Abstract:
An image sensor having a photo-detector and a reset contact that are electrically connected by a discharge path disposed between the reset contact and the photo-detector. The photo-detector has a depletion region for receiving and collecting radiation charges that are discharged through the discharge path to the reset contact. In one implementation, the reset of the photo-detector to a known potential is achieved by applying a high reset voltage to the reset contact that causes a reset depletion region to form beneath the reset contact. The outer perimeter of the reset depletion region defines a reset junction. The reset junction and the photo-detector junction are of the same polarity. As the high reset voltage is increased at the reset junction, the reset depletion region merges via punch through with the photo-detector&#39;s depletion region to create the discharge path. The voltage on the reset contact is increased beyond the expected potential of the photo-detector so that a potential difference is established across the discharge path and charges are swept away from the photo-detector via the discharge path. At the end of the reset, the potential on the reset junction is reduced and the depletion regions separate and the photo detector is left with a fixed potential. In this manner the kTC noise associated with the reset through a MOSFET switch is eliminated as the charge is transferred through the merging of two depletion areas.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates generally to electronic imaging devices and, in particular, to an electronic imaging device that suppresses reset noise in an image sensor. 
     2. Related Art 
     Photosensor image processing in camera and video systems commonly utilize CMOS image sensors that have cost and power advantages over other technologies such as charge coupled devices (CCD). A conventional CMOS image sensor has a photo-detector that is reset to a known potential after the readout of each image by a NMOS FET acting as a reset switch. When the NMOS FET reset switch is “switched off,” charge left in the NMOS FET channel drifts back to the photo-detector and produces reset noise. A common measure of reset noise is the product of the Boltzman&#39;s constant “k”, temperature “T”, and capacitance “C” (typically known as kTC) and represents an uncertainty about the voltage on the photo-detector following a reset. 
     CMOS imager sensors typically utilize off-chip signal processing to improve signal to noise (S/N) performance and compensate for the reset noise generated by a conventional NMOS FET acting as a reset switch. In addition, utilization of a conventional NMOS FET as a reset switch adds a significantly large capacitance component to the photodiode because of the FET&#39;s moderately doped p-well being in direct contact with the more heavily doped drain implant. This increased capacitance results in a loss of sensitivity in the CMOS image sensors. 
     Additionally, the sub-micron fabrication technology utilized in conventional NMOS FET fabrication is not optimized to reduce junction leakage. Junction leakage in a MOS FET results from an increased electric field associated with a shallow junction, Arsenic implant damage, and gate induced drain leakage. Furthermore, when the gate threshold is too low, which is the typical case for the conventional sub-micron NMOS FET, continuous soft resets results due to sub-threshold leakage. Junction leakage associated with poor junction optimization and continuous soft resets in a CMOS image sensor contribute to reset noise and a loss of sensitivity at low light levels. What is needed in the art is an approach to reduce reset noise, typically the dominant source of noise, in CMOS imager sensor without reducing the area available for light collection. 
     SUMMARY 
     A CMOS imager with a discharge path to suppress reset noise is provided. The CMOS imager has a discharge path and a reset contact electrically connected to the photo-detector. The discharge path may enable charge flow between the reset contact and the photo-detector. The CMOS image sensor suppresses reset noise by utilizing an image sensor that has a discharge path, rather than utilizing a conventional CMOS device, such as a NMOS FET, as a reset switch. 
     The reset of a photo-detector to a known potential is achieved by applying a high reverse bias to a reset node that is in close proximity to the photodiode junction. The reset node junction and the photodiode junction are of the same polarity. As the bias is increased, the depletion regions of the reset junction and the photodiode junction merge to establish a common potential. The potential on the reset junction is removed and the depletion regions separate at the end of the reset leaving the photodiode in isolation as a reverse biased junction with a fixed potential. In this manner, the kTC noise associated with the reset through a NMOS FET reset switch is eliminated. 
    
    
     Other systems, methods features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying clams. 
     BRIEF DESCRIPTION OF THE FIGURES 
     The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principals of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. 
     FIG. 1 is a block diagram illustrating an exemplary image sensor in accordance with an example implementation of the invention. 
     FIG. 2 is a cross sectional view of the exemplary image sensor of FIG.  1 . 
     FIG. 3 is a cross sectional view of the exemplary image sensor in FIG. 1 during a reset operation. 
     FIG. 4 is a cross sectional view of another exemplary image sensor. 
     FIG. 5 is a cross sectional view of the exemplary image sensor depicted in FIG.  4 . 
     FIG. 6 is a cross sectional view of the exemplary image sensor of FIG. 4 during a reset operation. 
     FIG. 7 is a cross sectional view of another exemplary image sensor. 
     FIG. 8 is a flow chart illustrating an exemplary image sensor reset process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a block diagram depicting an image sensor  100  is illustrated. Image sensor  100  has a photodiode photo-detector  102  with a floating capacitive charge (shown as a capacitor  104 ). The photo-detector  102  is electronically connected to a voltage source  106  and a buffer/amplifier  112  at a terminal  108  and an electrical ground  110  at the other end of the photo-detector  102 . The buffer/amplifier  112  is electrically connected to the terminal  108  and a select switch  114 . A reset contact  116  is electrically connected to a reset threshold voltage supply  120 . The output circuitry for the image processor, such as a buffer/amplifier  112  and a select switch  114  for accessing the photo-detector  102  during a read operation is shown, but a description of the operation of buffer/amplifier  112  and select switch  114  is not necessary for an understanding of the invention. 
     The image sensor  100  has a photo-detector  102 , such as a photodiode, photogate, photocapacitor or other device that is capable of converting electromagnetic radiation into a signal. The photo-detector  102  converts received radiation charges into corresponding electrical signals that are read by an image processor (not shown). The capacitor  104  represents the floating capacitive charge that accumulates in the photo-detector  102  due to sensing radiation prior to a reset or a read operation. To facilitate receiving and collecting charges, the photo-detector  102  may be electrically connected to a voltage source  106 , resulting in the photo-detector having a reverse bias. 
     Applying the reset threshold voltage  120  to the reset contact  116  creates the discharge path  118 . The discharge path  118  is shown in FIG. 1 in phantom to denote that it is created during reset of the photo-detector  102 , and not while the sensor  100  is being sampled or read. When created, the discharge path  118  is preferably disposed between the photo-detector  102  and the reset contact  116 . In addition, the reset contact  116  is preferably disposed in close proximity to photo-detector  102  to limit the required reset threshold voltage level applied to the reset contact  116  to create the discharge path  118 . 
     In FIG. 2, a cross sectional view of the image sensor  100  of FIG. 1 is shown. The photo-detector  102  (FIG.  1 ), includes a photo-detector node  202  (FIG.  2 ), formed with a substrate  204  and electrically connected to a voltage source  106  via terminal  108 . A reset contact  116  is present and electrically connected to a reset threshold voltage supply  112 . The photo-detector node  202  preferably has a polarity opposite the polarity of substrate  204  such that the photo-detector node  202  and the substrate  204  define a detector junction  206 . The detector junction  206  has a depletion region  208  that is subject to an electric field based on the differing polarities of the photo-detector node  202  and the substrate  204 . The electric field of the depletion region  208  is consistent with the reverse biasing potentials across the photo-detector  102  (FIG.  1 ). Radiation  214  (FIG.  2 ), that enters the depletion region  208  of the detector junction  206  creates charge carriers or electron-hole pairs  210  (only electrons shown for clarity) which are swept to one side of the depletion region  208  (e.g. electrons swept to positive side of electric field). A charge carrier  212  created when radiation is absorbed in the substrate  204 , may diffuse to the depletion region  208  to also be collected by the photo-detector node  202 . 
     FIG. 3, a cross sectional view of the image sensor  100  during a reset operation is shown. The reset threshold voltage from the reset voltage supply  112  is applied to the reset contact  116  having a polarity opposite to another polarity found in the substrate  204  underlying the reset contact  116 . A reset depletion region  302  (e.g., majority carrier holes repelled by positive potential on reset contact  116  when substrate  204  is p-type semiconductor resulting in a low resistive path to the reset contact  116 ) forms beneath the reset contact  116 . The outer perimeter of the reset depletion region  302  defines a reset junction  304 . When the reset threshold voltage is increased to a predetermined level on reset contact  116 , the reset depletion region  302  extends further into the substrate  204  to punch through or merge with the detector junction  206 , creating the discharge path  306  between the reset contact  116  and the photo-detector node  202 . 
     The photo-detector node  202  formed with the substrate  202  and electrically connected to the voltage source  106  via terminal  108  and detects radiation  214 . The radiation  214  enters the depletion region  208  of the detector junction  206  resulting in charge carriers or electron-hole pairs  210  that end up in the depletion region  208 . The accumulated charge, including charged carrier  212 , collected by the photo-detector node  202  are released via the discharge path  306  through the reset contact  116 . 
     In FIG. 4, an illustration of another embodiment of an image sensor  400  is shown. Image sensor  400  has a photo-detector  402  with floating capacitive charge (shown as a capacitor)  404 . The photo-detector  402  is electrically connected to a voltage source  406  and a buffer/amplifier  408  at a terminal  410  and an electrical ground  412  at the other end of the photo-detector  402 . The output circuitry for the image processor, such as a buffer/amplifier  408  and a select switch  414  for accessing the photo-detector  402  during a read operation is shown, but a description of the operation of buffer/amplifier  408  and select switch  414  is not necessary for an understanding of the invention. The buffer/amplifier  408  is electrically connected to the terminal  410  and a select switch  414 . A reset contact  416  is electrically connected to a reset threshold voltage supply  418  and a reset implant  420 . Upon application of the reset threshold voltage from  418  to the reset contact  416 , a discharge path  422  is created. The discharge path  422  is shown in FIG. 4 in phantom to denote that it is created during reset of the photo-detector  402  and not while the sensor  400  is being sampled. 
     In FIG. 5, a cross sectional view of the image sensor  400  is shown. The reset implant  420 , preferably a diode, has a reset node  502  that is electrically connected to the reset contact  416 . The reset node  502  is in the substrate  504  in close proximity to photo-detector node  506  enabling the discharge path  422  (FIG.  4 ), to be created between the reset node  502  (FIG.  5 ), and the photo-detector node  506  by applying the reset threshold voltage from the reset threshold voltage supply  418  to the reset contact  416 . In the current implementation, the reset node  502  has a polarity that is opposite to the polarity of the substrate  504  such that the reset node  502  and the substrate  504  define a reset junction  508 . 
     Substrate  504  preferably comprises a dopant level that is higher than a dopant level for the reset node  502  in order to provide a sufficient resistance between the reset node  502  and the photo-detector node  506  and electrically isolate one from the other. Thus, the reset implant  420  may be reversed bias or left floating while the reset threshold voltage from the reset threshold voltage supply  418  is applied to the reset contact  416 . In either case, when a sufficient amount of charge carriers accumulates in the reset node  502 , a discharge path  422  (FIG.  4 ), is created. The carriers diffuse to a portion of the substrate  504 , shown in FIG. 5, as the region between the reset node  502  and the photo-detector node  506 . The reset node  502  “punches” through the depletion region  512  (created around detector junction  510  and formed when voltage from the voltage source  406  is applied via terminal  410 ) to the photo-detector node  506  creating the discharge path  422  (FIG.  4 ). In this exemplary implementation of the invention, the substrate  504  (FIG.  5 ), has a p-type dopant while the reset node  502  and the photo-detector node  506  each has a n-type dopant. It is appreciated that the invention may also be implemented with a substrate having an n-type region and the reset node  502  with the photo-detector node  506  having a p-type region. In addition, the photo-detector node  506  is preferably more lightly doped then the substrate  504  to provide a sufficient resistance between the detector junction  510  and the reset node  502  requiring a significant reset threshold voltage level before the discharge path  422  (FIG.  4 ), is created. A significant reset threshold voltage limits the occurrence of a soft reset and thus an erroneous reading of the sensor. 
     In FIG. 6, a cross sectional view of the image sensor  400  during a reset operation is shown. A reset threshold voltage from the reset threshold voltage supply  418  is applied to the reset contact  416 . In this instance, the discharge path  602  between the reset node  502  and the photo-detector node  506  is created and disposed between the detector junction  510  and the reset junction  508 . When the reset threshold voltage from the reset threshold voltage supply  416  is increased to a predetermined level, the reset depletion region  606  extends further into the substrate  504  to punch through or merge with the depletion region  512 . The depletion region  512  around the photo-detector node  506  is created by the voltage source  406  applying a voltage to terminal  410  that is electrically connected to the photo-detector node  506 . The merging of the reset depletion region  606  and the depletion region  512  creates the discharge path  602  between the reset contact  416  and the photo-detector node  506 . The discharge path  602  allows the accumulated charge on the photo-detector node  610  to be released through the reset contact  416 . 
     In FIG. 7, a cross sectional view of yet another implementation of an image sensor  700  is shown. The image sensor  700  includes a reset strip  702 , in lieu of a reset node  502  (FIG.  5 ), that is electrically connected to the reset contact  704  (FIG.  7 ), via a reset implant  706 . The reset contact  704  is connected electronically to the reset threshold voltage supply  708  in addition to the reset implant  706 . The photo-detector node  710  is connected electronically to a voltage source  720  by a terminal  722 . The reset strip  702  is disposed within the substrate  708  in close proximity to the photo-detector node  710 . The discharge path  712  is created by applying the reset threshold voltage from the reset threshold voltage supply  718  to the reset contact  704 . The reset strip  702  is preferably buried below the photo-detector node  710 . It is appreciated by one skilled in the art that in an alternative implementation an epitaxial layer that has properties similar to the substrate  708  may be grown over the reset strip  702  during fabrication to bury the reset strip  702  below the photo-detector element  710 . Thus, the substrate  708  and epitaxial layer may be utilized interchangeably without limiting the invention. 
     To facilitate creation of the discharge path  712 , the reset strip  702  has a polarity that is opposite to the polarity of the substrate  708  so that the reset strip  702  and the substrate  708  define a reset junction  714 . In this instance, the discharge path  712  is disposed between the detector junction  716  and the reset junction  714 . The substrate  708  preferably has a dopant level that is higher than a dopant level in the reset strip  702  to provide a sufficient resistance between the reset strip  702  and the photo-detector node  710  to electrically isolate one from the other. Thus, the reset strip  702  may be reversed bias or left floating while the reset threshold voltage from the reset threshold voltage supply  718  is applied to the reset contact  704 . Again, when a sufficient amount of charge carriers accumulates in the reset node  706  to diffuse a portion of the substrate  708  region between the reset strip  702  and the photo-detector node  710 , the reset strip  702  “punches” through the region to the photo-detector node  710  creating the discharge path  712 . 
     In an exemplary implementation of the invention, the substrate  708  also has a p-type dopant while the reset strip  702  and the photo-detector node  710  each has a n-type dopant. It is contemplated that the invention could be accomplished with a substrate having an n-type region and the reset strip  702  and the photo-detector node  710  having a p-type region. In addition, the photo-detector node  710  is preferably more lightly doped then the substrate  708  to provide a sufficient resistance between the respective junctions such that a significant reset threshold voltage level applied at the reset threshold voltage supply  718  is required before the discharge path  710  is created. 
     The sensor  700  may also comprise a plurality of photo-detector nodes  710 . In the implementation shown in FIG. 7, each one of the plurality of the photo-detector node  710  is electrically associated with the reset strip  702  such that a single reset may be performed on a corresponding row or column of photo-detector nodes  710  of sensor  700 . Therefore, a sensor fabricated in accordance with this invention may yield higher fill factor as more sensors may be accommodated on the same size die. 
     In FIG. 8, a flow chart of an exemplary image sensor reset process is shown. The process starts  800  when a first reset threshold voltage from the reset threshold voltage supply  112  (FIG.  3 ), is applied  802  (FIG.  8 ), to the reset contact  116 , of the image sensor  100 . The application of the reset threshold voltage results in the creation of discharge path  306  between the reset depletion region  302  and the photo-detector node  202  of the image sensor  100 . The charge on the photo-detector node  202  (FIG.  3 ), is released  806  (FIG.  8 ), through the discharge path  306  (FIG.  3 ). Because a conventional NMOS FET reset switch is not being utilized to perform the reset or drain the charge on the respective photo-detector, there is no random noise component associated with a NMOS FET reset switch, such as a large drain/channel capacitance that prevents a more complete discharge of the photo-detector. Therefore, the typical need to perform a correlated double sampling for each read operation to eliminate the respective contribution to reset noise associated with the NMOS FET reset switch is eliminated. In other words, because the reset operation in accordance with the invention is not susceptible to the thermal or temporal factors that create the uncertainty as to the level of voltage remaining on the photo-detector node  202  following a reset is substantially reduced. Once the image sensor  100  is reset once and then read once to determine the residual charge on the photo-detector node  202  following a reset, the residual charge can be stored and utilized thereafter as an offset correction to a read operation. Alternatively, an imager using image sensor  100  may perform uncorrelated double sampling to correct for any predicable residual offset voltage following a read of sensor  100 . 
     The reset operation may also include applying a second reset threshold voltage  806  (FIG.  8 ), from the reset threshold voltage supply  112  (FIG.  3 ), to the reset contact  116  to allow the photo-detector node  202  to reach or exceed substantially the same voltage potential as the reset contact  116 . The potential increases on the reset junction  206  (FIG.  3 ), independently of the photo-detector node  202  at the start. As the potential on this reversed biased junction increases, the depletion region  302  extends further into the substrate  204  until the depletion regions of the reset junction  302  and the detector junction  208  merge via punch through. At this point the potentials on the reset junction  304  and detector junction  208  rapidly reach equilibrium. Since there are variations in alternative embodiments having a reset node  502  (FIG.  5 ), or reset strip  702  (FIG.  7 ), the spacing and the doping of the reset node  502  (FIG.  5 ), or reset strip  702  in the alternate embodiments, the exact voltage at which junctions will merge vary with fabrication. Therefore, the potential on the reset contact is preferably increased beyond the minimum required for the depletion regions to merge  806  (FIG.  8 ), to ensure that the potentials reach equilibrium. Upon the depletion regions merging, a charge on the photo-detector node  202  (FIG.  3 ), is swept through the discharge path  306 , towards the reset contact  116  in  808  (FIG.  8 ). The potential on the reset contact  116  (FIG.  3 ), is lowered to maintain the electric field across the discharge path  306  and prevent the discharge path  306  from collapsing before the charge on the photo-detector mode  202  is substantially removed  810  (FIG.  8 ). The voltage on the reset contact is reduced so the discharge path  306  at the photo-detector node  202  end collapses before the reset contact  116  end and charges in the discharge path  306  are swept towards the reset contact  116  in  812  (FIG.  8 ). The potential that remains on the photo-detector after the discharge path collapses is equal to the potential at which the depletion regions separate and processing ends  814 . It is contemplated that the potential remaining on the photo-detector may be at or near zero voltage such that the reset results in a complete discharge. Alternatively, the potential remaining on the photo-detector node  202  may be an offset that is significant enough to effect the dynamic range of an imager utilizing image sensor  100  but still providing the advantage of increased sensitivity as the uncertainty of reset noise is substantially suppressed. In an alternate embodiment where the image sensor has a reset node  502  (FIG.  5 ), or a reset strip  702  (FIG.  7 ), the reset contact  116  may be grounded to avoid soft resets. The processing ends  814  upon the discharge path collapses. 
     While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention.