Patent Publication Number: US-2003234344-A1

Title: Logarithmic mode CMOS image sensor with reduced in-pixel fixed

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
     [0001] This application claims the priority benefit of Taiwan application Ser. No. 91113451, filed Jun. 20, 2002 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of Invention  
       [0003] The present invention relates to a logarithmic mode CMOS image sensor. More particularly, the present invention relates to a logarithmic mode CMOS image sensor with reduced in-pixel fixed pattern noise.  
       [0004] 2. Description of Related Art  
       [0005] An ideal image sensor should have a high dynamic range (for increasing the range of illumination), a low dark current (applicable in a dim environment) and low power consumption. Furthermore, image data must be able to be captured quickly and randomly accessed. At present, the most commonly used image sensor is the charge-coupled device (CCD). Although most CCD devices generally have a high dynamic range and a low dark current, special processes demanded for manufacturing the CCD device result in very expensive. Moreover, circuits inside the CCD require a high driving voltage supply. Hence, considerable power is consumed by each CCD cell.  
       [0006] Relatively speaking, a CMOS image sensor has properties including high quantum efficiency, low read-out noise, high dynamic range and random access capacity. Moreover, a CMOS image sensor is a hundred percent compatible with other CMOS fabrication process. In other words, a CMOS image sensor has none of the drawbacks of a charge-coupled device. Furthermore, rapid progress in the technique for fabricating CMOS devices also has reduced the production cost of a CMOS sensor considerably. Improved manufacturing methods have also reduced pixel size and overall power consumption. Consequently, many low-end imaging products use CMOS image sensors instead of the CCD device.  
       [0007] A logarithmic mode CMOS image sensor is one type of CMOS image sensor. FIG. 1 is a simplified circuit diagram of a conventional logarithmic mode CMOS image sensor. As shown in FIG. 1, a logarithmic mode CMOS image sensor mainly includes three NMOS transistors  101 ,  103 ,  105  and a photodiode  107 . The gate of the transistor  101  is tied to the high voltage Vdd. The voltage between the gate terminal and the source terminal of the transistor  101  response to the drain current in a logarithmic fashion. Hence, the output voltage V out  and the photocurrent induced by the photodiode  107  have a logarithmic relationship. With this arrangement, the logarithmic mode CMOS image sensor has a very high dynamic range. In addition, image contrast is very strong. However, the read-out voltage is often interfered by excessive in-pixel fixed patterned noise due to a threshold voltage variation of MOS transistor resulting from fabrication. Moreover, correlated double sampling (CDS) technique cannot be applied to eliminate fixed pattern noise in a logarithmic mode CMOS image sensor.  
       SUMMARY OF THE INVENTION  
       [0008] Accordingly, one object of the present invention is to provide a logarithmic mode CMOS image sensor capable of eliminating in-pixel fixed pattern noise inside the logarithmic mode CMOS image sensor through correlated double sampling (CDS) technique. Ultimately, quality of captured images is improved.  
       [0009] To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a logarithmic mode CMOS image sensor capable of eliminating in-pixel fixed pattern noise. The image sensor includes a first MOS transistor, a second MOS transistor, a third MOS transistor, a fourth MOS transistor, a photodiode and a current source. The first MOS transistor has a gate terminal, a first connection terminal and a second connection terminal. The gate terminal of the first MOS transistor and the first connection terminal are tied at a first node point. The first node point is connected to a terminal with a high voltage, such as the highest voltage, in the circuit. The second MOS transistor has a gate terminal, a first connection terminal and a second connection terminal. The gate terminal of the second MOS transistor and the second connection terminal of the first MOS transistor are tied at a second node point. The first connection terminal of the second MOS transistor is also connected to the terminal with the highest voltage. The third MOS transistor has a gate terminal, a first connection terminal and a second connection terminal. The gate terminal of the third MOS transistor is tied to a row select signal terminal and the first connection terminal of the third MOS transistor is tied to the second terminal of the second MOS transistor. The second connection terminal of the third MOS transistor is a voltage output terminal. The fourth MOS transistor has a gate terminal, a first connection terminal and a second connection terminal. The gate terminal of the fourth MOS transistor is tied to a control signal terminal and the first connection terminal of the fourth MOS transistor is tied to the second node point. The photodiode has a first connection terminal and a second connection terminal. The first connection terminal of the photodiode is tied to the second connection terminal of the fourth MOS transistor and the second connection terminal of the photodiode is tied to a ground terminal.  
       [0010] In the preferred embodiment of this invention, the first MOS transistor, the second MOS transistor, the third MOS transistor and the fourth MOS transistor are N-channel MOS transistors. Note that the fourth MOS transistor can be a P-channel MOS transistor.  
       [0011] This invention also provides a method of operating a logarithmic mode CMOS image sensor with minimal in-pixel fixed pattern noise. First, the logarithmic mode CMOS image sensor initiates a signal voltage sampling when the fourth MOS transistor receives an enable signal through the control terminal. Secondly, the logarithmic mode CMOS image sensor initiates load voltage sampling when the fourth MOS transistor receives a disable signal through the control terminal. Next, using CDS circuit to subtract the two signals. A final signal is the obtained.  
       [0012] In brief, adding one more MOS transistor to a conventional logarithmic mode CMOS image sensor circuit can eliminate most part of in-pixel fixed pattern noise.  
       [0013] It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014] The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,  
     [0015]FIG. 1 is a simplified circuit diagram of a conventional logarithmic mode CMOS image sensor,  
     [0016]FIG. 2 is a simplified circuit diagram of a logarithmic mode CMOS image sensor for eliminating in-pixel fixed pattern noise according to one preferred embodiment of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0017] Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.  
     [0018]FIG. 2 is a simplified circuit diagram of a logarithmic mode CMOS image sensor for eliminating in-pixel fixed pattern noise according to one preferred embodiment of this invention. As shown in FIG. 2, the logarithmic mode CMOS image sensor comprises four NMOS transistors  201 ,  203 ,  205 ,  211 , a photodiode  207  and a current source  209 .  
     [0019] The following is a description of the operating principles of the logarithmic mode CMOS image sensor. A few definitions are defined as follows: V g201  represents the voltage at the gate terminal of the NMOS transistor  201 , V s201  represents the voltage at the source terminal of the NMOS transistor, V th201  represents the threshold voltage of the NMOS transistor  201  and so on, and I 209  represents current provided by the current source  209 .  
     [0020] According to the drain current formula for an NMOS transistor operating in a sub-threshold region, the current I flowing through the NMOS transistor is given by:  
             I   =       I   0     ·     Exp        [         V   g201     -     V   s201     -     V   th201         nV   T       ]                 (   1   )                       
 
     [0021] where I 0  is the leakage current flowing through the NMOS transistor  201  when V g201 −V s201 =0V and V T  is the thermal voltage (equivalent to KT/q). From equation (1),  
           n                   V   T                     ln        [     I     I   0       ]         =       V   g201     -     V   s201     -         V   th201     ·   In                   other                 words         ,       V   s201     =       V   g201     -     V   th201     -     n                   V   T                       ln        [     I     I   0       ]       .                         
 
     [0022] Since  
     [0023] V g201  is almost equivalent to a high voltage, such as the highest voltage, in the circuit V dd , hence  
               V   s201     =       V   dd     -     V   th201     -     n                   V   T                     ln        [     I     I   0       ]                   (   2   )                       
 
     [0024] Since the NMOS transistor  203  operates in the saturated region, drain current flowing through the NMOS transistor  203  will almost equal the current I 209  provided by the current source  209 . Thus, the current I 209  is almost equal to the source current of the NMOS transistor  203 . According to the source current formula for an NMOS transistor operating in the saturated region, source current through the NMOS transistor  203  is ½ K P203 (V g203 −V s203 −V th203 ) 2 , which is equal to I 209 . In other words,  
         I   209     =       1   2                K   P203          (       V   g203     -     V   s203     -     V   th203       )       2     .                     
 
     [0025] By deduction,  
             2        I   209         K   P203         =       V   g203     -     V   s203     -       V   th203     .                     
 
     [0026] Under an ideal condition, V s203  is equivalent to the output voltage V out  from the logarithmic mode CMOS image sensor. Hence,  
               V   out     =       V   s203     =       V   g203     -     V   th203     -         2        I   209         K   P203                     (   3   )                       
 
     [0027] where K P203  is a device parameter for the NMOS transistor  203 . Since V s201 =V g203 , substituting equation (2) into equation (3) gives:  
               V   out     =       V   dd     -     V   th201     -     n                   V   T                     ln        [     I     I   0       ]         -     V   th203     -         2        I   209         K   P203                   (   4   )                       
 
     [0028] When the photodiode  207  picks up weak illumination (the NMOS transistor  201  operates in the sub-threshold region) and the NMOS transistor  211  receives an ‘OFF’ control signal, a current I, that is, a reverse bias current I rev1  flows into the node point  1 . Hence, the output voltage V out11  or the load voltage during correlated double sampling (CDS) of the logarithmic mode CMOS image sensor is given by the formula:  
               V   out11     =       V   dd     -     V   th201     -     n                   V   T                     ln        [       I   rev1       I   0       ]         -     V   th203     -         2        I   209         K   P203                   (   5   )                       
 
     [0029] When the photodiode  207  picks up weak illumination and the NMOS transistor  211  receives an ‘ON’ control signal, a current I, equivalent to the photocurrent I P1  generated by the illuminated photodiode  207  and the reverse bias current I rev1  to the node point  1  (leakage current from the photodiode without illumination). Hence, the output voltage V out12  or the signal voltage during correlated double sampling (CDS) of the logarithmic mode CMOS image sensor is given by the formula:  
               V   out12     =       V   dd     -     V   th201     -     n                   V   T                     ln        [         I   P1     +     I   rev1         I   0       ]         -     V   th203     -         2        I   209         K   P203                   (   6   )                       
 
     [0030] Therefore, when the photodiode  207  picks up weak illumination while the NMOS transistor  211  receives either an enable or a disable control signal to sample load voltage V out11  and signal voltage V out12  through the CDS circuit, a pure signal voltage is obtained by subtraction, that is:  
                      V   out12     -     V   out11            =     n                   V   T                     ln        [         I   P1     +     I   rev1         I   rev1       ]                 (   7   )                       
 
     [0031] Similarly, when the photodiode  207  picks up highillumination (the NMOS transistor  201  operates in the saturation region) and the NMOS transistor  211  receives an ‘OFF’ control signal, a current I, that is, a reverse bias current I rev1  flows into the node point  1 . Hence, the output voltage V out21  of the logarithmic mode CMOS image sensor is given by the formula:  
               V     out                 21       =       V   dd     -     V   th201     -     n                   V   T          ln        [       I   rev1       I   o       ]         -     V   th203     -         2        I   209         K   P203                   (   8   )                       
 
     [0032] When the photodiode  207  picks up high illumination and the NMOS transistor  211  receives an ‘ON’ control signal, a current I, the output voltage V out22  of the logarithmic mode CMOS image sensor is given by the formula:  
               V     out                 22       =       V   dd     -     V   th201     -         2        (       I   P1     +     I   rev1       )         K   P201         -     V   th203     -         2        I   209         K   P203                   (   9   )                       
 
     [0033] Therefore, when the photodiode  207  picks up highillumination while the NMOS transistor  211  receives either an enable or a disable control signal to match the CDS circuit for reset voltage V out11  and signal voltage V out12  sampling, a pure signal voltage is obtained after subtraction, that is:  
                      V     out                 22       -     V     out                 21              =           2        (       I   P1     +     I   rev1       )         I   o         -     n                   V   T          ln        [       I   rev1       I   o       ]                   (   10   )                       
 
     [0034] According to equations (7) and (10), fixed pattern noise due to threshold voltage variation is canceled. Hence, the output voltage V out  of the logarithmic mode CMOS image sensor responds to the photocurrent generated by the photodiode  207  under illumination in a logarithmic fashion. By controlling the row select signal, signals are transmitted from the NMOS transistor to a correlated sampling circuit for sampling. After the sampled signal voltage V out12 , V out22  and load voltage V out11 , V out21  are subtracted, any mismatch between the threshold voltage of various transistors inside the logarithmic mode CMOS image sensor array are canceled out so that quality of output voltage is improved.  
     [0035] Note that the highest voltage V dd  is selected according to the manufacturing technique used in fabricating the logarithmic mode CMOS image sensor. For example, if the line width of the fabrication process is 0.25 μm, the highest voltage V dd  is about 2.5V. If the line width is 0.35 μm, the highest voltage V dd  is about 3.3V. If the line width is 0.5 μm, the highest voltage V dd  is 5V.  
     [0036] In summary, by adding one more MOS transistor, when the logarithmic mode CMOS image sensor circuit is performing thecorrelated double sampling, the threshold voltage variation produced in the pixel and most part of in-pixel fixed pattern noise can be effectively eliminated while the output voltage is exported. Hence, output voltage is stabilized and image quality is improved.  
     [0037] It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.