Abstract:
A pixel circuit with a dual gate PMOS is formed by forming two P +  regions in an N −  well. The N −  well is in a P +  type substrate. The two P +  regions form the source and drain of a PMOS transistor. The PMOS transistors formed within the N −  well will not affect the collection of the photo-generated charge as long as the source and drain potentials of the PMOS transistors are set at a lower potential than the N −  well potential so that they remain reverse biased with respect to the N −  well. One of the P +  regions used to form the source and drain regions can be used to reset the pixel after it has been read in preparation for the next cycle of accumulating photo-generated charge. The N −  well forms a second gate for the dual gate PMOS transistor since the potential of the N −  well 12 affects the conductivity of the channel of the PMOS transistor. The addition of two NMOS transistors enables the readout signal to be stored at the gate of one of the NMOS transistors thereby making a snapshot imager possible. The circuit can be expanded to form two PMOS transistors sharing a common drain in the N −  well.

Description:
[0001]     This is a Continuation Application of U.S. patent application Ser. No. 11/068,365 filed on Feb. 28, 2005, which is hereby incorporated in its entirety, and assigned to a common assignee. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     (1) Field of the Invention  
         [0003]     This invention relates to a CMOS pixel comprising an N −  well formed in a P +  epitaxial silicon layer with a dual gate PMOS transistors formed in the N −  well.  
         [0004]     (2) Description of the Related Art  
         [0005]     U.S. Pat. No. 6,147,362 to Keyser describes a high performance pixel for active matrix electronic displays. The pixel combines a compact mesa-isolated PMOS access transistor with a novel, area efficient high voltage device.  
         [0006]     U.S. Pat. No. 6,127,697 to Guidash describes an active pixel sensor comprising a substrate of a first conductivity type having a surface containing PMOS and NMOS implants that are indicative of a sub-micron CMOS process, a photodetector formed at a first depth from an implant of a second conductivity type that is opposite the first conductivity type on the surface, and a gate on the surface adjacent to the photodetector. The photodetector is formed by an implant of the second conductivity type that is deeper and more lightly doped than implants used within the sub-micron CMOS process.  
         [0007]     U.S. Pat. No. 5,923,369 to Merrill et al. describes an active pixel sensor cell array in which a differential amplifier amplifies the output of each cell. The output of the differential amplifier is fed back to one of its inputs. The use of the differential amplifiers reduces fixed pattern noise in the image data generated by reading the array.  
         [0008]     U.S. Pat. No. 5,917,547 to Merrill et al. describes an active pixel sensor array in which a two stage amplifier amplifies the output of each cell. The two stage amplifier design reduces fixed pattern noise in the image data generated by reading the array.  
       SUMMARY OF THE INVENTION  
       [0009]     Active pixel sensors, APS, are of particular value in digital imaging systems because they can be fabricated using standard CMOS, complimentary metal oxide semiconductor, processing and because they have lower power consumption than CCD, charge coupled device, imagers. As CMOS process parameters shrink, the analog performance of minimum size transistors deteriorates. It is desirable to have transistors in the semiconductor well forming the pixel which can be drawn to a size large enough to improve the analog performance without impacting the area under which signal-generated carriers, such as photo-generated carriers, will be generated. This is a problem using N +  regions with V DD  bias acting as drains to form the pixel.  
         [0010]     It is a principle objective of this invention to provide a CMOS pixel circuit formed in an N −  well with a dual gate PMOS, P channel metal oxide semiconductor, transistor formed in an N −  well wherein any of the P + + regions used to form the PMOS transistor can be used to reset the pixel.  
         [0011]     It is another principle objective of this invention to provide a CMOS pixel circuit formed in an N −  well with a dual gate PMOS transistor formed in an N −  well with two NMOS, N channel metal oxide semiconductor, transistors used to read the pixel.  
         [0012]     It is another principle objective of this invention to provide a CMOS pixel circuit formed in an N −  well with two dual gate PMOS transistors formed in an N −  well with four NMOS transistors used to read the pixel.  
         [0013]     These objectives are achieved by forming an N −  well in a P −  epitaxial silicon layer. P +  regions are then formed in the N −  well to form the source and drain of a PMOS, P channel metal oxide semiconductor, transistor. The PMOS transistors formed within the N −  well will not affect the collection of signal generated carriers as long as the source and drain potentials of the PMOS transistors are set at a lower potential than the N −  well potential so that they remain reverse biased with respect to the N −  well. Typically, but not necessarily, the signal generated carriers are photo-generated carriers. Any of the P +  regions used to form the source and drain regions can be used to reset the pixel after it has been read in preparation for the next cycle of accumulating signal-generated carriers. The N −  well forms a second gate for the dual gate PMOS transistor since the potential of the N −  well  12  affects the conductivity of the channel of the PMOS transistor.  
         [0014]     The drain of the PMOS transistor can be connected to ground potential and thereby require one less conducting line to operate each pixel.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  shows a top view of a number of N −  wells formed in a P −  epitaxial silicon layer.  
         [0016]      FIG. 2A  shows a cross section view of an N −  well pixel with a PMOS transistor formed therein and a schematic view of an NMOS transistor used to read the pixel.  
         [0017]      FIG. 2B  shows a schematic view of the circuit of  FIG. 2A .  
         [0018]      FIG. 3A  shows a cross section view of an N −  well pixel with a PMOS transistor and an N +  region formed therein and a schematic view of a two NMOS transistor circuit used to read the pixel.  
         [0019]      FIG. 3B  shows a schematic view of the circuit of  FIG. 3A .  
         [0020]      FIG. 4A  shows a cross section view of an N− well pixel with two PMOS transistors formed therein and a schematic view of four NMOS transistors used to read the pixel.  
         [0021]      FIG. 4B  shows a schematic view of the circuit of  FIG. 4A .  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     Refer now to  FIGS. 1-4B  of the drawings for a description of the preferred embodiments of this invention.  FIG. 1  shows a top view of a number of N −  wells  12  formed of N −  type silicon in a P +  type silicon substrate  10 . Typically, but not necessarily, the P +  type silicon substrate  10  is a P +  type epitaxial silicon layer.  FIG. 1  shows four N −  wells  12  as an example however the actual number will be larger or smaller, typically smaller, arranged in an array. Each N −  well  12  forms a PN junction diode with the surrounding P −  silicon material. The N −  wells  12  are biased such that the potential of the N −  wells  12  are higher than the P −  silicon material  10  and the PN junction is back biased. This back biased PN junction forms a pixel which can accumulate carriers generated by an external signal to be read during a readout period. Typically, but not necessarily, the external signal is optical radiation and the carriers are photo-generated carriers.  
         [0023]      FIG. 2A  shows a cross section view of one of the N −  wells  12  and the surrounding P −  silicon material  10 . As shown in  FIG. 2A , a first P +  type silicon region  14  and a second P +  type silicon region  16  are formed in the N −  well  12 . The first P +  type silicon region  14  forms the source and the second P +  type silicon region  16  forms the drain of a PMOS, P channel metal oxide semiconductor, transistor  19 . A gate oxide  18  is formed over the channel  28  of the PMOS transistor  19 . A gate electrode  20  is formed on the gate oxide  18 . The N −  well  12  is biased at the highest potential in the circuit when the pixel is reset. This will allow the N −  well  12  region to collect all the signal-generated electrons within a diffusion length of the N −  well  12  and P +  substrate junction. Biasing the N −  well  12  at the highest potential in the circuit during reset allows a circuit tolerant to a 100% fill factor for the pixel. In this example the highest potential in the circuit is the V DD  potential and is between about 4.5 and 5.5 volts, usually 5.0 volts. As shown in  FIG. 2  the P −  substrate is held at ground potential by means of a P +  contact  21  into the P −  substrate which is held at ground potential. Either the first P +  region  14  or the second P +  region  16  can be used to reset the pixel by raising the potential of the selected P +  region to V DD  while the pixel is being reset and then returning the selected P +  region to ground potential while the pixel is accumulating signal-generated electrons.  
         [0024]      FIGS. 2A and 2B  show a circuit for reading and resetting the pixel using a single NMOS, N channel metal oxide semiconductor, transistor  22  per pixel. Like reference numbers are used to denote like circuit elements in  FIGS. 2A and 2B . As shown in  FIG. 2A , the drain of the NMOS transistor  22  is connected to an output node  24  and the source of the NMOS transistor  22  is connected to the source  14  of the PMOS transistor  19 . The drain  16  of the PMOS transistor  19  is connected to a reset node  30 . The gate of the NMOS transistor  22  is connected to a select node  26 . During pixel reset the NMOS transistor  22  is turned off and the reset node  30  is raised from ground potential to a potential of V DD  to reset the pixel. This sets the potential of the pixel to V DD -V PB  where V PB  is the potential drop across the junction between the drain  16  of the PMOS transistor and the N −  well. The NMOS transistor is turned on and off by means of a potential applied to the select node  26 . After the reset of the pixel has been completed the reset node  30  is returned to ground potential and the NMOS transistor  22  remains turned off while the pixel accumulates signal-generated carriers. Since the source  14  of the PMOS transistor  19  is floating and the drain  16  of the PMOS transistor  19  is at ground potential during the charge accumulation period the PMOS within the N −  well  12  will not impact the collection of the signal-generated carriers by the pixel. After the accumulation period has been completed the NMOS transistor is turned on and the charge accumulated by the pixel can be read by detecting the signal at the output node  24 .  
         [0025]     Alternatively the drain  16  of the PMOS transistor  19  can be permanently connected to ground potential by holding the reset node  30  at ground potential. This has the advantage of eliminating the need for a separate reset line to be bussed to the pixel. In this configuration during reset the NMOS transistor  22  is turned on and the output node  24  is set to V DD  This brings the source  14  of the PMOS transistor  19  to very nearly V DD  potential thereby resetting the pixel. After the pixel has been reset the NMOS transistor is turned off while the pixel accumulates signal-generated carriers. As before, since the source  14  of the PMOS transistor  19  is floating and the drain  16  of the PMOS transistor  19  is at ground potential during the charge accumulation period, the PMOS within the N −  well  12  will not impact the collection of the signal-generated carriers by the pixel. After the accumulation period has been completed the charge accumulated by the pixel is read. One method of reading the pixel is to turn the NMOS transistor on and detect the charge accumulated by the pixel at the output node  24 .  
         [0026]     The potential of the N −  well  12  and the floating PMOS source  16  will change based on the amount of signal-generated carriers accumulated by the pixel during the charge accumulation period. For readout of the accumulated charge the body effect can be utilized to form a dual gate PMOS transistor  19  using the PMOS transistor  19  as a source follower. This is shown schematically in  FIG. 2B  showing the NMOS transistor  22  having a source connected to the output node  24  and the gate connected to a select node  26 . The reset node  30  is either connected to ground or used for resetting the pixel. The N −  well  12  forms a second gate for the dual gate PMOS transistor  19  since the potential of the N −  well  12  affects the conductivity of the channel  28  of the PMOS transistor  19 , see  FIG. 2A . The gate  20  of the PMOS transistor  19  can be used as a gain control in this case.  
         [0027]     There are several readout circuits that can be used with the pixel with the embedded gate PMOS transistor  19  of this invention.  FIGS. 3A and 3B  show an example of one of these circuits. Like reference numbers are used to denote like circuit elements in  FIGS. 3A and 3B . In this example as in the previous example, as shown in  FIG. 3A , a first P +  region  14  forms the source and a second P +  region  16  forms the drain of a PMOS transistor  19  formed in the N −  well  12 . The N −  well is formed in a P −  substrate  10 . A gate oxide  18  is formed over the channel  28  of the PMOS transistor  19  and a gate electrode  20  is formed on the gate oxide  28 . The drain  16  of the PMOS transistor  19  is connected to a reset node  30 , and the P −  substrate  10  is held at ground potential by means of a P +  contact  21  in the P −  region  10 . As in the previous example, the source of a first NMOS transistor  22  is connected to the source  14  of the PMOS transistor  19  and the drain of the first NMOS transistor  22  is connected to an output node  24 . As shown in  FIG. 3A  an N +   34  region is formed in the N −  well  12  and connected to the source of a second NMOS transistor  32 . The drain of the second NMOS transistor  32  is connected to the gate of the first NMOS transistor. The gate of the second NMOS transistor  32  is connected to the source of the first NMOS transistor  22 . The diode  31  in  FIG. 3B  represents the N +  region  34  and N −  well junction  12  in  FIG. 3A . The potential at the cathode of the diode  31  is the potential of the N −  well and is the signal to be read after the pixel has completed a charge accumulation cycle.  
         [0028]     During the reset operation the gate  20  of the PMOS transistor  19  is held at ground potential and the reset node  30  is held at V DD  potential. In this example V DD  is the highest potential in the circuit and is between about 4.5 and 5.5 volts, typically 5.0 volts. This turns the PMOS transistor  19  on, sets the N −  region  12  to a potential of nearly V DD , V DD  minus a small built in potential, and turns the second NMOS transistor  32  on. This built in potential is the potential drop across the P +  source and N −  well junction. This also turns first NMOS transistor  22  off since the potential at the gate of the first NMOS transistor  22  is less than the potential at the source of the first NMOS transistor  22 . The reset node  30  is then returned to V DD  potential turning the PMOS transistor  19  off to begin charge integration. If the potential of the gate  20  of the PMOS transistor  19  is modulated the charge conversion gain can be varied. The second NMOS transistor  32  remains on, because the forward bias remains greater than the threshold voltage. The first NMOS transistor  22  remains off because the potential at the gate of the first NMOS transistor  22  remains less than the potential at the source of the first NMOS transistor  22 . Since the first NMOS transistor  22  is off during the reset operation the potential of the output node  24  does not matter.  
         [0029]     After the pixel has been reset the signal-generated carriers will reduce the potential of the N −  well  12  and the floating source  14  of the PMOS transistor  19 . When the pixel is read the potential of the gate  20  of the PMOS transistor  19  is ramped from V DD  to ground potential. When the potential of this gate  20  becomes less than the potential at the source  14  of the PMOS transistor  19  minus the threshold voltage of the second NMOS transistor  32  the PMOS transistor  19  turns on. This will pull the potential of the source  14  of the PMOS transistor  19  down to ground potential and reverse bias the diode  31 , see  FIG. 3B . This causes the second NMOS transistor  32  to turn off and the signal level, the potential of the N −  well  12 , is stored at the gate of the first NMOS transistor  22 . During the readout cycle the gate  20  of the PMOS transistor  19  can be used as a gain adjust control.  
         [0030]     The ramping of the potential of the gate  20  of the PMOS transistor  19  can be used to detect the pixel signal level, the potential of the N −  well, and can also be used in conjunction with a timer for a basic analog to digital converter. The timer is started at the time the potential at the gate  20  of the PMOS transistor  19  begins to ramp from V DD  toward ground potential. The time at which the PMOS transistor turns on is a digital representation of the signal detected by the pixel. This time can be stored for future use. If the pixels are arranged in an array of rows and columns with a global timer is at the bottom of each column, the times at which the PMOS transistor in each pixel of a selected row turns on stored gives a digital representation of the signal and forms a basic analog to digital converter.  
         [0031]     Since the potential of the N −  well  12  is stored at the gate of the first NMOS transistor  22  a snapshot imager with in pixel storage can be realized with the addition of a third NMOS transistor  90  with the gate of the third NMOS transistor  90  connected to a sequential row addressing circuit  91  and the source of the third NMOS transistor  90  connected to the output node  24 . Since the gate of the first NMOS transistor  22  stores the potential of the N −  well  12  in a non destructive fashion, an array of rows and columns of pixels can integrate for an identical time duration and store individual pixel signals at the gate of the first NMOS transistor  22  of each pixel in the array. Using the third NMOS transistor  90  as a readout transistor having a gate connected to a sequential row addressing circuit  91  each row can be selectively read out through a single output using a raster scan.  
         [0032]     This basic circuit block can be repeated and used for on pixel correlated double sampling, CDS. This embodiment is shown in  FIGS. 4A and 4B . Like reference numbers are used to denote like circuit elements in  FIGS. 4A and 4B .  FIGS. 4A and 4B  show two dual gate PMOS transistors in a single N −  well. As shown in  FIG. 4A , a first P +  type silicon region  40 , a second P +  type silicon region  42 , and a third P+ type silicon region  44  are formed in the N −  well  12 . The first P +  type silicon region  40  forms the source of a first PMOS transistor  56  and the third P +  type silicon region  44  forms the source of a second PMOS transistor  60 . The second P +  region  42  forms the drain of both the first PMOS transistor  56  and the second PMOS transistor  60 . A first gate oxide  46  and first gate electrode  52  are formed over the channel of the first PMOS transistor  56 . A second gate oxide  48  and second gate electrode  50  are formed over the channel of the second PMOS transistor  60 . As in previous embodiments, the N −  well  12  is biased at the highest potential in the circuit when the pixel is reset. This will allow the N −  well  12  region to collect all the signal-generated electrons within a diffusion length of the N −  well  12  and P −  substrate junction. In this example the highest potential in the circuit is the V DD  potential. In this example V DD  is between 4.5 and 5.5 volts, usually 5.0 volts. As shown in  FIG. 4A  the P −  substrate is held at ground potential by means of a P +  contact  21  into the P −  substrate which is held at ground potential. The pixel is reset by raising the potential of the reset node  58 , connected to the second P +  region  42 ,  58 , to V DD  while the pixel is being reset and then returning the reset node  58  to ground potential while the pixel is accumulating signal-generated electrons.  
         [0033]     The second P +  region  42 , which forms a common drain of the first  56  and second  60  PMOS transistors, is connected to the reset node  58 , and the P −  substrate  10  is held at ground potential by means of a P +  contact  21  in the P +  region  10 . The source of a first NMOS transistor  70  is connected to the source  40  of the first PMOS transistor  56  and the drain of the first NMOS transistor  70  is connected to a first output node  78 . As shown in  FIG. 4A  a first N +  region  82  is formed in the N −  well  12  and connected to the source of a second NMOS transistor  72 . The drain of the second NMOS transistor  72  is connected to the gate of the first NMOS transistor  70 . The gate of the second NMOS transistor  72  is connected to the source of the first NMOS transistor  70 . The source of a third NMOS transistor  74  is connected to the source  44  of the second PMOS transistor  60  and the drain of the third NMOS transistor  74  is connected to a second output node  80 . As shown in  FIG. 4A  a second N +  region  84  is formed in the N −  well  12  and connected to the source of a fourth NMOS transistor  76 . The drain of the fourth NMOS transistor  76  is connected to the gate of the third NMOS transistor  74 . The gate of the fourth NMOS transistor  76  is connected to the source of the third NMOS transistor  74 .  
         [0034]      FIG. 4B  shows a schematic diagram of the circuit shown in  FIG. 4A  for easier understanding of the operation of the circuit of  FIGS. 4A and 4B . A first diode  83  in  FIG. 4B  represents the first N +  region  28  and N −  well  12  junction in  FIG. 4A . A second diode  85  in  FIG. 4B  represents the second N +  region  84  and N −  well  12  junction in  FIG. 4A . The potential at the cathodes of the first diode  83  and second diode  85  is the potential of the N −  well and is the signal to be read after the pixel has completed a charge accumulation cycle.  
         [0035]     During the reset operation the potentials of the first gate  52  of the first PMOS transistor  56  and the second gate  50  of the second PMOS transistor  60  are set at ground potential while the potential of the reset node  58  is raised from ground potential to V DD . After the reset has been completed the potential at the second gate  50  is raised to V DD  while potential of reset node  58  remains at V DD  and the potential of the first gate  52  remains at ground. This stores the reference voltage on the PN junction between the N −  well  12  and the P +  substrate  10  the at the gate of the third NMOS transistor  74 . The potential of the reset node  58  is then returned to ground potential with the potential at the second gate  50  held at V DD  and the charge integration cycle begins. During the charge integration cycle the voltage across the PN junction between the N −  well and the P −  substrate decreases and the potential of the first gate  52  increases as charge is accumulated. At the end of the charge integration cycle the potential of the second gate  50  is returned to ground potential, the reset node  58  remains at ground potential and the voltage across the PN junction between the N −  well  12  and the P −  substrate  10 , from which the signal generated charge can be determined, is stored at the gate of the first NMOS transistor  70 . The difference in potential between the second output node  80  and the first output node  78  gives an image signal with reduced noise and reduced pixel to pixel non-uniformity to accomplish on pixel correlated double sampling, CDS.  
         [0036]     As in the previous example, since the potentials at the gates of the first  70  and third  74  NMOS transistors are stored in a non destructive fashion a snapshot imager with in pixel storage can be realized with the addition of a fifth NMOS transistor  92 , with the gate of the fifth NMOS transistor  92  connected to a sequential row addressing circuit  93  and the source of the fifth NMOS transistor  92  connected to the first output node  78 , and a sixth NMOS transistor  94 , with the gate of the sixth NMOS transistor  94  connected to a sequential row addressing circuit  95  and the source of the sixth NMOS transistor  94  connected to the second output node  80 , as shown in  FIGS. 4A and 4B . As in the previous example, an array of rows and columns of pixels can integrate for an identical time duration and store individual pixel signals at the gates of the first  70  and third  74  NMOS transistors of each pixel in the array. Using the fifth  92  and sixth  94  NMOS transistors as readout transistors having their gates connected to a sequential row addressing circuits,  93  and  95 , each row can be selectively read out using a raster scan.  
         [0037]     In this invention an N −  well formed in a P −  substrate is used to form the junction for accumulating signal generated carriers. Those skilled in the art will readily recognize that the invention will work equally well using a P −  well in an N −  substrate. In this case P +  regions are replaced by N +  regions, N +  regions are replaced by P +  regions, P −  regions are replaced by N −  regions, N −  regions are replaced by P −  regions, P regions are replaced by N regions, N regions are replaced by P regions, PMOS transistors are replaced by NNOS transistors, NMOS transistors are replaced by PMOS transistors, and the highest voltage in the circuit is replaced by the lowest voltage in the circuit.  
         [0038]     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.