Abstract:
A pixel sensor cell of improved dynamic range and a design structure including the pixel sensor cell embodied in a machine readable medium are provided. The pixel cell comprises a coupling transistor that couples a capacitor device to a photosensing region (e.g., photodiode) of the pixel cell, the photodiode being coupled to a transfer gate and one terminal of the coupling transistor. In operation, the additional capacitance is coupled to the pixel cell photodiode when the voltage on the photodiode is drawn down to the substrate potential. Thus, the added capacitance is only connected to the imager cell when the cell is nearing its charge capacity. Otherwise, the cell has a low capacitance and low leakage. In an additional embodiment, a terminal of the capacitor is coupled to a “pulsed” supply voltage signal that enables substantially full depletion of stored charge from the capacitor to the photosensing region during a read out operation of the pixel sensor cell. In various embodiments, the locations of the added capacitance and photodiode may be interchanged with respect to the coupling transistor. In addition, the added capacitor of the pixel sensor cell allows for a global shutter operation.

Description:
This application is related to co-pending and co-assigned U.S. patent application Ser. No. 11/687,245, filed Mar. 16, 2007, currently pending. 
     FIELD OF THE INVENTION 
     The present invention relates generally to pixel image sensors. More particularly, the present invention relates to an improved image cell having improved dynamic range by use of large capacitance when the cell approaches its charge capacity, and a design structure including the pixel image cell embodied in a machine readable medium. 
     DESCRIPTION OF THE PRIOR ART 
     The requirements for current solid-state image sensors, e.g., CMOS image sensors, are that they be highly sensitive, have a high S/N ratio and a high resolution. However it is desirous to provide pixel image sensor devices with greater dynamic range. Current devices are currently limited in total dynamic range. At the low end, devices can not collect less than 1 electron. At the high end, devices are limited to collect up to 30,000 electrons (i.e., approximately 5 fC calculated as 2 fF*2.5V, where 2.5 volts is the bias of the sensing photodiode or photosensing device, fF is femtoFarads and is the capacitance of the cell, and C is coulombs (charge)). 
     Dynamic range may be increased by utilizing a higher bias voltage, but doing so may negatively impact power and junction leakage. Utilizing a higher capacitance may also increase dynamic range. However, this is likely to increase dark current due to higher doping levels and an increase in tunnel current (See for example, the reference to S. M. Sze “Physics of Semiconductor Devices” 2 nd  edition p. 529). 
     Current attempts at extending the dynamic range of CMOS image sensors include the provision of an image sensor having a lateral overflow capacitor in a pixel, which integrates the overflowed charges from a fully depleted photodiode during the same exposure, as was described in the reference to Nana Akahane, et al. entitled “A Sensitivity and Linearity Improvement of a 100 dB Dynamic Range CMOS Image Sensor Using a Lateral Overflow Integration Capacitor”, 2005 Symposium on VLSI Circuits Digest of Technical Papers, 4-900784-01-X, pp. 62-65. 
       FIG. 1  depicts a prior art circuit topology  10  as described in the VLSI Circuits reference by Akahane et al. As shown in  FIG. 1 , the prior art pixel circuit  10  includes a fully depleted photodiode device (PD), a floating diffusion to convert the charge to the voltage (FD), a charge transfer switch (M 1 ), an overflow photoelectron integration capacitor (CS), a switch between the floating diffusion FD and the overflow capacitor CS (M 3 ), a reset switch (M 2 ), a source follower amplifier (M 4 ) and a pixel select switch (M 5 ). In operation, the pixel cell circuit topology  10  in  FIG. 1  collects charge in the photodiode, lets charge overflow through the transfer switch M 1  and M 3  into the extra capacitor CS for extended dynamic range. The dynamic range in this embodiment reaches 100 dB while keeping a high sensitivity and a high S/N ratio in low and very bright lights. 
     Additional prior art circuit topologies may be found in references U.S. Pat. Nos. 6,204,524, 6,429,470, and 6,852,591 representative of pixel image cell topologies that extend dynamic range by adding a capacitor directly onto the photodiode without any intervening transistor device. 
     A further circuit design as described in U.S. Patent Application Serial No. 2005/0110884, shows a capacitor device linked to the transfer gate and provides a storage node for a pixel, allowing for kTC noise reduction prior to readout. The pixel may be operated with the shutter gate on during the integration period to increase the amount of time for charge storage by a pixel. 
     It would be desirable to provide a pixel circuit that exhibits improved dynamic range by including a capacitor device that is controllably coupled to the photosensitive charge accumulator element (e.g., photodiode). 
     Moreover, as global shutter operation is the next major feature to be added to CMOS imagers, it would be desirable to provide a pixel circuit that exhibits improved dynamic range by including a capacitor device controllably coupled to the photosensitive charge accumulator element and that additionally includes global shutter extensions which are not addressed in prior art circuit topologies. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a pixel image device having improved extended dynamic range, particularly, by enabling adaptive coupling of a large capacitance via a coupling transistor to the pixel cell when the voltage on the photodiode is drawn down to the substrate potential. Thus, in this manner, the added capacitance is only connected to the imager cell when the cell is nearing its charge capacity. Otherwise, the cell has a low capacitance and low leakage. 
     Thus, according to one embodiment of the invention, there is provided a pixel image cell that relies upon the sub-threshold leakage characteristics of a transistor to enable charge carrier leakage into an added capacitor when the voltage on the photodiode is drawn down to the substrate potential. This sub-threshold leakage then continues to fill up the capacitor with its significantly larger (e.g., 2 fF-100 fF) capacitance allowing a 2-50× dynamic range increase. 
     Further, there is provided a pixel image cell that exhibits improved dynamic range that relies upon the sub-threshold leakage characteristics of a coupling transistor to leak additional accumulated charge into an added capacitor when the voltage on the photodiode is drawn down to the substrate potential and, that further is equipped with global shutter extensions. 
     Besides providing a pixel cell imager topology having improved dynamic range advantage, it additionally offers the ability to use true correlated double sampling with both photodiode read and the capacitor read operations. 
     According to one aspect of the invention, there is provided a pixel sensor cell and method of operation. The pixel sensor cell comprises: 
     a photosensitive element for receiving incident light for a pixel; 
     a capacitor device for storing charge, the capacitor having first and second terminals; 
     a capacitor coupling transistor device including a first terminal connected to the first terminal of the capacitor device and a second terminal connected to the photosensitive element; 
     wherein the capacitor coupling transistor device is biased so as to enable additional charge carriers to leak to the capacitor device via the capacitor coupling transistor device when a sufficient number of charge carriers have accumulated at the photosensitive element thereby extending the sensor cell&#39;s dynamic range of operation. 
     Moreover, further to this embodiment, the capacitor device is enabled for global shutter operation, whereby the charge carriers at the photosensitive element is fully depleted into the capacitor during a period of shutter operation. 
     In still a further embodiment, a voltage supply is provided for supplying a voltage Vp to the second terminal of the capacitor device, the voltage Vp biasing the capacitor device to facilitate charges being leaked thereto via the capacitor coupling transistor device. 
     According to another aspect of the invention, there is provided a pixel sensor cell and method of operating a pixel sensor cell of increased dynamic range, the sensor cell having a photosensitive element for receiving incident light for the pixel. The method comprises: 
     providing a capacitor device for storing charge, the capacitor having first and second terminals; 
     providing a capacitor coupling transistor device including a first terminal connected to the first terminal of the capacitor device and a second terminal connected to the photosensitive element; and, 
     biasing the capacitor coupling transistor device so as to enable additional charge carriers to leak to the capacitor device via the capacitor coupling transistor device when a sufficient number of charge carriers have accumulated at the photosensitive element, thereby increasing the sensor cell&#39;s dynamic range of operation. 
     Further to this aspect of the invention, there is provided the additional step of biasing the second terminal of the capacitor device with a voltage to facilitate leaking of charges via the capacitor coupling transistor device. 
     In another aspect of the invention, a design structure embodied in a machine readable medium is also provided that includes:
         a photosensitive element for receiving incident light for a pixel;   a capacitor device for storing charge, said capacitor having first and second terminals;   a capacitor coupling transistor device including a first terminal connected to said first terminal of said capacitor device and a second terminal connected to said photosensitive element;   wherein said capacitor coupling transistor device is biased so as to enable additional charge carriers to leak to said capacitor device via said capacitor coupling transistor device when a sufficient number of charge carriers have accumulated at said photosensitive element thereby extending a dynamic range of operation of said sensor cell.       

     In another aspect of the invention, a design structure embodied in a machine readable medium for designing, manufacturing, or testing a design is also provided that includes:
         a photosensitive element for receiving incident light for a pixel;   a capacitor device for storing charge, said capacitor having first and second terminals, wherein said capacitor device enables for global shutter operation, whereby said photosensitive element is fully depleted onto the capacitor device after a period of shutter operation;   a capacitor coupling transistor device including a first terminal connected to said first terminal of said capacitor device and a second terminal connected to said photosensitive element, said capacitor coupling transistor device being biased so as to enable additional charge carriers to leak to said capacitor device via said capacitor coupling transistor device when a sufficient number of charge carriers have accumulated at said photosensitive element, thereby increasing a dynamic range of operation of said sensor cell, and wherein said second terminal of said capacitor coupling transistor device is connected to said first terminal of said capacitor device and a first terminal of said capacitor coupling transistor device is connected to said photosensitive element; and,   a voltage supply for providing a voltage Vp to said second terminal of said capacitor device, said voltage Vp biasing said capacitor device to facilitate charges being transferred via said capacitor coupling transistor device, wherein a voltage across said photosensitive element drops as charge accumulates at said element, said capacitor coupling transistor device being enabled to transfer said additional charge carriers when voltage across said transistor device becomes sufficiently large.       

     Advantageously, the invention is advantageously employed in integrated circuits and electronic devices that employ arrays of pixel imager cells of 3T and 4T and extended designs, using CMOS semiconductor manufacturing technology. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, features and advantages of the present invention will become apparent to one skilled in the art, in view of the following detailed description taken in combination with the attached drawings, in which: 
         FIG. 1  depicts an example pixel image cell circuit  10  according to the prior art; 
         FIG. 2  depicts the novel pixel cell  100  of the invention that has increased dynamic range; 
         FIG. 3  depicts an alternate embodiment  150  of the novel pixel cell  100  of the invention described with respect to  FIG. 2  that exhibits increased dynamic range according to the invention by implementing a pulsed voltage to bias capacitor C 1 ; and, 
         FIG. 4  depicts an alternate embodiment  150 ′ of the novel pixel cell  150  of the invention depicted in  FIG. 3  wherein the locations of the capacitor and photodiode are interchanged. 
         FIG. 5  is a flow diagram of a design process used in semiconductor designing, manufacturing and/or testing. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the invention, a new pixel image device is provided that extends dynamic range above and beyond those of conventional image cell devices. 
       FIG. 2  depicts the novel pixel cell  100  of the invention that has increased dynamic range as compared to conventional pixel cells. As shown in the circuit of  FIG. 2 , there is depicted the four (4) device pixel design  100  typically referred to in literature as a 4 device active pixel sensor (APS). The pixel device  100  consists of a first transfer device labeled T 1  with a transfer gate, whose one diffusion is the photo diode  150  which collects the incident light, and the other diffusion of the transfer device T 1  is referred to as the floating diffusion  200 . The floating diffusion  200  is connected to the source  250  of a Reset device T 4  having a reset gate, and the drain  275  of the reset device T 4  is connected to Vdd which also serves as the reset voltage. Note that sometimes the drain of the reset device is connected to a separate reference voltage Vref, different from Vdd, and serves as the reset voltage. The voltage of the reset gate of device T 4  is commonly 4V and it should be at least one threshold voltage higher than the desired reset voltage. The source  200  of the transfer device T 1  is additionally connected to the gate of N-channel MOS readout transistor T 3 . The drain  300  of the readout device T 3  is connected to Vdd and the source  350  of the nFET readout device T 3  is connected to the drain of N-channel MOS transistor row select device T 2 , whose gate is connected to a Row Select line  340 . The source of the row select nFET T 2  is connected to a column output line  345 . 
     Further to the embodiment of the invention depicted in  FIG. 2 , there is provided the additional circuit addition  399  including a large capacitor device C 1   400  connected to the cell via a further transistor device T 5 . Particularly, a first terminal of the capacitor C 1  is connected to the source or drain diffusion of the coupling transistor T 5 . In operation, when the cell  100  is nearing its charge capacity the large capacitor C 1  is switched in circuit; otherwise, the cell  100  has a low capacitance and low leakage. While each of the transistors depicted in  FIG. 2  are nFET (nMOSFET) devices, it is understood that one or more pFET devices may be incorporated in the same circuit with the polarities of all voltages, e.g., Vdd, modified accordingly. 
     The operation of the pixel arrangement shown in  FIG. 2  is as follows: during a Reset operation, both the transfer transistor device T 1  and the reset transistor device T 4  are high, the reset voltage is transferred to the photo diffusion  200 . The capacitor coupling T 5  device is additionally turned on. As a result of turning on devices T 1 , T 4  and T 5 , the voltage across D 1  is pinning voltage and the voltage across C 1  is equal to the pinning voltage. 
     During photo accumulation, with incident light on the photo diffusion, there is a charge integration period formed by the resulting photocurrent generated by the charge formation at the photo diode D 1  which decreases the voltage across the diode. The following circuit configuration is established for photo accumulation: Reset transistor device T 4  and transfer device T 1  are turned OFF and device T 5  is set to a bias voltage, e.g., applied to the gate  125  of device T 5 . For example, T 5  may be set to about gnd to 0.5V above gnd., or a value sufficient to allow the transistor to leak current to C 1  when sufficient carriers have been accumulated on the photodiode. An amount of photocarriers that may initiate the leakage may be on the order of 10 k ē, for example. The bias may comprise a ground potential but other bias voltage points for gate bias  125  of transistor T 5  may be implemented. Thus, in  FIG. 2 , it is seen that the cell  100  relies upon the sub-threshold leakage characteristics of transistor T 5  to leak excess carriers into the capacitor C 1  when the voltage on the photodiode is drawn down close to the substrate potential, i.e., Vgs of T 5  starts to approach Vt (Vg=potential on T 5 , and Vs=potential on D 1  so Vgs=vg(t 5 )−vs(d 1 ) which is adjustable by the gate potential applied to T 5 . This sub-threshold leakage will then continue to fill up the capacitor with its significantly larger (about 2 fF-100 fF) capacitance allowing about a 2-50× dynamic range increase. Capacitor C 1  may comprise a capacitance value that is identical to the capacity of the photodiode anywhere from 2 femtofarads at the low end to 100 femtofarads at the high end. The capacitor devices may be formed using standard CMOS semiconductor manufacturing techniques and may comprise a trench capacitor, a MIMcap, and the like. It may for some applications be advantageous to use a pinned photodiode or pinned photogate as well. 
     The next operation is a read operation which comprise a multi-step process: 1) during read, the transfer device T 1 , reset device T 4  and capacitor coupling device T 5  are each turned OFF. The row select transistor device T 2  is turned ON so as to enable measurement of a null signal at the output line  345  for use in Correlated Double Sampling (CDS); 2.) then the reset transistor T 4  is turned on bringing the potential at node  200  (floating diffusion node) to a potential below Vdd. (This potential is determined by both the Gate potential of the transistor T 4  and the Vt of transistor T 4 .); 3.) The reset transistor is then turned off. (which may will couple node  200  down (a couple of tenths of a volt.)); 4.) Then, the potential on node  200  is read through the readout circuitry through transistors T 3  and T 2  to the column circuits and this potential is stored on a capacitor or like charge storage device; 5) then, during a second part of the read operation, reset device T 4  and coupling device T 5  remain OFF and then transfer device T 1  is turned on along with row select transistor device T 2 . At this point charge at the floating diffusion on transistor T 3  is measured, i.e., read the voltage at the source of T 3  commensurate with the voltage accumulated at the gate of T 3  representing the charge accumulation at the photodiode D 1 . This result is stored on a second capacitor or like charge storage device in the column circuit and the desired signal is the measured by subtracting the potentials on the two stored capacitors; 6) during a next step, the floating node, i.e., the source diffusion  200  at the Transfer device T 1  is reset again by turning ON transfer device T 1  and Reset device T 4 . During this step, coupling device T 5  remains OFF; 7) then, in a final read step, the charge that had been collecting on C 1   400  is read by turning each of coupling device T 5 , transfer device T 1  and row select device T 2  ON and measuring the charge on the capacitor C 1   400  using the readout transistor device T 3 , i.e., read the voltage at the source of T 3  commensurate with the voltage accumulated at the gate of T 3  representing the excess charge accumulation at the capacitor C 1 . 
     Optionally, the read of the capacitor may proceed like the reading of the photodiode with a reset operation, storage of the results on the column capacitor or like charge storage means, transfer the signals using T 5  and T 1  and proceed to read the results on a second column capacitor or like charge storage means, and then subtract the two for the final measurement. This may or may not be performed when excess charges are stored on the overflow capacitor C 1  as there is usually less need to worry about the smaller read and reset noises that are being cancelled out by using the CDS technique. 
     In an alternate embodiment of the invention, the pixel sensor circuit  100  of  FIG. 2  may be configured for alternate use, particularly, the timing may be modified for electronic shutter use whereby the integration time (photodiode exposure) time of the photodiode is controlled and all pixels are simultaneously exposed and stored in parallel. As conventionally known, the shutter feature may be activated by a mechanical shutter that stops or gates the accumulation of charge at the photodiode. In this embodiment, the circuit of  FIG. 2  is programmed for operation as follows: As in the first embodiment of  FIG. 2 , the same reset operation is performed whereby both the transfer transistor device T 1  and the reset transistor device T 4  are high (i.e., ON), the reset voltage is transferred to the photo diffusion  200 . The capacitor coupling device T 5  is additionally turned ON. As a result of turning on devices T 1 , T 4  and T 5 , the voltage across D 1  is pinning voltage and the voltage across C 1  is equal to the pinning voltage. Then, in a further step, transistor devices T 4  and T 1  are turned OFF, when the electronic shutter is commenced. During this period of shutter operation, charge is accumulated on D 1  and C 1  while T 5  remains ON. Then, coupling device T 5  is turned OFF when the electronic shutter is to be turned off. Thus, storing the potential of the photodiode and capacitor on the capacitor C 1 . Any further charges accumulated in the photodiode will not influence the potentials on C 1 . To read the pixel value, the photodiode device D 1  is first reset by keeping T 5  turned OFF while transistors T 1  and T 4  are ON. The null value is then read on the floating diffusion (CDS operation) by turning off T 1  and T 4  and turning on T 2 . Then, the charge on the capacitor device C 1  is read by turning ON each of transistors T 5 , T 1  and T 2  while keeping transistor device T 4  OFF. 
       FIG. 3  depicts a modification of the novel pixel cell  100  of the invention that has increased dynamic range as compared to conventional pixel cells. Particularly, the circuit  150  of  FIG. 3 , is identical to the circuit modified for electronic shutter use with the difference being that it is configured for pulsed mode of operation. That is, in the circuit  150  of  FIG. 3 , a pulsed voltage power supply providing pulsed signal Vp is provided at one terminal  402  of the capacitor C 1  for altering the voltage potential of the C 1  in order to obtain all charge out of D 1 . Thus, during a power reset operation, the value of Vp at terminal  402  is low, e.g., at ground potential, and the voltage across D 1  and C 1  equals the pinning potential. Then, during photo accumulation in the embodiment of the circuit  150  in  FIG. 3 , the reset device T 4  and transfer device T 1  are turned OFF, while the gate  125  of T 5  is set to a bias that allows the transistor to leak current to C 1  when sufficient carriers have been accumulated on the photodiode. It is understood that the gate bias may be ground but other bias points are possible. Then, the voltage Vp at terminal  402  is stepped up, i.e., raised to a higher value greater than ground in a range of 0.5 v to 3.3 v, which in operation, raises the potential voltage at the node N 1  at the other terminal  404  of the capacitor C 1  making it easier for charges to leak across T 5  onto T 1 , i.e., enables more charge out of D 1  to be accumulated. 
     To read the pixel value after charge accumulation in the circuit  150  of  FIG. 3 , a read operation is performed which comprises a multi-step process: 1) during read, the transfer device T 1 , reset device T 4  and capacitor coupling device T 5  are each turned OFF while the voltage Vp is high. The row select transistor device T 2  is turned ON so as to enable measurement of a null signal at the output line  345  for use in correlated double sampling; 2) then, during a second part of the read operation, reset device T 4  and coupling device T 5  remain OFF and then transfer device T 1  is turned on along with row select transistor device T 2 . Charge is transferred from the photodiode to the floating diffusion. At this point, charge at the floating diffusion on transistor T 3  is measured, i.e., read the voltage at the source of T 3  commensurate with the voltage accumulated at the gate of T 3  representing the charge accumulation at the photodiode D 1 ; 3) during a next step, the floating node, i.e., the source diffusion  200  at the Transfer device T 1  is reset again by turning ON transfer device T 1  and Reset device T 4 . During this step, coupling device T 5  remains OFF; Optionally, the reset value may be read again using T 3 . 4) then, in a final read step, the charge that had been collecting on C 1   400  is read by turning each of coupling device T 5 , transfer device T 1  and row select device T 2  ON while the voltage Vp is brought low, measuring the charge on the capacitor C 1   400  using the readout transistor device T 3 , i.e., read the voltage at the source of T 3  commensurate with the voltage accumulated at the gate of T 3  representing the excess charge accumulation at the capacitor C 1 . For this application, a pinned photodiode or a pinned photogate may be ideal for capacitor C 1  to avoid introducing excess noise from the Vp signal. 
     The circuit  150  of  FIG. 3  may be additionally modified for electronic shutter use by configuring it for pulsed mode of operation. That is, in the circuit  150  of  FIG. 3 , the reset operation is as described herein; then the reset device T 4  and transfer device T 1  are turned OFF when the shutter starts. At this point, charge begins accumulating on D 1  and C 1  by turning ON coupling transistor T 5  while the value of Vp at the capacitor terminal  402  is low, e.g., ground. Then, after photo accumulation in the alternate embodiment of operating the circuit  150  in  FIG. 3  designed for use with electronic shutter, the voltage Vp at terminal  402  is stepped up, i.e., raised to a higher value greater than ground, which in operation, raises the potential voltage (ideally, above the pinning potential of the diode D 1 ) at the node N 1  at the other terminal  404  of the capacitor C 1  making charges generated in D 1  conduct across T 5  and onto C 1 , leaving D 1  fully depleted. Then, T 5  is turned OFF when the electronic shutter is to be turned off. Then, to read the pixel value, the photodiode device D 1  is first reset by keeping T 5  turned OFF while transistors T 1  and T 4  are ON. Then, the charge on the capacitor device C 1  is read by turning ON each of transistors T 5 , T 1  and T 2  while voltage value Vp is brought low keeping transistor device T 4  OFF. 
     A further modification to the pixel cell structure  150  of  FIG. 3  is depicted in  FIG. 4  which shows a circuit  150 ′ that has switched the positions of the capacitor and the diode. That is, in the circuit  150 ′ of  FIG. 4 , the reset operation is achieved by turning on T 11 , T 4  and T 5  with Vp set to a low condition. Then, the reset device T 4  and transfer device T 1  are turned OFF, while device T 5  is turned ON when the shutter starts. At this point, charge begins accumulating on D 1  and C 1  by turning ON coupling transistor T 5  while the value of Vp at the capacitor terminal  402  is low, e.g., ground. Then, after photo accumulation in the alternate embodiment of operating the circuit  150 ′ in  FIG. 4  designed for use with electronic shutter, the voltage Vp at terminal  402  is stepped up, i.e., raised to a higher value greater than ground, which in operation, raises the potential voltage (preferably above the pinning potential of diode D 1 ) at the other terminal  404  of the capacitor C 1  driving all the charges to conduct from D 1  through T 5  to C 1 . Then, T 5  is turned OFF when the electronic shutter is to be turned off. Then, to read the pixel value, i.e., read the charge at C 1 , the operations performed as follows: 1) the floating diffusion node  200  is reset by tuning on T 4  (while T 1  and T 5  are off); 2) then, the reset device T 4  is turned OFF. The row select transistor device T 2  is turned ON so as to enable measurement of a null signal at the output line  345  for use in correlated double sampling; 3) then, during a second part of the read operation, reset device T 4  and coupling device T 5  remain OFF and then transfer device T 1  is turned on and then off again and row select transistor device T 2  is ON. At this point charge at the floating diffusion on transistor T 3  is measured. This completes a shuttered read. For this application as well, a pinned photodiode or a pinned photogate may be ideal for capacitor C 1  to avoid introducing excess noise from the Vp signal. 
     Further, with respect to application of the global shutter option, it may be preferable that D 1  have a low pin potential. This will enable all charges to transfer onto capacitor C 1  while the coupling device gate of T 5  is turned ON although these charges will be collected by D 1 . In an optional embodiment, a blooming path may be includes such that the charge accumulated on D 1  will bloom to another location (not capacitor C 1 ) during the remainder of the exposure. 
     It is further understood that, for global (electronic) shutter application to work in the embodiments of  FIGS. 3 and 4 , the capacitor C 1   400  needs to be insensitive to light exposure. That is, by implementing a light shield or providing a metal cap layer, the capacitor structure may be rendered insensitive to light exposure. 
     It is further understood that the embodiments of the invention as depicted in  FIGS. 2-4  may be devised for pixel cell configuration that are considered alternate extensions to the 4T cells depicted, e.g., they could equally apply to switched rail 3T, 4T and shared structures (4T4S, 3T4S, etc.). 
       FIG. 5  shows a block diagram of an example design flow  500 . Design flow  500  may vary depending on the type of IC being designed. For example, a design flow  500  for building an application specific IC (ASIC) may differ from a design flow  500  for designating a standard component. Design structure  520  is preferably an input to a design process  510  and may come from an IP provider, core developer, or other design company, or may be generated by the operator of the design flow, or from other sources. Design structure  520  shown in  FIGS. 2-4  comprises the novel pixel cell  100  (and/or alternative embodiments  150  and  150 ′) in the form of schematics or HDL, a hardware-description language (e.g., Verilog, VHDL, C, etc.). Design structure  520  may be a text file or a graphical representation of the pixel cell  100 . Design process  510  preferably synthesizes (or translates) the pixel cell  100  into a netlist  580 , where netlist  580  is, for example, a list of wires, transistors, logic gates, control circuits, I/O, models, etc. that describes the connections to other elements and circuits in an integrated circuit design and recorded on at least one of machine readable medium. This may be an iterative process in which netlist  580  is resynthesized one or more times depending on design specifications and parameters for the circuit. 
     Design process  510  may include using a variety of inputs; for example, inputs from library elements  530  which may house a set of commonly used elements, circuits, and devices, including models, layouts, and symbolic representations for a given manufacturing technology (e.g., different technology nodes, 32 nm, 45 nm, 90 nm, etc.), design specifications  540 , characterization data  550 , verification data  560 , design specifications  570 , and test data files  585  (which may include test patterns and other testing information). Design process  510  may further include, for example, standard circuit design processes such as timing analysis, verification, design rule checking, place and route operations, etc. One of ordinary skill in the art of IC design can appreciate the extent of possible electronic design automation tools and applications used in design process  510  without deviating from the scope and spirit of the invention. The design structure of the invention is not limited to any specific design flow. 
     Design process  510  preferably translates embodiments of the invention, as shown in  FIGS. 2-4 , along with any additional integrated circuit design or data into a second design structure  590 . Design structure  590  resides on a storage medium in a data format used for the exchange of layout data of integrated circuits (e.g., information stored in a GDSII (GDS2), GL1, OASIS, or any other suitable format for storing such design structures). Design structure  590  may comprise information such as, for example, test data files, design content files, manufacturing data, layout parameters, wires, levels of metal, vias, shapes, data for routing through the manufacturing line, and any other data required by a semiconductor manufacturer to produce embodiments of the invention, as shown in  FIGS. 2-4 . Design structure  590  may then proceed to a stage  595  where, for example, design structure  590 : proceeds to tape-out, is released to manufacturing, is released to a mask house, is sent to another design house, is sent back to the customer, etc. 
     The invention has been described herein with reference to particular exemplary embodiments. Certain alterations and modifications may be apparent to those skilled in the art, without departing from the scope of the invention. The exemplary embodiments are meant to be illustrative, not limiting of the scope of the invention.