Abstract:
A pixel for an imaging device is described. The pixel includes a photosensitive device provided within a substrate for providing photo-generated charges, a circuit associated with the photosensitive device for providing at least one pixel output signal representative of the photo-generated charges, the circuit includes at least one operative device that is responsive to a first control signal during operation of the associated circuit and a pump circuit. The pump circuit may include substrate pumps, charge pumps and/or voltage pumps. The pixel may also be embedded in an imaging system.

Description:
FIELD OF THE INVENTION 
   The invention relates to a CMOS imager having an array of image sensing cells and to the driving signals, which operate the cells. In particular, the present invention relates to the use of a variety of pumps in CMOS imagers. 
   BACKGROUND OF THE INVENTION 
   CMOS imagers are low cost imaging devices. A filly compatible CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits would be beneficial to many digital applications such as, for example, in cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detection systems, image stabilization systems and data compression systems for high-definition television. 
   CMOS imagers have a low voltage operation and low power consumption; CMOS imagers are compatible with integrated on-chip electronics (control logic and timing, image processing, and signal conditioning such as A/D conversion); CMOS imagers allow random access to the image data; and CMOS imagers have lower fabrication costs as compared with, for example, the conventional CCD since standard CMOS processing techniques can be used. Additionally, low power consumption is achieved for CMOS imagers because only one row of pixels at a time needs to be active during the readout and there is no charge transfer (and associated switching) from pixel to pixel during image acquisition. On-chip integration of electronics is particularly advantageous because of the potential to perform many signal conditioning functions in the digital domain (versus analog signal processing) as well as to achieve a reduction in system size and cost. 
   A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including either a photogate or a photodiode overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate. A readout circuit is connected to each pixel cell and includes at least an output field effect transistor formed in the substrate and a charge transfer section formed on the substrate adjacent the photogate or photodiode having a sensing node, typically a floating diffusion node, connected to the gate of an output transistor. The imager may include at least one electronic device such as a transistor for transferring charge from the underlying portion of the substrate to the floating diffusion node and one device, also typically a transistor, for resetting the node to a predetermined charge level prior to charge transference. 
   In a CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the floating diffusion node accompanied by charge amplification; (4) resetting the floating diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion node. The charge at the floating diffusion node is typically converted to a pixel output voltage by a source follower output transistor. The photosensitive element of a CMOS imager pixel is typically either a depleted p-n junction photodiode or a field induced depletion region beneath a photogate. For photodiodes, image lag can be eliminated by completely depleting the photodiode upon readout. 
   CMOS imagers of the type discussed above are generally known as discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip” IEEE Journal of Solid-State Circuits, Vol. 31(12) pp. 2046-2050, 1996; Mendis et al, “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3) pp. 452-453, 1994 as well as U.S. Pat. No. 5,708,263 and U.S. Pat. No. 5,471,515, which are herein incorporated by reference. 
   It should be understood that the CMOS imager may include a photodiode or other image to charge converting device, in lieu of a photogate, as the initial accumulator for photo-generated charge. 
     FIG. 1  illustrates a block diagram for a CMOS imager having a pixel array  200 .  FIG. 2A  shows a 2×2 portion of pixel array  200 . Pixel array  200  ( FIG. 1 ) comprises a plurality of pixels arranged in a predetermined number of columns and rows. The pixels of each row in array  200  are all turned on at the same time by a row select line, e.g., line  86  (see  FIG. 2A ), and the pixel signal output, V out , of each column is selectively clocked onto a column select line, e.g., V out , line  42  (see  FIG. 2A ). A plurality of row and column lines are provided for the entire array  200 . The row lines are selectively activated by the row driver  210  in response to row address decoder  220  and the column select lines are selectively activated by the column driver  260  in response to column address decoder  270 . Thus, a row and column address is provided for each pixel. The CMOS imager is operated by the control circuit  250  which controls address decoders  220 ,  270  for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry  210 ,  260  which apply driving voltage to the drive transistors of the selected row and column lines. 
   The operation of the charge collection of the CMOS imager is known in the art and is described in several publications such as Mendis et al., “Progress in CMOS Active Pixel Image Sensors,” SPIE Vol. 2172, pp. 19-29, 1994; Mendis et al., “CMOS Active Pixel Image Sensors for Highly Integrated Imaging Systems,” IEEE Journal of Solid State Circuits, Vol. 32(2), 1997; and Eric R, Fossum, “CMOS Image Sensors: Electronic Camera on a Chip,” IEDM Vol. 95 pages 17-25 (1995) as well as other publications. These references are incorporated herein by reference. 
   The use and operation of a V cc  charge pump for CMOS Imagers is described in U.S. Pat. No. 6,140,630, incorporated in its entirety herein by reference. 
   Prior art CMOS imagers are not without their shortcomings. For example, these CMOS imagers experience leakage in the transfer gate. Furthermore, it would be desirable to provide a variety of pumps including pixel voltage pump so that the CMOS imager array operating voltage could be different from a periphery supply voltage, positive and/or negative pumps, and substrate pumps. 
   SUMMARY OF THE INVENTION 
   The deficiencies of the prior art are overcome by driving one or more of the reset gate, transfer gate (if used) and the row select gate with one or more pumps. A voltage pump provides a higher voltage than the supply voltage V dd  to improve the gating operation of the reset, transfer (if used) and row select transistors. By overdriving one or more of the gates of the reset, transfer and row select transistors with the output of a voltage pump, pixel to pixel fabrication differences in electrical characteristics of these transistors can also be avoided. Moreover, if a photogate is used to acquire image charges this too may be overdriven by an output voltage from a voltage pump. The above are examples of gates that can benefit from a voltage pump but should not be taken to be limiting. 
   Additionally, incorporation of a negative pump to CMOS imager gates such as a reset gate, a row select gate or a transfer gate (if used) allows the current off, I off , performance of these gates to improve as well as the overall image performance of the CMOS imager to improve. This also allows the gate length to shrink and more die/wafer is achieved without sacrificing imager performance. The above are examples of gates that can benefit from a negative pump but should not be taken to be limiting. 
   Additionally, a substrate pump is described, where the pixels of the array are linked through the substrate. 
   The above and other advantages and features of the invention will be more clearly understood from the following detailed description which is provided in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a CMOS active pixel sensor chip; 
       FIG. 2A  is a representative pixel layout showing a 2×2 pixel layout according to one embodiment of the present invention; 
       FIG. 2B  shows a generalized signal applied to any gate of a CMOS imager; 
       FIG. 2C  shows a repeating clock voltage which can be applied to any gate of a CMOS imager; 
       FIG. 2D  is an exemplary embodiment of an external V dd  supply as the input to five separate internal pumps; 
       FIG. 2E  is an exemplary embodiment of an external V dd  supply at a lower voltage as the input to five separate internal pumps; 
       FIG. 2F  is an example of an external V dd  supply applied to a positive high voltage pump and a negative low voltage pump; 
       FIG. 3  is an exploded view of a four transistor (4T) pixel of  FIG. 2  using a V aa-pix  charge pump in accordance with the present invention; 
       FIG. 4  is an exploded view of a 3T pixel using a V aa-pix  charge pump in accordance with the present invention; 
       FIG. 5  is an exploded view of a 3T pixel using negative substrate pump in accordance with the present invention; 
       FIG. 6A  is an exploded view of a 4T pixel using a negative gate pump in accordance with the present invention; 
       FIG. 6B  is an example of a timing diagram for a reset gate and a transfer gate; 
       FIG. 7  is a processor system including a CMOS imager constructed in accordance with any of the embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will now be described with reference to the figures. Reference is now made to  FIG. 2A . This figure shows a portion of an active pixel array constructed according to the present invention in which respective charge pumps  300 ,  301  and  302  are used to supply the gate voltages for the reset transistor, transfer gate transistor and row select transistors  31 ,  29  and  38 . As shown in  FIG. 3 , reset transistor  31  is formed by n+ region  30  and n+ region  34  and controlled by RST signal  32 . Transfer transistor  29  is formed by n+ region  26  and n+ region  30  and controlled by TX signal  28 . In  FIG. 2A , charge pump  303  is shown for providing a gate voltage to a photogate  24  for charge transfer. Charge pump  304  is shown for providing a voltage pump to a N+ junction, which is the V dd  pixel supply junction in this case. This figure shows a 2×2 array of pixels for simplification. It should be understood that the invention is directed to a M×N multiple pixel array of any size. 
   The operation of the  FIG. 2A  pixel array will now be described. Photodetectors  14  of a row of pixels are coupled via their respective row select transistors  38  to column line  42 . The photodetector selected by a row decoder via line  86  will provide electrical current depending upon the voltage at the gate of source follower transistor  36  supplied by floating diffusion node  30 . As noted, the gate of transistor  36  controls the current through load transistor  39  (not shown in  FIG. 2A ) and in consequence the voltage on column line  42 . 
   Signal ROW SELECT turns row select transistor  38  on. The voltage controlled by the row select signal on line  86  is a charge pump  302  output voltage. Row select line  86  is connected to charge pump  302  to overdrive the row select transistor  38 , that is, the gate voltage of transistor  38  is higher than the V dd  supply voltage. In a V dd  system, charge pump  302  will supply V pump &gt;V dd  volts to the gate of row select transistor  38 . In the absence of a charge pump on the reset gate, the reset gate RST turns on reset transistor  31 , which causes the floating diffusion node  30  to be reset to a potential of V dd -V th , where V th  is the threshold voltage of the reset transistor  31 . The actual gate  32   a  to transistor  31  is supplied by charge pump  300  to overdrive the gate of the reset transistor  31  with a voltage of V pump &gt;V dd  to achieve higher floating diffusion voltage reset value on node  30  at V dd . By having a higher reset voltage available at node  30 , a wider dynamic response range is available for the pixel output signal and variation in the voltage at which the floating diffusion node  30  is reset due to the reset transistor  31  V th  variation is reduced. 
   The photogate  24  is also supplied from a charge pump  303 , ensuring that all possible collected charge for an image signal is stored in the imager substrate beneath the photogate until it is to be transferred out of the collection area. 
   The  FIG. 2A  circuit shows use of a transfer gate  28   a  and associated transfer transistor  29 . If the CMOS imager cells uses a transfer transistor, then the transfer gate  28   a  voltage is also supplied from a charge pump  301  in response to transfer signal TX, once again ensuring that the transfer transistor is overdriven to its on state and eliminating the V th  voltage drop which normally occurs. The charge pump on the transfer gate enables improved charge transfer between the photosensor and the floating diffusion. The operation of the  FIG. 2A  circuit to acquire, transfer and output pixel charge is otherwise as previously described. 
   The charge pumps  300  and  301  provide voltage to the reset gate  32   a  and transfer gate  28   a  at a potential which is greater than the supply potential V dd . The pumped voltage enhances the performance of the transfer and reset transistors. In order to turn “on” the various transistors of the pixel array, a gate voltage to the transistor must exceed a source or drain voltage (depending on the type of transistor) such that V pump &gt;V dd . However, the threshold voltage (V th ) may differ for each transistor of a pixel array due to manufacturing imperfections. As a consequence, when all transistors of the array are turned “on” or “off” using the voltage supply potentials to supply control signals to the gates of the transistors, some transistors which are turned “on” are more “on” than other transistors thereby inconsistently transferring and/or amplifying the pixel charges transferred to the pixel output line  42 . Likewise, some of the transistors which are turned “off” are more “off” than other transistors causing leakage. This is reflected as an improper output of signals reflecting the charges collected by the photodetector circuit  14 . 
   The charge pumps  300 ,  301  and  302  help to overcome the inconsistent on/off threshold voltages (V th ) of the transistors by overdriving the gates with voltages which ensure that they turn on or off as required, regardless of manufacturing inconsistencies. The charge pump  303  ensures that the maximum possible charges are collected in the collection region beneath the photogate. 
   While multiple charge pumps  300 ,  301 ,  302 ,  303  and  304  are shown in  FIG. 2A  for the entire CMOS pixel array, it should be understood that a single charge pump having multiple controlled output voltages may be used for the entire CMOS imager and for associated logic circuits. Also, individual charge pumps may be used for different portions of the imager circuit and for the associated logic circuits. Also, while the charge pumps  300 ,  301 ,  302 ,  303 ,  304  are shown supplying voltage for the reset gate, the transfer gate, the row select gate and the photogate and V dd  supply, it should be understood that a charge pump may be used for one or more of these gates to achieve a benefit over conventional CMOS imagers which do not use a charge pump. It is understood that the present invention is not limited to the examples described herein. More complex 5T, 6T, 7T CMOS imagers are contemplated supporting global shutter, high dynamic range, and dual conversion gain applications. Pumped gates or diffusion will be advantageous in these applications as well. That is, a common charge pump source could be used to supply the high state voltage level to all pumped clocked gates (e.g. reset, row select, transfer, photogate and V dd  supply) so long as V pump &gt;V dd . 
   The particular construction of the charge pump is not critical to the invention and many circuits circuit can be used. Representative output voltages of charge pumps  300 ,  301 ,  302  and  303  are 4.0, 4.0 and 4.0, respectively, for a 3.3 volt V dd  supply and assuming that the V th  of each of these transistors is less than 0.7 volts. While it is advantageous to have V pump ≧V dd +V th  it is not required or limiting. The photogate pump when turned on by the positive clock pulse can be at a pumped voltage such that V pump &gt;V dd . When the clock voltage applied to the photogate returns to its low or off-state voltage that off-state may be pumped low so that the gate sees a negative voltage. All of the other gates of the CMOS imager may benefit from having a negative pumped voltage applied to turn the transistor off. The negative voltage can be any value so long as it is lower than a reference ground (0V) potential. It should be understood that the output of voltage charge pumps  300 ,  301 ,  302 ,  303  and  304  may vary, individually, depending upon the V dd  and/or V ss  supply as well as the V th , of the individual transistors. For collecting charge in the photogate, the charge pump  303  is configured to supply an output voltage V pgp  where V pgp  is greater than input voltage V dd . 
     FIG. 2B  shows a generalized signal applied to any gate of a CMOS imager.  FIG. 2C  shows a repeating clock voltage which can be applied to any gate. The high state voltage  205  is pumped above V dd  (V pump &gt;V dd ). The clock voltage applied returns to a low or off-state voltage  215  resulting in a pumped voltage that is below ground (0V). 
   The above discussion has described the circuit for an exemplary 2×2 pixel shown in  FIG. 2A . It is desirable for an additional pump circuit to supply voltage V aa-pix  to diffusion node  34  and through diffusion node  34  to floating diffusion node  30  so that the CMOS imager operating voltage could be different from a periphery supply voltage. The pump circuit includes a V aa-pix  charge pump, which is supplied by external power supply V dd . The pump circuit outputs a new supply voltage that is booted. The new booted V aa-pix supply then is used to supply all of the pixels. This permits the CMOS imager array to operate at a different voltage than the periphery. 
   As described above from V dd  a voltage V aa-pix  is created using a pump circuit such that V aa-pix &gt;V dd . The present invention also encompasses the situation where V pump &lt;V dd  using a regulated power supply that is less than the supplied voltage source V dd . In the alternative V dd  could be a high state voltage such as depicted in  FIG. 2B . From this high state voltage a regulated voltage V reg  can be created, where the regulated voltage is a low state voltage. In this instance, V dd  could supply the array and V reg  could supply the periphery where 0&lt;V reg &lt;V dd . 
     FIG. 2D  is an example of an external V dd  supply  115  as the input to five separate internal pumps, V aa-pix  pump  120 , photogate pump  125 , row select pump  130 , transfer gate pump  135  and reset pump  140 . These could be pumps to independently supply V pump &gt;V dd  and/or they could supply a negative off-state voltage to the various shown clocked voltages driving the array transistor gates and diffusions of imager array  110 . A regulator  145  is also shown providing a regulated voltage to the imager circuits in the periphery, such as periphery circuit  1  ( 150 ). If the external supply, is, for example, 3.3 volts, then the five shown array pumps can produce clocked voltages to the array such that V pump &gt;V dd . In our examples if the V th  of the array transistors is 0.7 volts then a reasonable V pump  high voltage to the array gates would be 4.0 volts. The pumps could also include negative pumps to control the off-state voltage of the voltage clocks supplying the array circuits. To conserve power, in this example, the regulator is supplied at V reg &lt;V dd  to support the circuits in the imager periphery. In this example V reg  is the range of 2.5 V to 1.2 V might be reasonable. Periphery circuit  2  ( 155 ) in this example is driven directly by the V dd  external supply. For example, periphery circuit  1  ( 150 ) could be digital circuits and periphery circuit  2  ( 155 ) could be analog circuits. In this example, the imager array  110  is provided with voltages greater than or equal to the supply voltage for the “high state” or “on” voltage of the array circuits. The periphery circuits  150 ,  155  are provided with voltages less than or equal to the supply voltage. 
   In  FIG. 2E  the external supply could be at a lower voltage of 2.5 volts. In this case it would be advantageous to have a periphery circuit pump  160  to increase the voltage supply to the analog circuits of periphery circuits  2  ( 155 ). It would also be possible to have a lower regulated voltage, V reg &lt;V dd  (1.2. 1.5, 1.8, 2.0, 2.2 V) supplying the digital circuits in periphery circuits  1 . 
     FIG. 2F  is an example of an external V dd  supply  115  applied to a positive high voltage pump  190 , a negative low voltage gate pump  195  and a negative substrate pump  197 . The negative substrate pump  197  supplies voltage to p-well and p-substrate  199 . The positive high voltage pump  190  and the negative low voltage pump  195  each supply a reset driver  165 , a row select driver  175 , a transfer gate driver  180 , a photogate driver  185  and a V aa-pix  driver  170 , each of which is coupled to the imager array  110 . In this example, the positive high voltage pump  190  also supplies voltage to periphery circuits  2  ( 155 ). Periphery circuits  1  ( 150 ) is supplied directly by the external V dd  supply  115 . 
   The advantageous operation of CMOS imagers is described using a four transistor (4T) CMOS imager. Actual CMOS imagers may contain fewer or more than four transistors. It is understood that the use of 4T CMOS imagers is not meant to limit the present invention to a 4T embodiment. If the CMOS imager requires more than four transistors, then some of those additional transistors will show improved performance by having their own pump. 
     FIG. 3  is an exploded view of an exemplary 4T pixel of the present invention illustrated in  FIG. 2A , where the pixel is formed using n-channel (n-ch) devices. Like components are labeled the same as in  FIG. 2A . N+ type region  34  is actively driven by V aa-pix  charge pump  100 , which gets its supply of voltage (charge) from V dd    105 . A V aa-pix  charge pump allows the CMOS imager to operate at higher voltages and, thus, achieve better image performance. The V aa-pix  charge pump permits lower voltage periphery and can be coupled with shorter transistor lengths to improve periphery performance. N+ type region  30  (floating diffusion node) is also supplied by V aa-pix  charge pump  100  through N+ diffusion node  34  via reset transistor  31 . Photodiode (PD)  26  is an n-type diffusion region. The n-ch devices are in a p-well. Substrate contact  20  may be ground (0V) or negative if a negative substrate pump is provided. The present invention also applies to an array containing n-ch transistors. 
     FIG. 4  is an exploded view of an exemplary 3T pixel of the present invention formed using n-ch devices. The 3T transistor pixel of  FIG. 4  is similar to the 4T pixel of  FIG. 3  except that there is no transfer transistor used in the 3T implementation.  FIG. 4  is appropriate for a V aa-pix  charge pump for any CMOS imager, 2T, 3T, 4T, 5T, or any type. PD  405  is n-type diffusion region, RST signal  410  controls a reset transistor formed by PD  405  and diffusion region  415 , which is an n+ diffusion region. N+ diffusion region  420  is actively driven by V aa-pix  charge pump  425 , which gets its supply of voltage (charge) from V dd    430 . The n-ch devices are in a p-well. The present invention also applies to an array containing p-ch transistors. 
     FIG. 5  is an exploded view of an exemplary 3T pixel using a negative substrate pump. The pixel is formed using n-ch devices. The PD  505  is diffusion n-type; diffusion region  515  is diffusion type n+. The diffusion region  520  under the substrate pump contact  526  is p+. Negative V substrate  pump  525  is also connected to ground  530  and the external power supply, V dd . Reset signal (RST)  510  controls the reset transistor formed by n-type diffusion region  505  and n+ diffusion region  515 , which supplies V aa-pix . The n-ch devices are in a p-well. All p-wells in the entire array are linked and the p-well attached to the negative substrate pump is connected to the array p-wells. The present invention also applies to an array containing p-ch transistors. 
   In another embodiment, a negative gate pump supplies a negative voltage, which is applied to gates such as reset and transfer gates. Specifically,  FIG. 6A  is an exploded view of a 4T pixel of the present invention, where the pixel is formed using n-ch devices. The gates ( 28   a ,  32   a ) of transfer transistor  29  and reset transistor  31  are driven by negative gate pump  650  via a transfer voltage driver  655  and a reset voltage driver  660  respectively. which gets it supply of voltage from V dd    105 . Both the transfer gate and the reset gate could see a negative pumped off-state voltage but they would have separate clocks in that instance. In this embodiment, the negative gate pump operates to drive the gate “off” harder in n-ch devices. The negative gate pump could also be applied to the row select gate or any gate on a CMOS imager and is not limited by the exemplary embodiments described herein. Typical gates used in CMOS imagers include but are not limited to reset devices, transfer devices, global shutter devices, storage devices, high dynamic range devices and lateral overflow drain devices.  FIG. 6B  is an example of a timing diagram for a reset gate and a transfer gate. In each case, the gates are supplied with a negative pumped voltage. 
   The present invention can be utilized within any integrated circuit which receives an input signal from an external source.  FIG. 7  illustrates an exemplary processing system  600  which may utilize a processor circuit comprising a CMOS imager constructed in accordance with any of the embodiments of the present invention disclosed above in connections with  FIGS. 1-6B . The processing system  600  includes one or more processors  601  coupled to a local bus  604 . A memory controller  602  and a primary bus bridge  603  are also coupled the local bus  604 . The processing system  600  may include multiple memory controllers  602  and/or multiple primary bus bridges  603 . The memory controller  602  and the primary bus bridge  603  may be integrated as a single device  606 . 
   The memory controller  602  is also coupled to one or more memory buses  607 . Each memory bus accepts circuits such as  608  which include at least one pixel  631  using the present invention. The imaging device, e.g. a CMOS Imager, may also be integrated with a memory card or a memory module and a CPU in accordance with the present invention. Examples of memory modules include single inline memory modules (SIMMs) and dual inline memory modules (DIMMs). The imaging device  608  may include one or more additional devices  609  (not shown). For example, in a SIMM or DIMM, the additional device  609  might be a configuration memory, such as a serial presence detect (SPD) memory. The memory controller  602  may also be coupled to a cache memory  605 . The cache memory  605  may be the only cache memory in the processing system. Alternatively, other devices, for example, processors  601  may also include cache memories, which may form a cache hierarchy with cache memory  605 . If the processing system  600  include peripherals or controllers which are bus masters or which support direct memory access (DMA), the memory controller  602  may implement a cache coherency protocol. If the memory controller  602  is coupled to a plurality of memory buses  607 , each memory bus  607  may be operated in parallel, or different address ranges may be mapped to different memory buses  607 . 
   The primary bus bridge  603  is coupled to at least one peripheral bus  610 . Various devices, such as peripherals or additional bus bridges may be coupled to the peripheral bus  610 . These devices may include a storage controller  611 , a miscellaneous I/O device  614 , a secondary bus bridge  615 , a multimedia processor  618 , and a legacy device interface  620 . The primary bus bridge  603  may also be coupled to one or more special purpose high speed ports  622 . 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  600 . 
   The storage controller  611  couples one or more storage devices  613 , via a storage bus  612 , to the peripheral bus  610 . For example, the storage controller  611  may be a SCSI controller and storage devices  613  may be SCSI discs. The I/O device  614  may be any sort of peripheral. For example, the I/O device  614  may be a local area network interface, such as an Ethernet card. The secondary bus bridge may be used to interface additional devices via another bus to the processing system. For example, the secondary bus bridge  616  may be a universal serial port (USB) controller used to couple USB bus devices  617  to the processing system  600 . The multimedia processor  618  may be a sound card, a video capture card, or any other type of media interface, which may also be coupled to additional devices such as speakers  619 . The legacy device interface  620  is used to couple legacy devices  621 , for example, older styled keyboards and mice, to the processing system  600 . In addition to pixel  631  which may contain a pump circuit of the present invention multimedia processor  618  of  FIG. 7  may also utilize an imaging device of the present invention including the CPU  601 . 
   The processing system  600  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  600  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  601  coupled to imaging device  608  and/or memory buffer devices  604 . These electronic devices may include, but are not limited to audio/video processors and recorders, gaming consoles, digital television sets, wired or wireless telephones, navigation devices (including system based on the global positioning system (GPS) and/or inertial navigation), and digital cameras and/or recorders. The modifications may include, for example, elimination of unnecessary components, addition of specialized devices or circuits, and/or integration of a plurality of devices. 
   In another embodiment, a negative pump supplies a negative voltage, which is applied to gates such as reset and transfer gates. In this embodiment, the negative pump operates to drive the gate “off” harder in n-ch devices. 
   In an alternative embodiment, a positive pump supplies a positive voltage, which is applied to gates such as reset and transfer gates. In this embodiment, the positive pump operates to drive the gate “off” harder in p-ch devices. 
   While the invention has been described and illustrated with reference to specific exemplary embodiments, it should be understood that many modifications and substitutions can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.