Abstract:
A solid state imaging device comprises a pixel unit, a first controller, and a second controller. The pixel unit has a two dimensional matrix of pixels and each of the pixels is arrayed in the matrix. The first controller independently controls the output unit row by row. The second controller independently controls the first reset unit row by row. The each of pixels comprises a photoelectric conversion element, a first reset unit, a charge retention unit, a transmitter, a second reset unit, and an output unit. The element converts incident light into a signal charge and accumulates the converted signal charge. The first reset unit resets the signal charge. The charge retention unit retains a signal charge output from the photoelectric conversion element. The transmitter transmits the signal charge to the charge retention unit. The second reset unit resets the signal charge. The output unit outputs a pixel signal in response to the signal charge to a vertical signal line.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a solid state imaging device in which a plurality of pixels having a photoelectric conversion element, which converts input light into signal charges and accumulates the converted signal charges, are arranged. 
         [0003]    Priority is claimed on Japanese Patent Application No. 2008-312726, filed Dec. 8, 2008, the content of which is incorporated herein by reference. 
         [0004]    2. Description of Related Art 
         [0005]    In the related art, a Metal Oxide Semiconductor (MOS) type solid state imaging device, which uses pixels having an amplification and reading function, is known.  FIG. 5  shows the structure of a pixel of a MOS type solid state imaging device. As shown in  FIG. 5 , a single pixel  100   a  includes a photodiode  101 , a transmission transistor  102 , a charge retention (or Floating Diffusion (FD)) section  103 , an FD reset transistor  104 , an amplification transistor  105 , and a selection transistor  106 . 
         [0006]    The photodiode  101  is photoelectric conversion element that converts input light into a signal charge and accumulates the converted signal charge. 
         [0007]    The transmission transistor  102  sends the signal charge, accumulated in the photodiode  101 , to the FD section  103 . The FD section  103  maintains the signal charge, which is sent from the photodiode  101  by the transmission transistor  102 . The FD reset transistor  104  resets the signal charge in the photodiode  101  and the FD section  103 . The amplification transistor  105  amplifies the level of a voltage of the FD section  103  and outputs the amplified voltage as a pixel signal. The selection transistor  106  outputs the pixel signal to a vertical signal line  114  when the pixel  100   a  is selected as a pixel that reads out a signal charge. Herein, other components except for the photodiode  101  are shaded from light. 
         [0008]    Inside the pixel  100   a , a pixel power line  110 , an FD reset line  111 , a transmission line  112 , and a selection line  113 , as well as the vertical signal line  114 , are arranged. The pixel power line  110  is a signal line for applying a supply voltage VDD, and is electrically connected to a drain side of the amplification transistor  105  and to a drain side of the FD reset transistor  104 . The FD reset line  111  is a signal line for applying FD reset pulses φRMi (i=1 to m), which are for resetting FD sections  103  of one row, and is connected to gates of FD reset transistors  104  of one row. 
         [0009]    The transmission line  112  is a signal line for applying row transmission pulses φTRi (i=1 to m), which are for sending signal charges of pixels of one row to the FD section  103  of each pixel, and is electrically connected to gates of transmission transistors  102  of the pixels of one row. The selection line  113  is a signal line for applying row selection pulses φSEi (i=1 to m), which are for selecting pixels of one row that read out pixel signals, and is electrically connected to gates of selection transistors  106  of the pixels of one row. Due to the structure of the pixel using the four transistors, a photoelectric conversion function, a reset function, an amplification and reading function, a temporary memory function, and a selection function are realized. 
         [0010]    The MOS type solid state imaging device has a pixel array in which pixels having the above-described structure are arrayed in a matrix consisting of m rows×n columns. The MOS type solid state imaging device uses a typical XY address reading method, which reads pixel signals from all of the pixels by reading pixel signals in a per row by sequentially selecting each row from first row to mth row using a vertical scanning circuit and a horizontal scanning circuit (not shown). 
         [0011]    In the typical XY address reading method, timings for sending signal charges to the FD sections  103  are different for each row. In more detail, in the first row in which the signal charges are initially read out and in the mth row in which the signal charges are read out for the last time, the greatest difference in the timings for reading the respective rows is as much as one frame. This causes a problem in that the image of a moving object is distorted if taken at high speed. 
         [0012]    As an approach to solve the foregoing problem in the typical XY address reading method, there is a global shutter reading method. The global shutter reading method will be described hereinafter with reference to  FIG. 6 . First, photodiodes  101  of pixels of all of the rows are reset by simultaneously outputting FD reset pulses φRM 1  to φRMm and row transmission pulses φTR 1  to φTRm of all of the rows from a vertical scanning circuit (not shown). Subsequently, after a signal accumulation period (i.e., an exposure period) is passed, the row transmission pulses φTR 1  to φTRm of all of the rows are simultaneously output from the vertical scanning circuit, and signal charges of the photodiodes  101  of the pixels of all of the rows, accumulated in the exposure period, are simultaneously sent to the FD sections  103 . The operation of a global shutter is performed in the above described processing. 
         [0013]    In addition, in the pixel structure as shown in  FIG. 5 , when strong light is incident on the photodiode  101 , a signal charge equal to or more than the maximum amount of charge, which can be charged in the photodiode  101 , is generated. This causes the problem of the so-called “blooming,” in which the signal charge flows out from the photodiode  101  and spills into the FD section  103 , adjacent pixels, or the like through the transmission transistor  102  or a channel stop area. The “blooming” also occurs in the case of performing a global shutter operation by once sending the signal charges obtained by the photodiodes  101  to the FD sections  103 , simultaneously across all of the pixels. 
         [0014]    When the “blooming” occurs, a white stripe shape or a white circular pattern is observed in a photographing image, thereby degrading image quality. In the case of performing the global shutter operation, after the signal charge is sent to the FD section  103 , an excessive charge occurred in the photodiode  101  also overflows into the FD section  103  in the pixel thereof. The excessive charge is then added to the signal charge sent to the FD section  103 , thereby destroying the signal. This, as a result, increases the influence of the blooming. 
         [0015]    As a method for suppressing the blooming from occurring in the case of performing the global shutter operation, there are the following methods. Specifically, Japanese Unexamined Patent Application Publication No. 2004-111590 and Japanese Unexamined Patent Application Publication No. 2005-42714 disclose a circuit structure shown in  FIG. 7 . With respect to the pixel  100   a  shown in  FIG. 5 , a Photodiode (PD) reset transistor  107 , which directly resets the photodiode  101 , is added to a pixel  100   b , as shown in  FIG. 7 . In addition, a PD reset line  115 , which applies a PD reset pulse φRPDi (i=1 to m) for resetting photodiodes  101  of one row, is added. The PD reset line  115  is connected to gates of PD reset transistors  107  of one row. In the disclosure of Japanese Unexamined Patent Application Publication No. 2004-111590 and Japanese Unexamined Patent Application Publication No. 2005-42714, a charge occurred in the photodiode  101  is output to the pixel power line  110  by turning on the PD reset transistor  107  using such a circuit structure, except for the exposure period. 
         [0016]    Below, the operation of this method is explained with reference to  FIG. 8 . First, PD reset pulses φRPD 1  to φRPDm of all of the rows, output from the vertical scanning circuit (not shown), are simultaneously converted from “H” level to “L” level, and the reset of the photodiodes  101  of the pixels of all of the rows is canceled, so that accumulation begins. Subsequently, after a certain signal accumulation period (i.e., an exposure period) is passed, row transmission pulses φTR 1  to φTRm of all of the rows are simultaneously output from the vertical scanning circuit, so that the signal charges, accumulated in the photodiodes  101  of the pixels of all of the rows in the exposure period, are simultaneously sent to the FD sections  103 . 
         [0017]    Afterwards, selection pulses φSEi are sequentially input row by row, and pixel signals are read out row by row using a horizontal reading circuit (not shown). Before these pixel signals are read out, the PD reset pulses φRPD 1  to φRPDm of all of the rows are simultaneously converted from “L” level to “H” level. Accordingly, the photodiodes  101  of the pixels of all of the rows are reset, and the charges occurred in the photodiodes  101  are discharged to the pixel power lines  110 . 
       SUMMARY 
       [0018]    In one aspect of the present invention, there is provided a solid state imaging device that includes at least: a pixel unit which has a two dimensional matrix of pixels, in which each of the pixels arrayed in the matrix comprises; a photoelectric conversion element which converts incident light into a signal charge and accumulates the converted signal charge, a first reset unit which resets the signal charge accumulated in the photoelectric conversion element, a charge retention unit which retains a signal charge output from the photoelectric conversion element, a transmitter which transmits the signal charge accumulated in the photoelectric conversion element to the charge retention unit, a second reset unit which resets the signal charge retained in the charge retention unit, and an output unit which outputs a pixel signal in response to the signal charge, retained in the charge retention unit, to a vertical signal line; a first controller which independently controls the output unit row by row; and a second controller which independently controls the first reset unit row by row. 
         [0019]    Moreover, preferably in the solid state imaging device according to the present invention, the first controller may control the output unit row by row so that pixel signals are sequentially output row by row to the vertical signal line, and the second controller may control the first reset unit row by row so as to reset the photoelectric conversion element when the pixel signals are output and to cancel the reset state of the photoelectric conversion element after the output of the pixel signals is completed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The above features and advantages of the present invention will be more apparent from the following detailed description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
           [0021]      FIG. 1  is a block diagram showing the structure of a solid state imaging device in accordance with a first exemplary embodiment of the invention; 
           [0022]      FIG. 2  is a timing chart showing the operation of the solid state imaging device in accordance with the first exemplary embodiment of the invention; 
           [0023]      FIG. 3  is a block diagram showing the structure of a solid state imaging device in accordance with a second exemplary embodiment of the invention; 
           [0024]      FIG. 4  is a timing chart showing the operation of the solid state imaging device in accordance with the second exemplary embodiment of the invention; 
           [0025]      FIG. 5  is a block diagram showing the structure of a MOS solid state imaging device of the related art; 
           [0026]      FIG. 6  is a timing chart showing the operation of the MOS solid state imaging device of the related art; 
           [0027]      FIG. 7  is a block diagram showing the structure of a MOS solid state imaging device of the related art; and 
           [0028]      FIG. 8  is a timing chart showing the operation of the MOS solid state imaging device of the related art. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes. 
         [0030]    Exemplary embodiments of the invention will now be described hereinafter with reference to the accompanying drawings. 
       First Embodiment 
       [0031]    First, a description will be given of a first exemplary embodiment of the invention.  FIG. 1  shows the structure of a solid state imaging device in accordance with this embodiment of the invention. The solid state imaging device shown in  FIG. 1  includes a pixel unit  200 , a vertical scanning circuit  300 , a horizontal signal reading circuit  400 , and a power supply  150 . The pixel unit  200  has a structure in which pixels  100   b  are arranged in the form of a 3×3 matrix. Description of the pixels  100   b  will be omitted since they have the same structure as shown in  FIG. 7 . 
         [0032]    The vertical scanning circuit  300  controls the driving of the pixel unit  200  with respect to each row. In order to perform the driving control, the vertical scanning circuit  300  includes unit circuits  301 - 1 ,  301 - 2 , and  301 - 3 , the number of which is the same as that of the rows. 
         [0033]    In addition, each unit circuit includes a controller  302 - i , a controller  303 - i , a controller  304 - i  (a second controller), and a controller  305 - i  (a first controller), where i is 1 to 3. 
         [0034]    The controller  302 - i  controls, row by row, FD reset pulses φRMi (i=1 to 3) for independently resetting FD sections  103  of one row. The controller  303 - i  independently controls, row by row, row transmission pulses φTRi (i=1 to 3), which are for transmitting signal charges of pixels of one row to FD sections  103  of each pixel. The controller  304 - i  independently controls, row by row, PD reset pulses φRPDi (i=1 to 3) for resetting photodiodes  101  of one row. The controller  305 - i  independently controls, row by row, row selection pulses φSEi (i=1 to 3) for selecting pixels of one row, which are for reading the pixel signals. The pixel signals of the pixels of the row selected by the row selection pulse φSEi are designed to be output to vertical signal lines  114 , each of which is provided in every column. 
         [0035]    The pixel signals of one row, output to the vertical signal lines  114 , are output from an output terminal  410  by the horizontal signal reading circuit  400 , in a time series in which the pixel signals are horizontally aligned in order. While  FIG. 1  shows FD reset lines  111 , transmission lines  112 , selection lines  113 , vertical signal lines  114 , and PD reset lines  115 , pixel power lines supplying a supply voltage VDD are not shown. 
         [0036]    Next, the operation of the solid state imaging device shown in  FIG. 1  will be described with reference to  FIG. 2 . First, with respect to all of the rows, the PD reset pulses φRPDi are set to “H” level, the row transmission pulses φTRi are set to “L” level, the FD reset pulses φRMi are set to “L” level, and the row selection pulses φSEi are set to “L” level, so that PD reset transistors  107  are turned on. Accordingly, the photodiodes  101  of all of the pixels are reset. The PD reset pulses φRPDi, the row transmission pulses φTRi, the FD reset pulses φRMi, and the row selection pulses φSEi are output from the vertical scanning circuit  300 . 
         [0037]    Subsequently, with respect to all of the rows, the PD reset pulses φRPDi are set to “L” level, and the PD reset transistors  107  are turned off, so that the photodiodes  101  start accumulation. When an intended accumulation time is passed, the FD reset pulses φRMi of all of the rows are set to “H” level, and the FD sections  103  are reset. Then, the row transmission pulses φTRi of all of the rows are set to “H” level, so that signal charges accumulated in the photodiodes  101  are sent to the FD sections  103 . In addition, right after the completion of the transmission of the signal charges, the PD reset pulses φRPi of all of the rows are set to “H” level, so that the photodiodes  101  of all of the pixels are reset. 
         [0038]    Subsequently, the row selection pulses φSEi for the first row are set to “H” level, so that the levels of optical signals in response to the signal charges are read out from the respective pixels of the first row. Then, the FD reset pulses φRM 1  of the first row are set to “H” level, so that the reset levels of the FD sections  103  of the respective pixels of the first row are read out. In the horizontal signal reading circuit  400 , differential processing between the optical signal levels and the reset levels is performed. As the differential processing is performed, output according to the signal charges occurred by the entering of light is obtained. Next, the pixel signals obtained by the foregoing differential processing are output from the output terminal  410  of the horizontal signal read circuit  400 , in a time series in which the pixel signals are horizontally aligned in order. The pixel signals of the first row are output according to the foregoing operation. After the completion of the operation of reading the pixels signals of the first row, the PD reset pulses φRPD 1  of the first row are set to “L” level, and the PD reset transistors  107  of the first row are turned off. 
         [0039]    Subsequently, the same operation as on the first row is performed on the second and third rows, and accordingly, the operation of reading the pixel signals of all of the pixels is completed. After the completion of the operation of reading the pixel signals of the second row, the PD reset transistors  107  of the second row are turned off, and after the completion of the operation of reading the pixel signals of the third row, the PD reset transistors  107  of the third row are turned off. 
         [0040]    According to the foregoing operation, the accumulation of the signals of all of the rows is started and completed at the same timing, and the global shutter operation can be realized. In addition, due to the control of the PD reset pulses φRPDi, the PD reset transistors  107  stay turned on until the pixel signals are read out. Thus, the signals are not destroyed by the blooming. After the pixel signals are read row by row, the PD reset transistors  107  are sequentially turned off row by row. It is therefore possible to reduce the period, in which the photodiodes  101  are reset, to be shorter than in the related art. This, as a result, can reduce the current of the supply voltage VDD consumed through the PD reset transistors  107 . 
         [0041]    This embodiment has been described with respect to the case in which the pixels are arrayed in three rows and three columns for the sake of simplicity. However, this is not intended to limit the present invention, but can be modified without departing from the object of the present invention. In addition, the off operation after the completion of reading the PD reset transistors  107  is not necessarily performed right after the reading of the pixel signals of that row. Rather, the off operation can be performed after the reading operation is performed on several rows. In addition, the off operation after the completion of the reading of the PD reset transistors  107  can be performed on several rows rather than on one row. 
       Second Embodiment 
       [0042]    Next, a description will be given of a second exemplary embodiment of the invention.  FIG. 3  shows the structure of a solid state imaging device in accordance with this embodiment. The solid state imaging device shown in  FIG. 3  also has an Analog to Digital (AD) converter  500  and a frame memory  600 , which are added with respect to the solid state imaging device shown in  FIG. 1 . Description of the other components will be omitted since they are the same as those in  FIG. 1 . 
         [0043]    Below, with reference to  FIG. 4 , a description will be given of the operation of the solid state imaging device shown in  FIG. 3 . First, with respect to all of the rows, PD reset pulses φRPDi are set to “H” level, row transmission pulses φTRi are set to “L” level, FD reset pulses φRMi are set to “L” level, and row selection pulses φSEi are set to “L” level, so that the PD reset transistors  107  are turned on. Thus, the photodiodes  101  of all of the pixels are reset. The PD reset pulses φRPDi, the row transmission pulses φTRi, the FD reset pulses φRMi, and the row selection pulses φSEi are output from the vertical scanning circuit  300 . 
         [0044]    Subsequently, the FD reset pulses φRM 1  for the first row are set to “H” level, and then the row selection pulses φSEi are set to “H” level, so that the reset levels of the FD sections  103  of the first row are output to the horizontal signal reading circuit  400 . These reset levels are output from the output terminal  410  of the horizontal signal reading circuit  400 , in a time series in which the reset levels are horizontally aligned in order. The output reset levels are AD-converted by the AD converter  500  and are retained in the frame memory  600 . With respect to the second and next rows, as in the first rows, the reset levels of the FD sections  103  are read out, and the reset levels of the FD sections  103  of all of the pixels are retained in the frame memory  600 . 
         [0045]    Subsequently, the PD reset pulses φRPDi of all of the rows are set to “L” level, and the PD reset transistors  107  are turned off, so that the photodiodes  101  start accumulation. When an intended accumulation time is passed, the row transmission pulses φTRi of all of the rows are set to “H” level, and thus the signal charges accumulated in the photodiodes  101  are sent to the FD sections  103 . In addition, right after the completion of the transmission of the signal charges, the PD reset pulses φRPDi of all of the rows are set to “H” level, so that the photodiodes  101  of all of the pixels are reset. 
         [0046]    Subsequently, as in the reading of the reset levels of the FD sections  103 , the row selection pulses φSEi of the first row are set to “H” level, and the optical signal levels of the first row are output to the horizontal signal reading circuit  400 . The optical signal levels are output from the output terminal  410  of the horizontal signal reading circuit  400  in a time series, in which the optical signal levels are horizontally aligned in order. 
         [0047]    The optical signal levels of the first row, output from the output terminal  410  of the horizontal signal reading circuit  400 , are AD-converted by the AD converter  500 . Next, only the signal components in response to the incident light are drawn by taking differences between the reset levels of the FD sections  103  of the first row and the optical levels of the first row, which are already retained in the frame memory  600 . 
         [0048]    Subsequently, the same operation as in the first row is performed on the second row and on the third row, thereby completing the operation of reading the pixel signals from all of the pixels. After the reading of the pixel signals of the second row is completed, the PD reset transistors  107  of the second row are turned off. After the reading of the pixel signals of the third row is completed, the PD reset transistors  107  of the third row are turned off. 
         [0049]    According to the foregoing operation, the accumulation of the signals of all of the rows is started and completed at the same timing, and the global shutter operation can be realized. In addition, due to the control of the PD reset pulses φRPDi, the PD reset transistors  107  stay turned on until the pixel signals are read out. Thus, the signals are not destroyed by the blooming. After the pixel signals are read row by row, the PD reset transistors  107  are sequentially turned off row by row. It is therefore possible to reduce the period in which the photodiodes  101  are reset to be shorter than in the related art. This, as a result, can reduce the current of the supply voltage VDD consumed through the PD reset transistors  107 . 
         [0050]    In addition, this embodiment can obtain the following effects. In the solid state imaging device in accordance with the first exemplary embodiment of the invention, reset noise (hereinafter, referred to as first reset noise) occurs as the FD reset pulses φRMi are set to “H” level and the FD reset transistors  104  are turned on within the exposure period as shown in  FIG. 2 . The first reset noise is applied to the reset level. In addition, in the reading period of each row, reset noise (hereinafter, referred to as second reset noise) occurs as the FD reset pulses φRMi are set to “H” level and thus the FD reset transistors  104  are turned on. The second reset noise is applied to the optical signal level. Even if differential processing is performed between the optical signal levels and the reset levels in order to remove the reset noises, it is impossible to completely remove the reset noises since there is no correlation between the first and second reset noises. 
         [0051]    Meanwhile, in the solid state imaging device in accordance with the second exemplary embodiment of the invention, the FD reset pulses φRMi are set to “H” level in the reset signal sequential reading period shown in  FIG. 4 , and thus the reset noise, generated by the turning on of the FD reset transistors  104 , is added to both the reset levels and the optical signal levels. Accordingly, the reset levels are removed by performing the differential processing between the optical signal levels and the reset levels, and thus it is possible to obtain higher SN signals when compared to those of the first exemplary embodiment. 
         [0052]    This embodiment has been described with respect to the case in which the pixels are arrayed in three rows and three columns for the sake of simplicity. However, this is not intended to limit the present invention, but can be modified without departing from the object of the present invention. In addition, the off operation after the completion of reading the PD reset transistors  107  is not necessarily performed right after the reading of the pixel signals of that row. Rather, the off operation can be performed after the reading operation performed on several rows. In addition, the off operation after the completion of the reading of the PD reset transistors  107  can be performed on several rows rather than just on one row. 
         [0053]    According to the present invention, the pixel signals can be sequentially output row by row by controlling the first reset section and the pixel signal output section independently row by row, the reset state of the photoelectric conversion elements can be canceled from the row in which the output of the pixel signals is completed. Accordingly, it is possible to reduce the period in which the photoelectric conversion elements are reset. 
         [0054]    While the exemplary embodiments of the invention have been described with reference to the accompanying drawings, the detailed constitutions of the invention are not limited to the foregoing embodiments but embrace changes in design to the extent that they do not depart from the concept of the invention.