CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 61/436,105, filed Jan. 25, 2011, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     Embodiments of the invention relate generally to improved semiconductor imaging devices and in particular to the manner of operating an array of pixels. 
     BACKGROUND OF THE INVENTION 
     A conventional four transistor (4T) circuit for a pixel  150  of a CMOS imager is illustrated in  FIG. 1 . The  FIG. 1  pixel  150  is a 4T pixel, where 4T is commonly used in the art to designate use of four transistors to operate the pixel. The 4T pixel  150  has a photosensor such as a photodiode  162 , a reset transistor  184 , a transfer transistor  190 , a source follower transistor  186 , and a row select transistor  188 . It should be understood that  FIG. 1  shows the circuitry for operation of a single pixel  150 , and that in practical use, there will be an M×N array of pixels arranged in rows and columns with the pixels of the array accessed using row and column select circuitry, as described in more detail below. 
     The photodiode  162  converts incident photons to electrons which are selectively passed to a floating diffusion stage node A through transfer transistor  190  when activated by the TX control signal. The source follower transistor  186  has its gate connected to node A and thus amplifies the signal appearing at the floating diffusion node A. When a particular row containing pixel  150  is selected by an activated row select transistor  188 , the signal amplified by the source follower transistor  186  is passed on a column line  170  to column readout circuitry. The photodiode  162  accumulates a photo-generated charge in a doped region of the substrate. It should be understood that the pixel  150  may include a photogate or other photon to charge converting device, in lieu of a photodiode, as the initial accumulator for photo-generated charge. 
     The gate of transfer transistor  190  is coupled to a transfer control signal line  191  for receiving the TX control signal, thereby serving to control the coupling of the photodiode  162  to node A. A voltage source Vpix is coupled through reset transistor  184  and conductive line  163  to node A. The gate of reset transistor  184  is coupled to a reset control line  183  for receiving the Rst control signal to control the reset operation in which the voltage source Vpix is connected to node A. 
     A row select signal (Row Sel) on a row select control line  160  is used to activate the row select transistor  188 . Although not shown, the row select control line  160  used to provide a row select signal (Row Sel) is coupled to all of the pixels of the same row of the array, as are the RST and TX lines. Voltage source Vpix is coupled to transistors  184  and  186  by conductive line  195 . A column line  170  is coupled to all of the pixels of the same column of the array and typically has a current sink  176  at its lower end. The upper part of column line  170 , outside of the pixel array, includes a pull-up circuit  111  which is used to selectively keep the voltage on the column line  170  high. Maintaining a positive voltage on the column line  170  during an image acquisition phase of a pixel  150  keeps the potential in a known state on the column line  170 . Signals from the pixel  150  are therefore selectively coupled to a column readout circuit ( FIGS. 2-4 ) through the column line  170  and through a pixel output (“Pix_out”) line  177  coupled between the column line  170  and the column readout circuit. 
     As known in the art, a value can be read from pixel  150  in a two step correlated double sampling process. First, node A is reset by activating the reset transistor  184 . The reset signal (e.g., Vpix) found at node A is readout to column line  170  via the source follower transistor  186  and the activated row select transistor  188 . During a charge integration period, photodiode  162  produces a charge from incident light. This is also known as the image acquisition period. After the integration period, transfer transistor  190  is activated and the charge from the photodiode  162  is passed through the transfer transistor to node A, where the charge is amplified by source follower transistor  186  and passed to column line  170  through the row select transistor  188 . As a result, two different voltage signals—the reset signal and the integrated charge signal—are readout from the pixel  150  and sent on the column line  170  to column readout circuitry where each signal is sampled and held for further processing as known in the art. Typically, all pixels in a row are readout simultaneously onto respective column lines  170  and the column lines may be activated in sequence for pixel reset and signal voltage readout. 
       FIG. 2  shows an example CMOS imager integrated circuit chip  201  that includes an array  230  of pixels and a controller  232 , which provides timing and control signals to enable reading out of signals stored in the pixels in a manner commonly known to those skilled in the art. Exemplary arrays have dimensions of M×N pixels, with the size of the array  230  depending on a particular application. The pixel signals from the array  230  are readout a row at a time using a column parallel readout architecture. The controller  232  selects a particular row of pixels in the array  230  by controlling the operation of row addressing circuit  234  and row drivers  240 . Signals corresponding to charges stored in the selected row of pixels and reset signals are provided on the column lines  170  to a column readout circuit  242  in the manner described above. The pixel signal read from each of the columns can be readout sequentially using a column addressing circuit  244 . Pixel signals (Vrst, Vsig) corresponding to the readout reset signal and integrated charge signal are provided as respective outputs Vout 1 , Vout 2  of the column readout circuit  242  where they are subtracted in differential amplifier  246 , digitized by analog to digital converter  248 , and sent to an image processor circuit  250  for image processing. 
       FIG. 3  shows more details of the rows and columns  249  of active pixels  150  in an array  230 . Each column includes multiple rows of pixels  150 . Signals from the pixels  150  in a particular column can be readout to sample and hold circuitry  261  associated with the column  249  (part of circuit  242 ) for acquiring the pixel reset and integrated charge signals. Signals stored in the sample and hold circuits  261  can be read sequentially column-by-column to the differential amplifier  246  which subtracts the reset and integrated charge signals and sends them to an analog-to-digital converter (ADC)  248 . 
       FIG. 4  illustrates a portion of the sample and hold circuit  261  of  FIG. 3  in greater detail. The sample and hold circuit  261  holds a set of signals, e.g., a reset signal and an integrated charge signal from a desired pixel. For example, a reset signal of a desired pixel on column line  170  is stored on capacitor  228  and the integrated charge signal is stored on capacitor  226 . 
     The operation of the circuits illustrated in  FIGS. 1-4  is now described with reference to the simplified signal timing diagram of  FIG. 5 . During an image acquisition/reset period  290 , the pull-up circuit  111  is enabled (via the PULLUP signal) to maintain the column line  170  at a high level and the signal on the row select line  160  is set to a logic low to disable the row select transistor  188  and isolate the pixel  150  from the column line  170 . 
     A readout period  298  for pixel  150  is separated into a readout period  292  for the readout of the reset signal, and a readout period  294  for the readout of the integrated charge signal. To begin the overall readout period  298 , the pull-up circuit  111  is disabled to no longer maintain the column line  170  at a high level and the signal on the row select line  160  is set to a logic high to enable the row select transistor  188  and couple the pixel  150  to the column line  170 . To begin the reset signal readout period  292 , the reset signal RST is enabled placing the reset voltage Vpix on node A which is transferred to the column line  170  via source follower transistor  186  and row select transistor  188  and stored in capacitor  228  when the SHR pulse is applied to switch  220  of the sample and hold circuit  261  ( FIG. 4 ). Thus, reset signal (Vrst) of the desired pixel  150  is sampled and stored on capacitor  228 . After the reset signal is stored, the reset readout period  292  ends. 
     After the reset readout period  292  ends, an integrated charge signal readout period  294  begins. Transfer transistor  190  is enabled by a transfer control signal Tx being pulsed on line  191 . The integrated charge which has been integrating at photodiode  162  is transferred onto Node A. Subsequently, the integrated charge signal on node A is transferred onto the column line  170  via source follower transistor  186  and row select transistor  188  and stored in capacitor  226  when an SHS signal is applied to switch  222  of the sample and hold circuit  261  ( FIG. 4 ). The SHS switch  222  ( FIG. 4 ) of the sample and hold circuit  261  is closed thereby storing an integrated charge pixel signal on capacitor  226 . The reset and integrated charge signals stored in the sample and hold circuit  261  for the column are now available for the differential readout circuit. The integrated charge signal readout period  294  and the readout period  298  is completed. As part of the next acquisition/reset period  296 , the pull-up circuit  111  is enabled to maintain the column line  170  at a high level and the signal on the row select line  160  is set to a logic low to disable the row select transistor  188  and isolate the pixel  150  from the column line  170 . 
     The circuitry described above requires space in an imager. However, there exists a need to reduce the size of imagers, and thus, it is desirable to eliminate circuitry from pixels, which helps reduce size and improves the pixel fill factor by permitting a larger area for the photodiode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an electrical schematic diagram of a conventional imager pixel. 
         FIG. 2  is a block diagram of a conventional imager chip. 
         FIG. 3  is a block diagram of an array of pixels illustrated in  FIG. 2  and an associated column readout circuit. 
         FIG. 4  is a conventional sample and hold circuit. 
         FIG. 5  is a simplified timing diagram associated with operation of the circuitry of  FIGS. 1-4 . 
         FIG. 6A  is an electrical schematic diagram of a pixel circuit where the reset transistor is controlled by a signal on the column line. 
         FIG. 6B  is an electrical schematic diagram of a pixel circuit similar to the pixel circuit shown in  FIG. 6A , except that the pixel circuit includes a pull down transistor, in accordance with an example of the present invention. 
         FIG. 7A  is a timing diagram associated with the pixel of  FIG. 6A . 
         FIG. 7B  is a timing diagram associated with the pixel of  FIG. 6B , in accordance with an example of the present invention. 
         FIG. 8  is an electrical schematic diagram of a pixel circuit, in accordance with another example of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to make and use them, and it is to be understood that structural, logical, or procedural changes may be made. 
     The embodiments described herein provide an improved imager and method of operation where the reset transistor is controlled by the signal on the column line. This control arrangement reduces the circuitry required to operate the pixel array of the imager. Dedicated reset control lines and corresponding row drivers are eliminated to reduce the area needed for a pixel and the associated circuitry. 
     Various imager pixel architectures having a column discharge are described in International Application Number PCT/US2008/063840, titled “Imager and System Utilizing Pixel with Internal Reset Control and Method of Operating Same,” which claims priority to U.S. application Ser. No. 11/802,200, filed on May 21, 2007. These applications are incorporated herein by reference in their entireties. 
     According to a first example described in the aforementioned applications, reference is now made to a pixel circuit shown in  FIG. 6A . As shown, the gate of the reset transistor  384  is coupled to and controlled by a signal on the column line  170  through a signal on the Pix_out line  177 . The pixel  350  is similar to pixel  150  of  FIG. 1  except that the gate of the reset transistor  384  is no longer coupled to a reset control line, but instead, the gate of the reset transistor  384  is coupled to the Pix_out line  177  through line  391 . The drivers and circuitry required to control and drive the dedicated reset control line  183  ( FIG. 1 ) are eliminated. In  FIG. 6A , the signal on the Pix_out line  177  is used to reset the floating diffusion node A, thus maintaining the operation of the 4T pixel. Thus, when the pull-up circuit  111  is enabled and applying a positive voltage to the column line  170 , a positive voltage is also applied through the Pix_out line  177  and line  391  to the gate of the reset transistor  384 . Applying a positive voltage to the gate of the reset transistor  384  activates the transistor  384  and couples the floating diffusion node A to the voltage source Vpix. 
     The remaining structures of pixel  350  and their operations correspond to like structures and their operations as described above with respect to  FIG. 1 . 
     The threshold of the reset transistor  384  affects the voltage of the floating diffusion node A (V FD ). If the threshold of reset transistor  384 , V rst     —     th , is zero (0), then subsequent to a reset operation, the voltage of the floating diffusion node A V FD , is equal to Vpix. If the reset transistor  384  threshold is not zero, then subsequent to a reset V FD  operation, the voltage on node A, V FD , is:
 
 V   FD   =Vpix−V   rs     —     th   (1)
 
     The operation of the circuit of  FIG. 6A  is now described with reference to the simplified signal timing diagram of  FIG. 7A . The timing diagram is illustrative of the timing of a readout of a pixel from a pixel array, as well as a portion of an acquisition/reset period that precedes and a portion of another acquisition/reset period that follows the readout period. This timing diagram of  FIG. 7A  is representative of the readout of each of the pixels from a pixel array. 
     Line  202  represents the SHR signal used to store a reset signal on a sample and hold capacitor for storing the reset signal. When SHR is logic high, switch  220  ( FIG. 4 ) is closed and capacitor  228  is coupled to the column line  170 . When SHR is logic low, switch  220  is open and capacitor  228  is uncoupled from the column line  170 . 
     Line  203  ( FIG. 7A ) represents the Tx control signal at a given time. When the Tx control signal is logic high, Tx transistor  190  ( FIG. 6A ) is activated and photodiode  162  is coupled to floating diffusion node A. When the Tx control signal is logic low, Tx transistor  190  is open and photodiode  162  is uncoupled from floating diffusion node A. Line  204  ( FIG. 7A ) represents the SHS signal used to store a integrated charge signal on a sample and hold capacitor for storing integrated charge signals. When SHS is logic high, switch  222  ( FIG. 4 ) is closed and capacitor  226  is coupled to the column line  170 . When SHS is logic low, switch  222  is open and capacitor  226  is uncoupled from the column line  170 . 
     Line  205  ( FIG. 7A ) represents the Row SeI signal at a given time. When the Row SeI signal is logic high, row select transistor  188  ( FIG. 6A ) is activated and pixel  350  is coupled to the column line  170 . When the Row SeI signal is logic low, row select transistor  188  is open and pixel  350  is uncoupled from the column line  170 . Line  206  ( FIG. 7A ) represents the PULLUP signal controlling the pull-up circuit  111  at a given time. When PULLUP is logic high, pull-up circuit  111  ( FIG. 6A ) is enabled and providing a voltage on column line  170 . When PULLUP is logic low, pull-up circuit  111  is disabled and not providing a voltage on column line  170 . Line  207  ( FIG. 7A ) represents the voltage on the Pix_out line  177  ( FIG. 6A ) at a given time. Line  208  ( FIG. 7A ) represents the voltage on the floating diffusion (FD) node A ( FIG. 6A ) at a given time. 
     During an acquisition/reset period  790 , the pull-up circuit  111  is enabled (logic high PULLUP signal) to maintain the column line  170  at a high level and the row select (Row SeI) signal on the row select line  160  is set to a logic low to disable the row select transistor  188  and isolate the source follower transistor  186  from the column line  170 . During acquisition/reset period  790 , the integrated charge signal is being accumulated by photodiode  162 . Also during the acquisition/reset period  790 , since the Pix_out line  177  is coupled to the column line  170 , the Pix_out line  177  is at a high level, which activates reset transistor  384 , thereby coupling floating diffusion node A to the reset voltage Vpix. Assuming that pull-up circuit  111  provides a 2.8V voltage and also assuming that there is no significant loss of voltage in the circuit, then when pull-up circuit  111  is at a high level and therefore Pix_put line  177  is at a high level, the voltage on Pix_out line  177  is equivalent to the voltage provided by the pull-up circuit, 2.8V. 
     As depicted in  FIG. 7A , the voltage on Pix_out line  177  ( FIG. 7A , line  208 ) during the acquisition/reset period  790  is 2.8V. Similarly, when a floating diffusion node A is reset to Vpix, the V FD  voltage on node A is 2.8V ( FIG. 7A , line  207 ), assuming no voltage loss in the circuit. In most implementations, the V FD  is related to the physical properties of the reset transistor, as indicated above with respect to Eq. (1). Thus, a reset signal is provided to the floating diffusion node A without a dedicated reset line such as the one shown in  FIG. 1 . 
     A readout period  798  for pixel  350  is separated into a readout period  792  for the readout of the reset signal, and a readout period  794  for the readout of the integrated charge signal. To begin the overall readout period  798 , the pull-up circuit  111  is disabled to no longer maintain the column line  170  at a high level and the Row SeI signal on the line  160  is set to a logic high to enable the row select transistor  188  and couple the pixel  350  to the column line  170 . 
     To begin the reset signal readout period  792 , the reset signal on floating diffusion node A is transferred to the column line  170  via source follower transistor  186  and row select transistor  188  and stored in capacitor  228  when the SHR pulse is applied to switch  220  of the readout circuit  242  ( FIG. 4 ). Thus, the reset signal (e.g., Vrst) of the desired pixel  350  is sampled and stored on capacitor  228 . After the reset signal is stored, the reset readout period  792  ends. 
     After the reset readout period  792  ends, the integrated charge signal readout period  794  begins. Transfer transistor  190  is enabled by a transfer control signal Tx being pulsed on line  191 . The integrated charge from photodiode  162  is transferred onto floating diffusion node A. Subsequently, the integrated charge signal on floating diffusion node A is transferred onto the column line  170  via source follower transistor  186  and row select transistor  188  and stored in capacitor  226  when the SHS signal is applied to switch  222  of the column readout circuit  242  ( FIG. 4 ). The SHS switch  222  of the column readout circuit  242  is closed thereby storing an integrated charge pixel signal on capacitor  226 . The reset and integrated charge signals stored in the sample and hold circuits  242  for the column are now available for the differential readout circuit  246  ( FIG. 2 ). The integrated charge signal readout period  794  and the readout period  798  is completed. 
     As depicted in  FIG. 7A , the voltage on Pix_out line  177  ( FIG. 7A , line  208 ) and the floating diffusion node A ( FIG. 7A , line  207 ) changes during the readout period  798 . During the reset readout period  792 , when the Row_sel is enabled the voltage on the Pix_out line  177  decreases due to the threshold voltage on source follower transistor  186 . The voltage on the gate of the reset gate  384  is also reduced, which builds a barrier for a potential wall on the floating diffusion node A equivalent to:
 
 V   B   =V   SF     —     th   (2)
 
     If V SF     —     th =0.8V, then the voltage on the Pix_out line  177  drops to 2.0V. 
     During the integrated charge signal readout period  794 , the voltage on the Pix_out line  177  decreases due to the transferring of the charge from the photodiode  162  to the floating diffusion node A equivalent to Q/C FD , where Q is the integrated charge of the photodiode  162  and C FD  is the capacitance of the floating diffusion node A. In the example of  FIG. 7A , Q/C FD =1, thus the voltage on Pix_out line  177  decreases 1.0 V. Correspondingly, the voltage on the Pix_out line and the reset gate  384  is reduced to 1.0V. 
     With the reduction of the voltage on the Pix_out line  177 , the barrier on the potential wall on the floating diffusion node A is
 
 Vc=V   SF     —     th   +Q/C   FD   (2),
 
and
 
 V   FD =1.8 V, as depicted in FIG.  7 A.
 
     As part of the next acquisition/reset period  796 , the pull-up circuit  111  is enabled to maintain the column line  170  at a high level and the signal on the row select line  160  is set to a logic low to disable the row select transistor  188  and isolate the pixel  350  from the column line  170 . Although not shown, node A of pixel  350  is reset by reset voltage Vpix during the acquisition/reset period  796  in a similar manner as described above, whereby the pull-up circuit  111  is enabled to maintain the column line  170  at a high level and the signal on the row select line  160  is set to a logic low to disable the row select transistor  188  and isolate the source follower transistor  186  of pixel  350  from the column line  170 . Similar to acquisition/reset period  790 , during acquisition period  796  the voltage on node A and on Pix_out line  177  is reset to 2.8V. 
     Therefore, the pixel can be operated without the need for a dedicated reset line and associated circuitry; in other words, it may have an internal reset operation. This can decrease the size required for the image sensor and corresponding circuitry. While the pixel shown in  FIG. 6A  has a number of advantages, when compared to a conventional  4 -T pixel architecture, it has fundamental limitations. One limitation is its slow readout time which limits its use in high speed applications. The Pix_out line  177  discharges slowly from 2.8V to 2.0V during the reset period of the readout cycle, as shown in signal line  208  in  FIG. 7A . 
     The column Pix_out line is first pulled up to a high voltage (2.8V) to reset the floating diffusion. This is a fast process. The Pix_out line, however, has to subsequently be discharged by a constant current provided by current mirror circuit  176 . This is a slow process that limits the readout speed of this imager pixel architecture. 
     A possible solution to tackle this problem may be to increase the discharging current to speed up the discharge process. One drawback of this approach is that power consumption is increased. The other drawback is that it lowers the SHR value of the pixel, which causes a reduction in full well capacity of the pixel if it is limited by the voltage swing of node A. 
     Another solution to tackle this problem is an internal reset architecture with enhanced column discharge (IRECD). A first embodiment of an IRECD device, in accordance to the present invention, is shown in  FIG. 6B , with its associated timing diagram shown in  FIG. 7B . 
     As shown in  FIG. 6B , an enhanced column discharge (ECD) is provided by the NMOS transistor pull down device, designated as  171 . One terminal of transistor  171  is connected to the column Pix_out line  177  (also line  170 ). The other terminal is connected to a column pull down voltage, V CPD . The gate of NMOS transistor  171  is controlled by a column pull down signal, CPD. 
     In the IRECD device, instead of relying on the constant current provided by a current mirror circuit to discharge the column Pix_out line, the present invention makes use of a constant voltage discharge to discharge the column Pix_out line. 
     Referring now to  FIG. 7B , the CPD line, designated as  209 , is activated immediately after disabling PULLUP line  206 . The CPD command causes a rapid discharge of the Pix_out signal  208 . The signal timing shown in  FIG. 7B  is similar to the timing shown in  FIG. 7A , except for the Pix_out signal  208 . In comparing  FIG. 7B  to  FIG. 7A , it will be appreciated that transistor  171  very rapidly pulls down the voltage of the Pix_out signal, while absence of transistor  171  allows a much slower current discharge through current mirror  176  of the Pix_out signal. 
     An important advantage of the IRECD circuit (device) is that the discharge settling time may be made even faster by choosing the appropriate pull down voltage. The present invention allows tuning for the best pixel performance, even after the chip is manufactured. In particular, if the pull down voltage is tuned such that it is close to the final SHR value, the settling time may be made even faster. This is similar to a critically damped circuit in a second-order system. 
     It will be appreciated that the present invention&#39;s use of an NMOS device to quickly bring the column line to its SHR value may also be applied to other imager architecture (e.g. regular 4-T pixel) to enhance their speed performance. Accordingly, an internal reset of the reset transistor need not be used, but the pull down transistor may, nevertheless, be used to speed up imager performance. 
       FIG. 8  depicts a pixel  550  according to a second embodiment of the present invention. The pixel  550  is similar to pixel  350  of  FIG. 6B  except that one source/drain of reset transistor  584  is coupled to floating diffusion node A and the other source/drain of reset transistor  584  is coupled to the pull up voltage on the column line  170  through Pix_out line  177 , i.e., the other source/drain of reset transistor  584  is coupled to the gate of reset transistor  584 . The method of operating the pixel  550  is similar to the method of operating pixel  350  as described above with respect to  FIG. 7B , except here the operating voltage for reset transistor  584  is also taken from the voltage on column line  170 . The arrangement of having a source/drain of reset transistor  584  coupled to the gate of reset transistor  584  also known as a diode connected transistor. 
     Although the embodiments described utilize a single pixel, they are not so limited and are also applicable to shared pixel arrays in which more than one photosensor from different pixels are switchably coupled to a common floating diffusion node. Descriptions of shared pixel arrays are provided in PCT/US2008/063840, which is incorporated herein by reference in its entirety. 
     While the invention has been described and illustrated with reference to specific example embodiments, it should be understood that many modifications and substitutions can be made. Although the embodiments discussed above describe specific numbers of transistors, photodiodes, conductive lines, etc., they are not so limited. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the claims.
A pixel circuit includes a photosensor and a floating diffusion node. A circuit is coupled to the floating diffusion node, for selectively providing a pixel output signal to a column line. A reset circuit, which resets the floating diffusion node, is configured to be activated by the column line. A pullup circuit is included for controlling the reset circuit through a signal on the column line. A discharge circuit, which is separate from the reset circuit, is used for discharging the pixel output signal on the column line. The discharge circuit includes a transistor having a first source/drain terminal coupled to the column line and a second source/drain terminal coupled to a fixed voltage level. The gate of the transistor activates the discharging of the column line.