Abstract:
A sensor may be formed with a transistor comprising a gate that has both n-type and p-type regions to increase the gate work function. In combination with moving the p-type well such that the p-type well only partially dopes the channel of the transistor, the increased gate work function further increases the reset voltage level required to create the reset channel without having to use high doping levels in the critical regions of the sensor structure including the photo-detector and the reset transistor. The source of the reset transistor is partially beneath the n-type region of gate, while the transistor&#39;s drain is partially beneath the p-type region of the gate. The channel has a p-type well portion and a substrate portion. This construction of the sensor may eliminate the reset noise associated with the uncertainty of whether the charge left in the transistor&#39;s channel will flow back towards the photo-detector after the transistor has been turned off.

Description:
This application is a continuation application of U.S. application Ser. No. 10/119,982, filed Apr. 10, 2002, now U.S. Pat. No. 6,902,945, which is a divisional application of U.S. application Ser. No. 09/680,036, filed Oct. 5, 2000, now U.S. Pat. No. 6,768,149, which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to electronic imaging devices and, in particular, to imagers having FET reset switches. 
     2. Related Art 
     Conventionally, CMOS imagers contain a number of photodiodes that are continuously queried and reset. The resetting of the photodiodes attempts to place each of the photodiodes into a known state (i.e. an expected voltage or charge level) and is commonly controlled by a N-channel Metal Oxide Semiconductor (NMOS) Field Effect Transistor (FET) acting as a reset switch. The NMOS FET has a drain implant that is in direct contact with a lighter doped P-well and protrudes into the material under the gate region of the NMOS FET. 
     The utilization of NMOS FETs acting as reset switches in CMOS imagers results in an additional source of noise, commonly known as reset noise (known as kTC noise). The typical construction of a NMOS FET allows charge to flow back to the drain and contributes to the reset noise in a CMOS imager. The length of time between resets and temperature changes affects the rate at which charge in a NMOS FET flows back to the drain and increases the reset noise when voltage is removed from the gate. The problem created by reset noise in a CMOS imager is that it causes uncertainty about the voltage values at the photodiodes after a reset. Attempts to compensate for reset noise in a NMOS FET have been generally unsuccessful due to charge redistribution that depends on the localized substrate noise (i.e. correlated double sampling measurements of the reset noise during a read operation). In addition, for signal readout circuits configured to integrate the photogenerated signal on the total sense capacitance, such as the source follower arrangement of Fry, et al., (IEEE JSSC, Vol. SC-5, No. 5, October 1970), is affected by the increased capacitance. The increased capacitance of a conventional reset FET decreases the electrical gain of the signal readout circuits. The decreased electrical gain results from the sense capacitance being the aggregate of the detector capacitance and various stray capacitances in compact pixel designs. The stray capacitances include, for example, the gate capacitance of the transistor gate driven by the photodiode cathode and the associated capacitance of the reset transistor. Therefore, the increased capacitance of a conventional NMOS FET reset switch results in optical degraded sensitivity for the CMOS imager due to both higher reset noise and lower electro-optical sensitivity. Thus, the use of known types of compensation for reset noise still results in a loss of sensitivity in the CMOS imager. 
     Additional reset noise problems occur due to the single chip construction of a conventional NMOS FET utilized as a reset switch in a CMOS imager. Construction of conventional NMOS FET utilize fabrication methods using sub-micron technology. As a result, the NMOS FET is susceptible to junction leakage. It is not uncommon for high leakage to occur from the increased electric field associated with a shallow junction, Arsenic implant damage, gate induced drain leakage, or a combination of all of the previous. The junction leakage of a conventional NMOS FET results in poor optimization and continuous soft resets during low light operation of a CMOS imager. Soft resets generate image lag because the charge that is not fully cleared from the photo-detector is subsequently added to the signal in the next integration period. The poor optimization and continuous soft resets significantly contributes to the reset noise and loss of sensitivity at low light level problems in a CMOS imager. Therefore, there is a need for a device and method to increase sensitivity at low light level while reducing the reset noise in CMOS imagers regardless of temperature and periods between resets of photodiodes while reducing junction leakage of the NMOS FET. 
     SUMMARY 
     The tapered threshold reset FET for a CMOS imager has a sensor having a transistor with a gate located partially over a source and partially over a drain having material between the source and drain beneath the gate of a predetermined length. The sensor also has a detection device that may be coupled to the drain by a signal path, where the material allows the detection device to be reset to a predetermined state. 
     Broadly conceptualized, the sensor may be formed with a reset transistor that reduces the capacitance of the photodiode. This may be accomplished by moving the p-type well, that isolates the source from the drain such that the p-type well partially dopes the channel of the transistor. The transistor may also be constructed to reduce reset noise through the use of the tapered reset operation. The tapered reset operation may include a reset transistor of relatively high impedance capable of suppressing the basic reset noise associated with the photodiode capacitance via an on-chip circuit and by using a channel implant that increases the reset voltage level for creating the reset channel. 
     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 claims. 
    
    
     
       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. In the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is a schematic diagram depicting an exemplary implementation of a sensor having a photo-detector and a reset switch in accordance with the invention. 
         FIG. 2  is a cross sectional view illustrating the sensor of  FIG. 1  having a FET transistor reset switch. 
         FIG. 3  is a cross sectional view illustrating another example of the sensor of  FIG. 1  having a FET transistor reset switch. 
         FIG. 4  is a cross sectional view illustrating still another exemplary implementation the sensor of  FIG. 1  having a FET transistor. 
         FIG. 5  is a flow diagram illustrating an example process for resetting the sensor of  FIG. 4 . 
         FIG. 6  is a signal diagram illustrating a tapered reset voltage applied to the FET of  FIG. 4 . 
         FIG. 7  is a flow diagram illustrating example process for resetting the sensor of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1 , a schematic diagram depicting a sensor  100  having a photo-detector  102  and a transistor  104  acting as a reset switch is shown. The sensor  100  includes a photo-detector  102  having of a p-n junction and the transistor  104  acting as a reset switch. The transistor has a source  106 , a gate  108 , and a drain  110 . The gate  108  of transistor  104  is electrically connected to a reset voltage supply  112 . The source  106  of transistor  104  is electrically connected to a reset voltage sink  114 , preferably ground. The drain  110  of transistor  104  is electrically connected to the anode  116  of the photo-detector  102  and a readout circuit  118 . The readout circuit  118  samples and processes the output of the photo-detector  102  during a read operation of the sensor  100 . However, one skilled in the art appreciates, that in an alternate embodiment the photo-detector may selectively be electrically associated with the source  106  as opposed to the drain  110  to facilitate a reset of the photo-detector  102 . 
     Upon a reset voltage being applied to the gate  108  of transistor  104  (acting as a reset switch) via the reset voltage supply  112 , a positive charge on the photo-detector  102  passes from the drain  110  to the source  106  of transistor  104  and ultimately to the reset voltage sink  114 . Once the reset operation is complete and the transistor  104  is turned off (i.e. the voltage on the gate  108  is removed), the remaining charge on the photo-detector  102  is measured and stored in memory (not shown) via readout circuit  118 . The stored charge value is utilized for correlated double sampling (CDS) in order to reduce reset noise via post processing and is a measure of offset error including the offset associated with the readout circuit  118  amplification, the random offsets generated by charge redistribution, and the classical reset noise. The offset error is described by the expression: 
               reset   ⁢           ⁢     noise   ⁢             ⁢             (   carriers   )       =         kTC   sense       e           
where “k” is Boltzman&#39;s constant, “T” is the temperature, “C sense ” is the sense capacitance and “e” is the electron charge. In  FIG. 1 , C sense  is comprised of the capacitance of photodiode  102 , the input capacitance of the readout circuit  118 , and the stray capacitance associated with the transistor  104  acting as a reset switch. Minimizing the capacitances reduces the maximum reset noise in sensor  100  with out utilizing the additional hardware and costs associated with CDS.
 
     Radiation or light is then permitted to accumulate on the photo-detector  102  for a predefined time (i.e. integration period), before the charge is read again via the readout circuit  118 . Ideally, the error in this second read is corrected by compensating for the earlier measurement for offset and reset noise. The reset error, however, may be significant and vary depending on the length of time of the reset and the construction of transistor  104  utilized as the reset switch. 
       FIG. 2  is an illustration of a cross sectional view of the sensor  100 . The sensor  100  has a photo-detector  102 , a FET transistor  104  reset switch. The transistor  104  has a gate  108  with a dielectric insulator  200 , a source  106 , and a drain  110  formed in a substrate  202 . The source  106  is formed within a p-type well  204  (p-type atoms include Phosphorus, Arsenic, Nitrogen Antimony, and Bismuth) and is partially beneath the gate  108 . The p-type well  204  is implanted or formed within the substrate  202 . The source  106  is preferably formed as a shallow surface implant on the p-type well  204 . The drain  110  is preferably formed with the substrate  202  to be partially beneath the gate  108 . The drain  110  is electrically associated with a photo-detector  102  formed with the surface implant of the drain  110 , deep implant  210 , and substrate  202 . Additionally, a person skilled in the art would recognize that the source  106  may selectively be interchanged with the drain  110  and associated with the photo-detector  102 . 
     The transistor  104  of sensor  100  further includes a material in the space defined by the separation of the source  106  from the drain  110  beneath the gate  108 . In an example implementation of the transistor  104 , the length  206  of the material is at least 20 percent longer than a process minimum. For example, the process minimum gate length for a conventional 3.3 volt logic process range is approximately 0.35 microns. The recommended minimum gate length to avoid exaggerated short channel effects would be approximately 0.4 microns. While increasing the length  206  of the material consumes additional die area, increasing the material length  206  by approximately 20 percent of the process minimum increases the potential required to deplete the reset channel. But, the increase in potential required to deplete the reset channel significantly decreases the likelihood of a soft reset during a read operation (i.e. sub-threshold leakage does not degrade low light operation) and promotes the proper functioning of the various forms of tapered reset. An additional result of increasing the length of the gate  108  is that the doping of the gate  108 , source  106 , drain  110 , and associated channel may be decreased. 
     The channel has a well portion and a shallow implant  208 . The implant  208  may be formed with a Boron dopant, but other implementations may selectively utilize hole-increasing dopants such as Aluminum, Gallium, Indium, and Thallium. The well portion of the channel constitutes a portion of the p-type well  204 . The implant  208  is disposed between the channel well portion  204  of the source  106  and the drain  110 . Additionally, one skilled in the art would appreciate, the channel implant  208  may be disposed between the channel well portion and the source  106  when the photo-detector  102  is formed to be operably associated with the source  106 . 
     The implant  208  is preferably formed to be sufficiently shallow such that the concentration of dopant near the channel surface under the gate  108  further increases the potential that must be applied to the gate  108  in order to deplete the reset channel and the implant  208  dose may be reduced to below the dopant level of the channel well portion. In this instance, because the implant  208  has a lower dopant level than the channel well portion, the drain  110  dose may be advantageously reduced from 3 e 13  cm −3  n-type, the typical level for a conventional NMOS FET, to approximately 6 e 12  cm −3  n-type. This has the additional advantage of reducing the capacitance of the photo-detector  102  relative to its volume and lowering junction leakage associated with arsenic implant damage and gate  108 . 
     In  FIG. 3 , a cross sectional view of an exemplary implementation of the sensor  300  of  FIG. 1  is illustrated. The FET transistor  302  has a gate  304 , a source  306 , and a drain  308 . The drain  308  of the transistor  302  is connected to the deep implant  316  of photo-detector  310 . The gate  304  has a n-type region  312 , a p-type region  314 , and a dielectric insulator  316 . The source  306  includes a p-type well  318  located in a substrate  320 . In this implementation, material is located between the p-type well  318  of the source  306  and the drain  308 . The length  322  of the material is defined by the separation of the source  306  from the drain  308  beneath the gate  304 . The length  322  of the material is preferably at least 20 percent longer than the process minimum. 
     As depicted in  FIG. 3 , the gate  304  has an n-type region  312 , and a p-type region  314  that form gate  304 . The gate  304  is preferably formed with polysilicon regions of opposite polarity (examples of polysilicons are; Boron, Aluminum, Gallium, Indium, Thallium, Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth). The potential that is applied to gate  304  in order to deplete the reset channel is increased (i.e. due to the resulting increase in the work function of the gate  304 ) without having to use high doping levels in the transistor  302 . 
     The source  306  is formed with a p-type well  318 , and partially beneath the n-type gate portion  312  of gate  304 . The p-type well  318  is diffused into or formed with the substrate  320 . The source  306  is preferably formed as a shallow surface implant on the p-type well  318 . 
     The drain  308  is formed with the substrate  320  and partially beneath the p-type region  314  of the gate  304 . The drain  308  is electrically associated with a photo-detector  310  that is formed by the deep implant  316  and the drain  308  in the substrate  320 . The drain  308  is preferably formed as a shallow surface implant on the deep implant  316  and underlying substrate  320 . 
     The material between the p-well  318  of the source  306  and drain  308  define a length  322  of a channel in the substrate  320 . The well portion of the channel constitutes a portion of the p-type well  318  of the source  306 . The channel substrate portion has a first conductivity type (e.g. n-type) while the p-well portion  318  has a second conductivity type (e.g. p-type). Therefore, because the p-well portion  318  is disposed away from the drain  308  (i.e. away from the photo-detector  310  side of the transistor), the capacitance typically associated with the p-type well  318  and drain junction in a conventional NMOS FET is effectively removed (i.e. channel substrate portion has the same conductivity type as the drain  308 ) that substantially suppresses reset noise. In addition, the drain  308  dose may be reduced from 3 e 13  cm −3  n-type, the typical level for a conventional NMOS FET, to approximately 2 e 12  cm −3  n-type since there is no need to overcome the high p-type doping of the p-well  318 . 
     In  FIG. 4 , a cross sectional view of still another example of an implementation of the sensor  400  having a FET transistor  402  is illustrated. The transistor  402  includes a gate  404 , a source  406 , a drain  408 . The gate  404  has an n-type region  410  and a p-type region  412  overlying dielectric insulator  414 . The source  406  includes a p-type well  416  formed in a substrate  418 . The drain  408  of transistor  402 , however, is formed with a p-type surface implant  420 . The p-type surface implant  420  is formed in the drain  408  partially beneath the p-type region  412  of the gate  404 , such that the drain  408  is not in direct contact with the surface of gate  404  (i.e., is in contact with the dielectric insulator  414  of the gate  404 ). The material under the dielectric insulator  414  has a length  422  defined by the separation of the p-type well  416  and drain  408  located beneath the gate  404 . 
     Turning to  FIG. 5  is a flow diagram illustrating an example process for resetting the sensor  100  shown in  FIG. 2 . The process begins  500  when a potential is applied to the gate  108  ( FIG. 2 ), resulting in the channel well portion being depleted  502  ( FIG. 5 ). The drain  110  is not in direct contact with the surface channel of gate  108  due to the insulator  200 . The gate  108  forces conduction (i.e. during a reset operation) away from the gate  108  edge nearest the drain  110 . Therefore, substantially no conduction occurs through the channel substrate  418  until the potential on the source  106  and the depleted channel p-well portion  204  is sufficient to punch through to the drain  110 . 
     As the applied potential to the gate  108  and the channel well portion is increased during the tapered reset (See  FIG. 6  for representative tapered reset voltage waveform). The depletion regions associated with the p-type well  204  of the source  106  and drain  110  merge below the implant  208  to accomplish the punch through of the channel  504  ( FIG. 5 ). Once punch through occurs, carriers are swept through the merged depletion region  506  from the drain  110  to the p-type well  204  of the source  106 . In other words, the potential or reset voltage applied to the gate  108  must be increased beyond the level required for the channel well portion to be depleted in order to punch through the substrate  202  portion. Once punch through is accomplished a electrical field is establish to release or diffuse the charge on the photo-detector through the created reset channel. The voltage applied to the gate  108  is reduced allowing the channel below the implant  208  to collapse starting at the drain  110  end of the channel before the source end. Thus, sweeping the charges away from the photo-detector  508 . Because there are very few minority carriers in the fully depleted channel, there is reduced thermal noise associated with this charge transfer process. Processing is completed  510  and the photo-detector  102  discharged when the voltage is removed from gate  108 . 
     In  FIG. 6 , a voltage plot is shown. The reset voltage  600  is applied and then tapered or lowered slowly, preferably over one clock cycle  602 , to gradually maintain the potential difference between the channel ends (i.e. between source and drain) such that the charge on the photo-detector is sufficiently removed. As the reset voltage is tapered or lowered, the portion of the channel created by punching through the channel substrate portion is pinched off or collapses before the depleted portion of the reset channel (i.e. the channel well portion) causing charges remaining in the reset channel to be swept towards the source and away from the drain or photo-diode  102 . 
       FIG. 7  is a flow diagram of another example process for resetting the sensor  100  is shown. The process begins  700  with the potential required to deplete a channel associated with a material between the source  204  ( FIG. 2 ), and the drain  110  under the gate  108  being increased  702 . A Boron implant  208  ( FIG. 2 ), is added to half of the reset channel that is nearest the photo-detector  210  during fabrication. The implant raises the surface threshold for creation of a depletion region in the transistor by approximately 0.8 volts or higher. A tapered voltage of  FIG. 6  is applied to the gate  108  causing a channel to be created  704  under the implant  208 . While the channel exist under the implant  208 , a path exist for a charge to flow from the photo detection device, such as a photo-detector  102  or photodiode, to the source  204  and eventually to the reset voltage sink  114  ( FIG. 1 ). Thus, the channel drains the charge  706  from the photo-detector  102  to the p-well  204  of the source  106 . Upon removal of the voltage at the gate  202  the channel between the source and drain is interrupted and the process is finished  208 . 
     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.