photo signal sample-and-hold (S/H) circuit SHS  151  for sampling and holding the Vsig output voltage, a reset signal S/H circuit SHR  152  for sampling and holding the Vrst output voltage, a differential amplifier  153 , and nodes C and D. As illustrated, column line  125  couples the output of the pixel at node B to the input of the pixel reading circuit at node C. 
         [0003]    The pixel  100  is operated by asserting the ROW control signal to cause the row select transistor  112  to conduct. The RESET control signal is asserted to cause a reset voltage from node A (e.g., VAAPIX) to be applied to charge storage node E. The RESET control signal is then deasserted. The pixel  100  outputs a reset signal Vrst through transistors  111  and  112 , which is sampled and held by circuit  152 . The photodiode  101  is exposed to light during a charge integration period, i.e., an exposure period. Upon completion of the integration, the accumulated charge is transferred to storage node E by transistor  114  (when TX is applied) causing the pixel  100  to output a photo signal Vsig through transistors  111  and  112  and sampled and held by circuit  151 . Both the reset signal Vrst and the photo signal Vsig are output at node B, albeit at different times. During the exposure, the photodiode  101  produces a current related to the amount of incident light. Charge accumulates at node P based on the intensity of the incident light and the amount of time the transfer transistor  114  is non-conducting. When the transfer transistor  114  becomes conductive, the charge accumulated at node P is transferred through the transfer transistor  114  to storage node E. 
         [0004]    As noted, the reset signal Vrst is sampled and held by the reset signal S/H circuit  152 , while the photo signal Vsig is sampled and held by the photo signal S/H circuit  151 . The sampled and held photo and reset signals are supplied as inputs to differential amplifier  153 , which generates the signal (Vrst−Vsig). The resulting amplified output signal is available at node D for analog-to-digital conversion. 
         [0005]      FIG. 2  is a block diagram of an imager  200  having a pixel array  201 . Pixel array  201  comprises a plurality of pixels  100  arranged in a predetermined number of columns and rows. Each pixel  100  of array  200  may have the architecture as shown in  FIG. 1  or other well-known pixel architectures. 
         [0006]    Typically, the imager  200  is operated on a rolling shutter basis, in which the rows of pixels are turned on at different times on a rolling basis; each pixel in a selected row respectively outputs its reset Vrst and photo Vsig signals at the same time. That is, a row of pixels from the array  201  is selected by the control circuit  250  by sending a row address from the control circuit  250  to the row decoder  220 . The row decoder  220  decodes the row address and operates the row driver  210 . The row driver  210  asserts the ROW control signal on a line coupled to the row select transistor  112  of each pixel in the selected row. 
         [0007]    The assertion of the ROW control signal causes the row select transistor  112  of each pixel  100  in the selected row to conduct. As previously described with respect to the processing performed within each pixel, this permits each pixel  100  in the selected row to output its reset Vrst and photo Vsig signals at node B, and for the pixel reading circuit  150  associated with each pixel to output a corresponding signal at node D. The control circuit  250  operates the column decoder  270  to cause the column driver circuit  260  to select a column from the selected pixels. The output from node D of the pixel in the selected column of the selected row is routed via node D′ to an analog to digital converter  280 , which converts the output to a digital value. The digital value is processed by an image processor  290 . Once the signals from each pixel of the selected row have been output, the control circuit  250  selects another row. This process is continued until every row of the array  201  has been processed. The imager  200  may include an output circuit  295  for outputting a digital signal corresponding to the complete image. The imager  200  may further include additional well known components, such as a lens assembly, which are not illustrated in order to avoid cluttering the figure. 
         [0008]    The above described rolling shutter operation is not suitable for imaging scenes with objects having significant motion because each row is effectively imaged at a different time. In such scenes, an object may have moved significantly during the processing time between the different selected rows. Additionally, there is often a need to precisely control integration (i.e., exposure) time of a pixel on a frame basis. Control of the integration time on a frame basis would permit more accurate exposure, particularly of images having relatively bright and/or relatively dark areas. Some imagers utilize mechanical shutters, which may be difficult to control precisely. Other images utilize electronic shutters, which include storage capacitors. Although electronic shutters can be more easily controlled, the use of capacitors has some disadvantages including, for example, a decreased pixel fill factor, decreased efficiency in pixel charge transfer, and increased susceptibility to noise. Accordingly, there is a need for a pixel architecture that includes an electronic shutter control capable of operating in a full frame mode, but which is relatively immune to the known disadvantages associated with a pixel architecture featuring a capacitor. 
       SUMMARY OF THE INVENTION 
       [0009]    Exemplary embodiments of the method and apparatus of the present invention provide a pixel architecture with an electronic shutter circuit comprising a shutter transistor and a pinned diode. The pinned diode is utilized as a charge storage device, while the shutter transistor is used along with a transfer transistor to control charge transfer from a photodiode to a source region coupled to the gate of a source follower transistor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The foregoing and other advantages and features of the invention will become more apparent from the detailed description of exemplary embodiments of the invention given below with reference to the accompanying drawings, in which: 
           [0011]      FIG. 1  illustrates a conventional pixel and associated pixel reading circuit; 
           [0012]      FIG. 2  illustrates a conventional imaging system; 
           [0013]      FIG. 3  illustrates a pixel in accordance with one embodiment of the present invention; 
           [0014]      FIG. 4A  illustrates an imager incorporating the pixel of  FIG. 3 ; 
           [0015]      FIG. 4B  is a timing diagram relating to the operation of the imager of  FIG. 4A ; and 
           [0016]      FIG. 5  illustrates a system incorporating the imager of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Now referring to the drawings, where like reference numerals designate like elements, there is shown in  FIG. 3 , a pixel  100 ′ having an architecture in accordance with one embodiment of the invention. The pixel  100 ′ includes several components found in pixel  100  ( FIG. 1 ), but the illustrated pixel  100 ′ includes additional circuitry in the form of an electronic shutter  300 . 
         [0018]    Node A of the pixel  100 ′ is coupled to a voltage source, which is typically a pixel array supply voltage VAAPIX. Node B is an output node. The pixel  100 ′ outputs its reset signal Vrst and the photo signal Vsig, at different times through node B. 
         [0019]    The photodiode  101  is typically a pinned photodiode, and produces photo generated charges, the number of which varies in accordance with incident light. The photo generated charges accumulate at node P. Typically, the incident light arrives from a subject after being focused by a lens assembly (not illustrated). 
         [0020]    The electronic shutter  300  includes a pinned diode  302  and a shutter control transistor  315 . The shutter control transistor  315  has one source/drain coupled via node P to the photodiode  101  and another source/drain coupled to a charge storage node E′. The gate of the shutter control transistor  315  accepts a control signal SH. The control signal SH is used to control the conductivity of the shutter control transistor  315 , and thus control whether photo generated charges accumulated at node P are transferred to the charge storage node E′. This shutter control is on a global basis for all imaging pixels of a pixel array. 
         [0021]    The charge storage node E′ is also coupled to the transfer transistor  114  and the pinned diode  302 . The pinned diode is also coupled to a predetermined voltage source. The predetermined voltage applied to pinned diode  302  is illustrated as ground potential, however, the predetermined voltage can be any fixed potential, for example, the substrate voltage (Vss). The pinned diode  302  can be any type of pinned diode, but in one exemplary embodiment the pinned diode  302  is a pinned photodiode similar to photodiode  101 . More specifically, the photodiode  101  receives incident light while the pinned diode  302  is blocked from receiving incident light. Accordingly, the photodiode  101  produces charges in response to incident light while the pinned diode, being opaque, does not. 
         [0022]    The reset transistor  110  has its source and drain coupled between node E and supply voltage node A, while the source follower transistor  111  has its source and drain coupled in series between node A and a source/drain of the row select transistor  112 . Another source/drain of the row select transistor  112  is coupled to node B. The source follower transistor  111  has its gate directly coupled to stage node E. The transfer gate transistor  114  acts as a switch to control the flow of charge from the charge storage node E′ to node E and the gate of the source follower transistor  111 . The shutter control transistor  315  acts as a switch between nodes P and E′. 
         [0023]      FIG. 4A  is a block diagram of an imager  200 ′ constructed in accordance with one exemplary embodiment of the invention. The imager  200 ′ includes similar components as imager  200  ( FIG. 2 ), however, imager  200 ′ features a pixel array  201 ′ having the pixels  100 ′ incorporating the electronic shutter  300 . The imager  200 ′ additionally includes a new control circuit  250 ′. 
         [0024]      FIG. 4B  is a timing diagram illustrating the timing sequence of control signals, which are controlled by the control circuit  250 ′ to operate the imager  200 ′. The SH control signal is a global signal common to each shutter control transistor  315  of each pixel  100 ′ in the pixel array  201 ′. The ROW-i, RST-i, TX-i, and OUT-i signals respectively represent the ROW control signal, RST control signal, TX control signal, and pixel output at node B of each pixel  100 ′ in the selected row. 
         [0025]    Time t 0  denotes the beginning of imaging operations for a new frame and the start of the integration period. At time t 0  each of the control signals SH, ROW-i, RST-i, TX-i is asserted low and there is no pixel output OUT-i. 
         [0026]    Time t 1  denotes the beginning of the global charge transfer period. The SH control signal is asserted high, causing the shutter control transistor  315  in each pixel  100 ′ of the pixel array  201 ′ to conduct. This permits, in each pixel  100 ′, photo generated charges that were accumulated during the integration period to be transferred to the charge storage node E′. 
         [0027]    Time t 2  denotes the end of the global charge transfer period. The SH control signal is asserted low, causing the shutter control transistor  315  in each pixel  100 ′ of the pixel array  201 ′ to not conduct. As a result, any additional photo generate charges produced after time t 2  are not added to the charge already accumulated at node E′. The new integration time starts once the SH control signal goes low. 
         [0028]    Time t 2  also denotes the beginning of a row read out operation. Accordingly, the ROW-i control signal is asserted high, causing the row select transistor  112  in each pixel  100 ′ of the selected row to conduct. Simultaneously, the RST-i control signal is also asserted high, causing the reset transistor  110  of each pixel  100 ′ in the selected row to conduct. As a result, each pixel  100 ′ in the selected row outputs a reset signal Vrst at node B. 
         [0029]    At time t 3 , the RST-i control signal is asserted low while the TX-i control signal is asserted high. As a result, the reset transistor  110  in each pixel  100 ′ of the selected row stops conducting and the transfer transistor  114  of each pixel  100 ′ of the selected row begins to conduct. As a result, in each pixel  100 ′ of the selected row, the charge accumulated during the integration period, which was stored at node E′, is coupled to the gate of the source follower transistor  111 , causing each pixel  100 ′ of the selected row to output the photo signal Vsig. 
         [0030]    Time t 4  denotes the end of row read out for the selected row. The ROW-i and TX-i control signals are each asserted low, causing the row select transistor  112  and transfer transistor  114  of each pixel  100 ′ in the selected row to stop conducting. 
         [0031]    The operations described above between time t 2  and time t 4  can then be repeated for a different one of the plurality of rows in the pixel array  201 ′, until each row in the pixel array  201 ′ has been read as described above. 
         [0032]      FIG. 5  shows system  500 , a typical processor system modified to include an imager  200 ′ ( FIG. 4 ) of the invention. The system  500  is exemplary of a system having digital circuits that could include image sensor devices. Without being limiting, such a system could include a computer system, camera system, scanner, machine vision, vehicle navigation, video phone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system, and data compression system. 
         [0033]    System  500 , for example a camera system, generally comprises a central processing unit (CPU)  511 , such as a microprocessor, that communicates with an input/output (I/O) device  506  over a bus  520 . Imaging device  200 ′ also communicates with the CPU  511  over the bus  520 . The system  500  also includes random access memory (RAM)  504 , and can include removable memory  514 , such as flash memory, which also communicate with the CPU  511  over the bus  520 . The imager  200 ′ may be combined with a processor, such as a CPU, digital signal processor, or microprocessor, with or without memory storage on a single integrated circuit or on a different chip than the processor. 
         [0034]    The present invention is therefore directed to a pixel architecture having an electronic shutter comprising a shutter transistor and a pinned diode. The pinned diode is used as a charge storage device to permit every pixel of the array to image simultaneously. The shutter transistor is used, in conjunction with the transfer transistor, to controllably isolate the imaged charge from the floating diffusion (node E). This permits the pixels output signals to be processed by the image processor in a conventional manner. Preferably, the transfer transistor and shutter transistor of any pixel are never simultaneously in a conducting state. 
         [0035]    While the invention has been described in detail in connection with the exemplary embodiments, it should be understood that the invention is not limited to the above disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alternations, 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.