Patent Publication Number: US-2023163229-A1

Title: Photoelectric conversion element and photoelectric conversion device

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a photoelectric conversion element and a photoelectric conversion device. 
     Description of the Related Art 
     As photoelectric conversion elements, APD (Avalanche Photo Diode) and SPAD (Single Photon Avalanche Diode) in which charge generated by incidence of photon is multiplied by avalanche breakdown are known. Japanese Patent Application Laid-Open No. 2018-088488 and Japanese Patent Application Laid-Open No. 2018-064086 disclose photoelectric conversion elements in which an n-type semiconductor region is arranged in an n-well of a photoelectric conversion region so as to be in contact with a p-type semiconductor region forming an avalanche multiplication region in order to easily collect generated charges in an avalanche multiplication region. 
     In a sensor requiring high-speed response such as a photoelectric conversion device having a distance measuring function of a ToF (Time of Flight) system, it is required to minimize variation in the collection time of charges generated by incidence of photons. However, the photoelectric conversion elements described in Japanese Patent Application Laid-Open No. 2018-088488 and Japanese Patent Application Laid-Open No. 2018-064086 are not necessarily preferable from the viewpoint of reducing variation in the collection time of charges generated by incidence of photons. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a photoelectric conversion element and a photoelectric conversion device excellent in high-speed response. 
     According to an embodiment of the present disclosure, there is provided a photoelectric conversion element provided in a semiconductor layer having a first surface and a second surface opposed to the first surface including a first semiconductor region of a first conductivity type, a second semiconductor region of a second conductivity type disposed closer to the second surface than the first semiconductor region and forming a p-n junction with the first semiconductor region, a third semiconductor region of the first conductivity type disposed closer to the second surface than the second semiconductor region and overlapping with the first semiconductor region and the second semiconductor region in a plan view, a fourth semiconductor region of the second conductivity type disposed closer to the second surface than the third semiconductor region and overlapping with the whole of a region where the first semiconductor region, the second semiconductor region, and the third semiconductor region are disposed in the plan view, a fifth semiconductor region of the second conductivity type disposed at a depth between the third semiconductor region and the fourth semiconductor region, and a sixth semiconductor region of the second conductivity type disposed so as to surround a region where the first semiconductor region, the second semiconductor region, the third semiconductor region, and the fifth semiconductor region are disposed in the plan view, and electrically connected to the fourth semiconductor region, wherein the fifth semiconductor region has an area smaller than an area of the third semiconductor region in the plan view and overlaps with the first semiconductor region in the plan view. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    and  FIG.  2    are block diagrams illustrating a schematic configuration of a photoelectric conversion device according to a first embodiment of the present invention. 
         FIG.  3    is a block diagram illustrating a configuration example of a pixel of the photoelectric conversion device according to the first embodiment of the present invention. 
         FIG.  4    is a perspective view illustrating a configuration example of the photoelectric conversion device according to the first embodiment of the present invention. 
         FIG.  5 A ,  FIG.  5 B , and  FIG.  5 C  are diagrams illustrating the basic operation of the photoelectric conversion unit in the photoelectric conversion device according to the first embodiment of the present invention. 
         FIG.  6    is a plan view illustrating a structure of a photoelectric conversion element in the photoelectric conversion device according to the first embodiment of the present invention. 
         FIG.  7    is a schematic cross-sectional view illustrating a structure of the photoelectric conversion element in the photoelectric conversion device according to the first embodiment of the present invention. 
         FIG.  8 A  and  FIG.  8 B  are diagrams illustrating a potential distribution and a charge transfer path inside the photoelectric conversion element according to a reference example. 
         FIG.  9 A ,  FIG.  9 B  and  FIG.  12 A  illustrate frequency distributions of time required to reach an avalanche multiplication region for charges generated inside the photoelectric conversion element according to the reference example. 
         FIG.  10 A  and  FIG.  10 B  are diagrams illustrating a potential distribution and a charge transfer path inside the photoelectric conversion element of the photoelectric conversion device according to the first embodiment of the present invention. 
         FIG.  11 A ,  FIG.  11 B , and  FIG.  12 B  illustrate frequency distributions of time required to reach an avalanche multiplication region for charges generated in the photoelectric conversion element of the photoelectric conversion device according to the first embodiment of the present invention. 
         FIG.  13    is a schematic cross-sectional view illustrating a structure of a photoelectric conversion element of a photoelectric conversion device according to a second embodiment of the present invention. 
         FIG.  14    is a plan view illustrating a structure of a photoelectric conversion element of a photoelectric conversion device according to a third embodiment of the present invention. 
         FIG.  15    is a schematic cross-sectional view illustrating a structure of a photoelectric conversion element of a photoelectric conversion device according to a fourth embodiment of the present invention. 
         FIG.  16    and  FIG.  17    are plan views illustrating a structure of a photoelectric conversion element in a photoelectric conversion device according to a fifth embodiment of the present invention. 
         FIG.  18    is a schematic cross-sectional view illustrating a structure of the photoelectric conversion element of the photoelectric conversion device according to the fifth embodiment of the present invention. 
         FIG.  19    is a schematic cross-sectional view illustrating a structure of a photoelectric conversion element of a photoelectric conversion device according to a modified example of the fifth embodiment of the present invention. 
         FIG.  20    is a block diagram illustrating a schematic configuration of a photodetection system according to a sixth embodiment of the present invention. 
         FIG.  21    is a block diagram illustrating a schematic configuration of a range image sensor according to a seventh embodiment of the present invention. 
         FIG.  22    is a schematic diagram illustrating a configuration example of an endoscopic surgical system according to an eighth embodiment of the present invention. 
         FIG.  23 A ,  FIG.  23 B , and  FIG.  23 C  are schematic diagrams illustrating a configuration example of a movable object according to a ninth embodiment of the present invention. 
         FIG.  24    is a block diagram illustrating a schematic configuration of a photodetection system according to the ninth embodiment of the present invention. 
         FIG.  25    is a flowchart illustrating the operation of the photodetection system according to the ninth embodiment of the present invention. 
         FIG.  26 A  and  FIG.  26 B  are schematic diagrams illustrating a schematic configuration of a photodetection system according to a tenth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings. The following embodiments are for embodying the technical concept of the present invention and are not limited thereto to. The sizes and positional relationships of the members illustrated in the drawings may be exaggerated for clarity of explanation. In the following description, the same components are denoted by the same reference numerals, and description thereof may be omitted. 
     First Embodiment 
     A schematic configuration of a photoelectric conversion device according to a first embodiment of the present invention will be described with reference to  FIG.  1    to  FIG.  4   .  FIG.  1    and  FIG.  2    are block diagrams illustrating a schematic configuration of a photoelectric conversion device according to the present embodiment.  FIG.  3    is a block diagram illustrating a configuration example of a pixel of the photoelectric conversion device according to the present embodiment.  FIG.  4    is a perspective view illustrating a configuration example of the photoelectric conversion device according to the present embodiment. 
     As illustrated in  FIG.  1   , the photoelectric conversion device  100  according to the present embodiment includes a pixel region  10 , a vertical scanning circuit unit  40 , a readout circuit unit  50 , a horizontal scanning circuit unit  60 , an output circuit unit  70 , and a control pulse generation unit  80 . 
     The pixel region  10  is provided with a plurality of pixels  12  arranged in an array so as to form a plurality of rows and a plurality of columns. Each pixel  12  may include a photoelectric conversion unit including a photoelectric conversion element and a pixel signal processing unit that processes a signal output from the photoelectric conversion unit, as will be described later. The number of pixels  12  constituting the pixel region  10  is not particularly limited. For example, the pixel region  10  may be constituted by a plurality of pixels  12  arranged in an array of several thousands of rows and several thousands of columns as in a general digital camera. Alternatively, the pixel region  10  may be constituted by a plurality of pixels  12  arranged in one row or one column. Alternatively, one pixel  12  may constitute the pixel region  10 . 
     In each row of the pixel array of the pixel region  10 , a control line  14  is arranged so as to extend in a first direction (lateral direction in  FIG.  1   ). The control line  14  is connected to the pixels  12  arranged in the first direction, and serves as a signal line common to these pixels  12 . The first direction in which the control line  14  extends may be denoted as a row direction or a horizontal direction. Each of the control lines  14  may include a plurality of signal lines for supplying a plurality of types of control signals to the pixels  12 . 
     In addition, in each column of the pixel array of the pixel region  10 , a data line  16  is arranged so as to extend in a second direction (vertical direction in  FIG.  1   ) intersecting with the first direction. The data line  16  is connected to the pixels  12  arranged in the second direction, and serves as a signal line common to these pixels  12 . The second direction in which the data line  16  extends may be referred to as a column direction or a vertical direction. Each of the data lines  16  may include a plurality of signal lines for transferring a digital signal of a plurality of bits output from the pixel  12  for each bit. 
     The control line  14  in each row is connected to the vertical scanning circuit unit  40 . The vertical scanning circuit unit  40  is a control unit having a function of receiving control signals output from the control pulse generation unit  80 , generating control signals for driving the pixels  12 , and supplying the generated control signals to the pixels  12  via the control lines  14 . A logic circuit such as a shift register or an address decoder may be used for the vertical scanning circuit unit  40 . The vertical scanning circuit unit  40  sequentially scans the pixels  12  in the pixel region  10  in units of rows, and sequentially outputs pixel signals of the pixels  12  to the readout circuit unit  50  via the data lines  16 . 
     The data line  16  in each column is connected to the readout circuit unit  50 . The readout circuit unit  50  includes a plurality of holding units (not illustrated) provided in correspondence with each column of the pixel array of the pixel region  10 , and has a function of holding the pixel signals of the pixels  12  of each column output from the pixel region  10  in units of rows via the data line  16  in the holding units of the corresponding columns. 
     The horizontal scanning circuit unit  60  is a control unit that receives a control signal output from the control pulse generation unit  80 , generates a control signal for reading out a pixel signal from a holding unit of each column of the readout circuit unit  50 , and supplies the control signal to the readout circuit unit  50 . A logic circuit such as a shift register or an address decoder may be used for the horizontal scanning circuit unit  60 . The horizontal scanning circuit unit  60  sequentially scans the holding units of the respective columns of the readout circuit unit  50 , and sequentially outputs the pixel signals held in the respective columns to the output circuit unit  70 . 
     The output circuit unit  70  includes an external interface circuit, and outputs the pixel signals output from the readout circuit unit  50  to the outside of the photoelectric conversion device  100 . The external interface circuit included in the output circuit unit  70  is not particularly limited. For example, SerDes (SERializer/DESerializer) transmission circuits such as LVDS (Low Voltage Differential Signaling) circuit and SLVS (Scalable Low Voltage Signaling) circuit may be applied to the external interface circuit. 
     The control pulse generation unit  80  is a control circuit for generating control signals for controlling the operation and timing of the operation of the vertical scanning circuit unit  40 , the readout circuit unit  50 , and the horizontal scanning circuit unit  60 , and supplying the generated control signals to each functional block. At least a part of the control signals for controlling the operation and timing of the operation of the vertical scanning circuit unit  40 , the readout circuit unit  50 , and the horizontal scanning circuit unit  60  may be supplied from the outside of the photoelectric conversion device  100 . 
     The connection mode of each functional block of the photoelectric conversion device  100  is not limited to the configuration example illustrated in  FIG.  1   , and may be configured as illustrated in  FIG.  2   , for example. 
     In the configuration example of  FIG.  2   , a data line  16  extending in the first direction is arranged in each row of the pixel array of the pixel region  10 . The data line  16  is connected to the pixels  12  arranged in the first direction, and serves as a signal line common to these pixels  12 . In addition, a control line  18  extending in the second direction is arranged in each column of the pixel array of the pixel region  10 . The control line  18  is connected to the pixels  12  arranged in the second direction, and serves as a signal line common to these pixels  12 . 
     The control line  18  in each column is connected to the horizontal scanning circuit unit  60 . The horizontal scanning circuit unit  60  receives control signals output from the control pulse generation unit  80 , generates control signals for reading out pixel signals from the pixels  12 , and supplies the generated control signals to the pixels  12  via the control lines  18 . Specifically, the horizontal scanning circuit unit  60  sequentially scans the plurality of pixels  12  in the pixel region  10  in units of columns, and outputs the pixel signal of the pixel  12  in each row belonging to the selected column to the data lines  16 . 
     The data line  16  of each row is connected to the readout circuit unit  50 . The readout circuit unit  50  includes a plurality of holding units (not illustrated) provided in correspondence with each row of the pixel array of the pixel region  10 , and has a function of holding the pixel signals of the pixels  12  of each row outputted from the pixel region  10  in units of columns via the data lines  16  in the holding units of the corresponding rows. 
     The readout circuit unit  50  receives the control signals output from the control pulse generation unit  80 , and sequentially outputs the pixel signals held in the holding units of the respective rows to the output circuit unit  70 . 
     Other configurations in the configuration example of  FIG.  2    may be similar to those in the configuration example of  FIG.  1   . 
     As illustrated in  FIG.  3   , each pixel  12  includes a photoelectric conversion unit  20  and a pixel signal processing unit  30 . The photoelectric conversion unit  20  may include a photoelectric conversion element  22  and a quenching element  24 . The pixel signal processing unit  30  may include a signal processing circuit  32 , a counter  34 , and a pixel output circuit  36 . 
     The photoelectric conversion element  22  may be an avalanche photodiode (hereinafter referred to as “APD”). An anode of the APD constituting the photoelectric conversion element  22  is connected to a node to which the voltage VL is supplied. A cathode of the APD constituting the photoelectric conversion element  22  is connected to one terminal of the quenching element  24 . A connection node between the photoelectric conversion element  22  and the quenching element  24  is an output node of the photoelectric conversion unit  20 . The other terminal of the quenching element  24  is connected to a node to which a voltage VH higher than the voltage VL is supplied. The voltage VL and the voltage VH are set such that a reverse bias voltage sufficient for the APD to perform the avalanche multiplication operation is applied. In one example, a negative high voltage is applied as the voltage VL, and a positive voltage as high as a power supply voltage is applied as the voltage VH. For example, the voltage VL is -30 V and the voltage VH is 1 V. 
     The photoelectric conversion element  22  may be formed of an APD as described above. By supplying a reverse bias voltage sufficient to perform the avalanche multiplication operation to the APD, charge generated by light incidence to the APD cause avalanche multiplication, and an avalanche current is generated. The operation modes in a state where a reverse bias voltage is supplied to the APD include a Geiger mode and a linear mode. The Geiger mode is an operation mode in which a voltage applied between an anode and a cathode is set to a reverse bias voltage larger than a breakdown voltage of the APD. The linear mode is an operation mode in which a voltage applied between an anode and a cathode is set to a reverse bias voltage close to or lower than a breakdown voltage of the APD. The APD operating in the Geiger mode is called SPAD (Single Photon Avalanche Diode). The APD constituting the photoelectric conversion element  22  may operate in the linear mode or the Geiger mode. 
     In the present embodiment, the anode of the APD is set to a fixed potential, and a signal is extracted from the cathode side. Therefore, the semiconductor region of the first conductivity type in which charge having the same polarity as the signal charge is a majority carrier is an n-type semiconductor region, and the semiconductor region of the second conductivity type in which charge having a polarity different from the signal charge is a majority carrier is a p-type semiconductor region. The carriers of the first conductivity type are electrons, and the carriers of the second conductivity type are holes. Note that the present invention is true even when the cathode of the APD is set to a fixed potential and a signal is extracted from the anode side. In this case, the semiconductor region of the first conductivity type in which charge having the same polarity as the signal charge is a majority carrier is a p-type semiconductor region, and the semiconductor region of the second conductivity type in which charge having a polarity different from the signal charge is a majority carrier is an n-type semiconductor region. Although the case where one node of the APD is set to a fixed potential is described below, potentials of both nodes may be varied. 
     In this specification, when the term “impurity concentration” is used, it means a net impurity concentration obtained by subtracting the amount compensated by the impurities of the opposite conductivity type. That is, the “impurity concentration” refers to the net doping concentration. A region where the doped concentration of the p-type impurity is higher than the doped concentration of the n-type impurity is a p-type semiconductor region. Conversely, a region where the doped concentration of the n-type impurity is higher than the doped concentration of the p-type impurity is an n-type semiconductor region. 
     The quenching element  24  has a function of converting a change in the avalanche current generated in the photoelectric conversion element  22  into a voltage signal. Further, the quenching element  24  functions as a load circuit (quenching circuit) at the time of signal multiplication by avalanche multiplication, and has a function of reducing a voltage applied to the photoelectric conversion element  22  to suppress avalanche multiplication. The operation in which the quenching element  24  suppresses avalanche multiplication is called a quenching operation. Further, the quenching element  24  has a function of returning the voltage supplied to the photoelectric conversion element  22  to the voltage VH by flowing a current corresponding to the voltage drop by the quenching operation. The operation in which the quenching element  24  returns the voltage supplied to the photoelectric conversion element  22  to the voltage VH is called a recharging operation. The quenching element  24  may be composed of a resistor, a MOS transistor, or the like. 
     The signal processing circuit  32  includes an input node to which an output signal of the photoelectric conversion unit  20  is supplied, and an output node. The signal processing circuit  32  has a function as a waveform shaping unit that converts an analog signal supplied from the photoelectric conversion unit  20  into a pulse signal. The signal processing circuit  32  may be configured by a logic circuit including a NOT circuit (inverter circuit), a NOR circuit, a NAND circuit, and the like. The output node of the signal processing circuit  32  is connected to the counter  34 . 
     The counter  34  includes an input node to which an output signal of the signal processing circuit  32  is supplied, an input node connected to the control line  14 , and an output node. The counter  34  has a function of counting pulses to be superimposed on a signal output from the signal processing circuit  32 , and holding a count value as a counting result. The signal supplied from the vertical scanning circuit unit  40  to the counter  34  via the control line  14  may include an enable signal for controlling a pulse counting period (exposure period), a reset signal for resetting a count value held by the counter  34 , and the like. The output node of the counter  34  is connected to the data line  16  via the pixel output circuit  36 . 
     The pixel output circuit  36  has a function of switching an electrical connection state (connection or disconnection) between the counter  34  and the data line  16 . The pixel output circuit  36  switches the connection state between the counter  34  and the data line  16  in accordance with a control signal supplied from the vertical scanning circuit unit  40  via the control line  14  (a control signal supplied from the horizontal scanning circuit unit  60  via the control line  18  in the configuration example of  FIG.  2   ). The pixel output circuit  36  may include a buffer circuit for outputting a signal. 
     The pixel  12  is typically a unit structure that outputs a pixel signal for forming an image. However, in the case where distance measurement using a time of flight (TOF) method or the like is intended, the pixel  12  need not necessarily be a unit structure that outputs a pixel signal for forming an image. That is, the pixel  12  may be a unit structure that outputs a signal for measuring the time at which light has reached and the amount of light. 
     One pixel signal processing unit  30  is not necessarily provided for each pixel  12 , and one pixel signal processing unit  30  may be provided for a plurality of pixels  12 . In this case, one pixel signal processing unit  30  may be used to sequentially perform signal processing of a plurality of pixels  12 . 
     The photoelectric conversion device  100  according to the present embodiment may be formed on one substrate, or may be configured as a stacked-type photoelectric conversion device in which a plurality of substrates is stacked. In the latter case, for example, as illustrated in  FIG.  4   , a sensor substrate  110  and a circuit substrate  180  may be stacked and electrically connected to each other to form a stacked-type photoelectric conversion device. At least the photoelectric conversion element  22  among the components of the pixel  12  may be disposed on the sensor substrate  110 . Further, the quenching element  24  and the pixel signal processing unit  30  among the components of the pixel  12  may be disposed on the circuit substrate  180 . The photoelectric conversion element  22 , the quenching element  24 , and the pixel signal processing unit  30  are electrically connected to each other via a connection interconnection provided for each pixel  12 . The circuit substrate  180  may further include the vertical scanning circuit unit  40 , the readout circuit unit  50 , the horizontal scanning circuit unit  60 , the output circuit unit  70 , the control pulse generation unit  80 , and the like. 
     The photoelectric conversion element  22  of each pixel  12 , and the quenching element  24  and the pixel signal processing unit  30  may be provided on the sensor substrate  110  and the circuit substrate  180  so as to overlap each other in a plan view. The vertical scanning circuit unit  40 , the readout circuit unit  50 , the horizontal scanning circuit unit  60 , the output circuit unit  70 , and the control pulse generation unit  80  may be arranged around the pixel region  10  formed by the plurality of pixels  12 . Here, the “plan view” refers to a view from a direction perpendicular to the surface of the sensor substrate  110 . 
     By configuring the stacked-type photoelectric conversion device  100 , the degree of integration of elements may be increased and high functionality may be achieved. In particular, by disposing the photoelectric conversion element  22 , and the quenching element  24  and the pixel signal processing unit  30  on different substrates, the photoelectric conversion element  22  may be disposed at high density without sacrificing the light receiving area of the photoelectric conversion element  22 , and the photon detection efficiency may be improved. 
     The number of substrates constituting the photoelectric conversion device  100  is not limited to two, and three or more substrates may be stacked to form the photoelectric conversion device  100 . 
     Although the sensor substrate  110  and the circuit substrate  180  are diced chips in  FIG.  4   , the sensor substrate  110  and the circuit substrate  180  are not limited to chips. For example, each of the sensor substrate  110  and the circuit substrate  180  may be a wafer. Further, the sensor substrate  110  and the circuit substrate  180  may be stacked in a wafer state and then diced, or may be stacked and bonded after each sensor substrate  110  and the circuit substrate  180  are formed into chips. 
     Next, a basic operation of the photoelectric conversion unit  20  in the photoelectric conversion device according to the present embodiment will be described with reference to  FIG.  5 A  to  FIG.  5 C .  FIG.  5 A  to  FIG.  5 C  are diagrams illustrating the basic operation of the photoelectric conversion unit in the photoelectric conversion device according to the present embodiment.  FIG.  5 A  is a circuit diagram of the photoelectric conversion unit  20  and the signal processing circuit  32 ,  FIG.  5 B  illustrates a waveform of a signal at an input node (node A) of the signal processing circuit  32 , and  FIG.  5 C  illustrates a waveform of a signal at an output node (node B) of the signal processing circuit  32 . Here, for simplicity of explanation, it is assumed that the signal processing circuit  32  is configured by an inverter circuit. 
     At time t0, a reverse bias voltage of a potential difference corresponding to (VH-VL) is applied to the photoelectric conversion element  22 . Although a reverse bias voltage sufficient to cause avalanche multiplication is applied between the anode and the cathode of the APD constituting the photoelectric conversion element  22 , there is no carrier that becomes a cause of avalanche multiplication in a state where no photon is incident on the photoelectric conversion element  22 . Therefore, no avalanche multiplication occurs in the photoelectric conversion element  22 , and no current flows through the photoelectric conversion element  22 . 
     At time t1, it is assumed that a photon enters the photoelectric conversion element  22 . When the photon is incident on the photoelectric conversion element  22 , an electron-hole pair is generated by photoelectric conversion, avalanche multiplication occurs using these carriers as a cause, and an avalanche multiplication current flows through the photoelectric conversion element  22 . When the avalanche multiplication current flows through the quenching element  24 , a voltage drop by the quenching element  24  occurs, and the voltage of the node A begins to drop. When the voltage drop amount of the node A increases and the avalanche multiplication stops at time t3, the voltage level of the node A does not drop any further. 
     When the avalanche multiplication in the photoelectric conversion element  22  stops, a current that compensates the voltage drop flows from the node to which the voltage VL is supplied to the node A via the photoelectric conversion element  22 , and the voltage of the node A gradually increases. Then, at time t5, node Ais settled to the original voltage level. 
     The signal processing circuit  32  binarizes the signal input from the node A according to a predetermined determination threshold value, and outputs the signal from the node B. Specifically, the signal processing circuit  32  outputs a Low-level signal from the node B when the voltage level of the node A exceeds the determination threshold value, and outputs a High-level signal from the node B when the voltage level of the node A is equal to or lower than the determination threshold value. For example, as illustrated in  FIG.  5 B , it is assumed that the voltage of the node A is equal to or lower than the determination threshold value during a period from the time t2 to the time t4. In this case, as illustrated in  FIG.  5 C , the signal level at the node B is Low-level during the period from the time t0 to the time t2, and during the period from the time t4 to the time t5, and is High-level during the period from the time t2 to the time t4. 
     Thus, the analog signal input from the node A is shaped into a digital signal by the signal processing circuit  32 . A pulse signal output from the signal processing circuit  32  in response to incidence of a photon on the photoelectric conversion element  22  is a photon detection pulse signal. 
     Next, a specific structure of the photoelectric conversion element  22  in the photoelectric conversion device  100  according to the present embodiment will be described with reference to  FIG.  6    and  FIG.  7   .  FIG.  6    is a plan view illustrating a structure of the photoelectric conversion element in the photoelectric conversion device according to the present embodiment.  FIG.  7    is a schematic cross-sectional view illustrating a structure of the photoelectric conversion element in the photoelectric conversion device according to the present embodiment. 
       FIG.  6    is a plan view of photoelectric conversion elements  22  of two pixels  12  arranged adjacent to each other among a plurality of pixels  12  constituting the pixel region  10 .  FIG.  7    is a cross-sectional view taken along line A-A′ of  FIG.  6   . 
     In this specification, the term “plan view” refers to a normal direction of a light incident surface (second surface  124 ) of the semiconductor layer  120  or a surface (first surface  122 ) opposite to the light incident surface described later.  FIG.  6    corresponds to a plan view of the semiconductor layer  120  from a side of a first surface  122 . In addition, a cross section refers to a plane parallel to a normal direction of the first surface  122  or the second surface  124  of the semiconductor layer  120 . When the first surface  122  or the second surface  124  of the semiconductor layer  120  is a rough surface when viewed microscopically, the plan view is defined with reference to the first surface  122  or the second surface  124  of the semiconductor layer  120  when viewed macroscopically. In this specification, the depth direction is a direction from the first surface  122  to the second surface  124  of the semiconductor layer  120 . Hereinafter, the first surface  122  may be referred to as a “front surface” and the second surface  124  may be referred to as a “back surface”. 
     For example, as illustrated in  FIG.  7   , the photoelectric conversion device according to the present embodiment may be configured as a stacked-type photoelectric conversion device in which the sensor substrate  110  and the circuit substrate  180  are stacked. 
     The sensor substrate  110  includes a semiconductor layer  120  having a first surface  122  and a second surface  124  opposed to the first surface  122 , and an interconnection structure layer  150  provided on a side of the first surface  122  of the semiconductor layer  120 . An optical structure layer  190  may be disposed over a side of the second surface  124  of the semiconductor layer  120 . The side of the second surface  124  of the semiconductor layer  120  provided with the optical structure layer  190  serves as a light receiving surface for receiving light to be detected. That is, the photoelectric conversion device according to the present embodiment is a so-called backside illumination type photoelectric conversion device. 
     The semiconductor layer  120  is formed by thinning a single crystalline silicon substrate, for example, and contains an n-type impurity or a p-type impurity at a predetermined concentration. Here, as an example, it is assumed that the semiconductor layer  120  is formed by thinning a p-type silicon substrate. 
     The semiconductor layer  120  includes n-type semiconductor regions  126  and  128  and p-type semiconductor regions  130 ,  132 ,  134  and  138 . The n-type semiconductor region  126  is disposed on the side of the first surface  122  of the semiconductor layer  120  in a cross-sectional view, and at least a part of the n-type semiconductor region  126  reaches the first surface  122  of the semiconductor layer  120 . The p-type semiconductor region  130  is disposed closer to the second surface  124  of the semiconductor layer  120  than the n-type semiconductor region  126 , and forms a p-n junction with the n-type semiconductor region  126 . The p-type semiconductor region  134  is disposed on the side of the second surface  124  of the semiconductor layer  120  in the cross-sectional view. The n-type semiconductor region  128  is disposed in a region between the p-type semiconductor region  130  and the p-type semiconductor region  134  so as to be separated from the p-type semiconductor region  134 . The p-type semiconductor region  138  is disposed in a region between the n-type semiconductor region  128  and the p-type semiconductor region  134 . That is, in the semiconductor layer  120 , the n-type semiconductor region  126 , the p-type semiconductor region  130 , the n-type semiconductor region  128 , the p-type semiconductor region  138 , and the p-type semiconductor region  134  are arranged in this order along the depth direction from the first surface  122  to the second surface  124 . 
     In a plan view, the p-type semiconductor region  134  overlaps the entire region where the n-type semiconductor regions  126  and  128  and the p-type semiconductor regions  130  and  138  are arranged. The p-type semiconductor region  132  is disposed so as to surround each of the regions in which the n-type semiconductor regions  126  and  128  and the p-type semiconductor regions  130  and  138  are provided in the plan view. The p-type semiconductor region  132  is disposed from the first surface  122  of the semiconductor layer  120  to a depth at which the p-type semiconductor region  134  is disposed, and is electrically connected to the p-type semiconductor region  134 . A region in the semiconductor layer  120  surrounded by the p-type semiconductor regions  132  and  134  is a well region (semiconductor region  136 ) in which the n-type semiconductor regions  126  and  128  and the p-type semiconductor regions  130  and  138  of one photoelectric conversion element  22  are arranged. Note that in the present embodiment, the conductivity type of the semiconductor region  136  is a p-type. 
     In the plan view, the n-type semiconductor region  128  is disposed inside the region defined by the p-type semiconductor region  132 . In the plan view, the n-type semiconductor region  126  is disposed in a region inside the n-type semiconductor region  128 . In the plan view, the p-type semiconductor region  130  is disposed in a region inside the n-type semiconductor region  126 . In the plan view, the p-type semiconductor region  138  is disposed in a region inside the n-type semiconductor region  128 . That is, the area of the p-type semiconductor region  138  in the plan view is smaller than the area of the n-type semiconductor region  128  in the plan view. The p-type semiconductor region  138  is positioned at a center portion of the photoelectric conversion element  22  in the plan view. Here, the center portion of the photoelectric conversion element  22  in the plan view may be a portion where an extraction portion of the signal charge from the photoelectric conversion element  22  (a portion where the n-type semiconductor region  126  and a cathode electrode  158  overlap each other in the plan view) is located in a plan view. 
     The semiconductor region  136  is disposed so as to surround the n-type semiconductor region  128  at a depth at which the n-type semiconductor region  128  is provided. The semiconductor region  136  is disposed between the n-type semiconductor region  128  and the p-type semiconductor region  132  at a depth at which the n-type semiconductor region  128  is provided. The semiconductor region  136  is disposed between the n-type semiconductor region  126  and the p-type semiconductor region  130 , and the p-type semiconductor region  132  at a depth where the n-type semiconductor region  126  and the p-type semiconductor region  130  are provided. The semiconductor region  136  extends in a region between the n-type semiconductor region  128  and the p-type semiconductor region  134  except a region where the p-type semiconductor region  138  is disposed. 
     The p-type semiconductor region  138  forms a p-n junction with the n-type semiconductor region  128 . The impurity concentration of the p-type semiconductor region  138  is desirably lower than the impurity concentration of the p-type semiconductor region  134 . In the cross-sectional view, an end of the p-type semiconductor region  138  on the side of the first surface  122  is positioned closer to the first surface  122  than an end of the n-type semiconductor region  128  on the side of the second surface  124 . 
     In the present embodiment, one photoelectric conversion element  22  includes the n-type semiconductor regions  126  and  128  and the p-type semiconductor regions  130 ,  132 ,  134  and  138 . The photoelectric conversion elements  22  arranged next to each other are electrically isolated from each other by the p-type semiconductor regions  132  and  134 . That is, the p-type semiconductor regions  132  and  134  form an isolation portion that electrically isolates the photoelectric conversion elements  22  from each other. A depletion layer formed in the p-n junction between the n-type semiconductor region  126  and the p-type semiconductor region  130  becomes an avalanche multiplication region. The n-type semiconductor region  128  serves to quickly collect charges generated in the semiconductor layer  120  in the avalanche multiplication region. The impurity concentration of the n-type semiconductor region  128  is lower than the impurity concentration of the n-type semiconductor region  126 . 
     In  FIG.  7   , although the n-type semiconductor region  128  is provided in contact with the p-type semiconductor region  130 , the n-type semiconductor region  128  may be provided apart from the p-type semiconductor region  130 . Although the p-type semiconductor region  134  is provided in contact with the second surface  124 , the p-type semiconductor region  134  may be provided apart from the second surface  124 . In the configuration example illustrated in  FIG.  6    and  FIG.  7   , although the n-type semiconductor region  128  is provided apart from the p-type semiconductor region  132 , the n-type semiconductor region  128  may be provided in contact with the p-type semiconductor region  132 . 
     The interconnection structure layer  150  includes an insulating layer  152  and an interconnection layer  154  disposed in the insulating layer  152 . The interconnection layer  154  includes an anode electrode (not illustrated) electrically connected to the p-type semiconductor region  132 , a cathode electrode  158  electrically connected to the n-type semiconductor region  126 , and a pad electrode  160  formed of an interconnection layer most distant from the semiconductor layer  120 . As illustrated in  FIG.  6   , the cathode electrode  158  is disposed at the center portion of the n-type semiconductor region  126  in the plan view. A plurality of cathode electrodes  158  may be disposed for one n-type semiconductor region  126 . 
     The circuit substrate  180  is stacked on the side of the interconnection structure layer  150  of the sensor substrate  110 . A bonding surface  170  in  FIG.  7    is a bonding portion between the sensor substrate  110  and the circuit substrate  180 . The circuit substrate  180  includes a semiconductor layer provided with elements such as transistors and an interconnection structure layer provided over the semiconductor layer.  FIG.  7    illustrates only pad electrodes  182  formed of the uppermost interconnection layer and a part of the interconnection layer  184  connected to the pad electrode  182  among the semiconductor layer and the interconnection structure layer constituting the circuit substrate  180  for simplification of the drawing. The sensor substrate  110  and the circuit substrate  180  may be bonded to each other by, for example, metal bonding between a metal member constituting the pad electrodes  160  and a metal member constituting the pad electrodes  182 . 
     For example, as illustrated in  FIG.  7   , the optical structure layer  190  may include a pinning film  192 , a planarization layer  194 , and a microlens layer including a plurality of microlenses  196 . The optical structure layer  190  may further include a filter layer (not illustrated). Various optical filters such as a color filter, an infrared light cut filter, and a monochrome filter may be applied to the filter layer. Instead of providing the p-type semiconductor region  134  in the semiconductor layer  120 , the pinning film  192  may be provided so as to be in contact with the p-type semiconductor region  132 . A known material may be applied to the pinning film  192 . The photoelectric conversion element  22  does not necessarily have to include the optical structure layer  190 . The optical structure layer  190  may include only a part of the above-described elements, or may further include other elements. 
     The photoelectric conversion element  22  generates an avalanche multiplication by a reverse bias voltage applied between the n-type semiconductor region  128  and the p-type semiconductor region  130  via the anode electrode (not illustrated) and the cathode electrode  158  using charges generated in the semiconductor layer  120  by photoelectric conversion as causes. The carriers generated by the avalanche multiplication are output to the outside of the photoelectric conversion element  22  via the cathode electrode  158 . Therefore, the response speed of the photoelectric conversion element  22  is improved as the charges generated in the semiconductor layer  120  may be collected in the avalanche multiplication region more quickly. 
     In this respect, in the photoelectric conversion element  22  according to the present embodiment, as described above, the n-type semiconductor region  128  whose impurity concentration is lower than that of the n-type semiconductor region  126  is disposed between the p-type semiconductor region  130  and the p-type semiconductor region  134 . Therefore, the potential of the n-type semiconductor region  128  becomes lower than the potential of the semiconductor region  136  for the signal charge, and it becomes possible to collect more signal charges in the avalanche multiplication region in a shorter time. 
     In the photoelectric conversion element  22  according to the present embodiment, the p-type semiconductor region  138  is further provided between the n-type semiconductor region  128  and the p-type semiconductor region  134 . The effect of providing the p-type semiconductor region  138  will be described below with reference to  FIG.  8 A  to  FIG.  12 B . 
       FIG.  8 A ,  FIG.  8 B ,  FIG.  10 A , and  FIG.  10 B  are diagrams illustrating a potential distribution and a charge transfer path inside the photoelectric conversion element  22 .  FIG.  9 A ,  FIG.  9 B ,  FIG.  11 A  and  FIG.  11 B  illustrate frequency distributions (probability distributions) of time required for charges generated in the photoelectric conversion element  22  to reach the avalanche multiplication region.  FIG.  8 A ,  FIG.  8 B ,  FIG.  9 A  and  FIG.  9 B  illustrate the case of a photoelectric conversion element in which the p-type semiconductor region  138  is not provided (reference example), and  FIG.  10 A ,  FIG.  10 B ,  FIG.  11 A  and  FIG.  11 B  illustrate the case of the photoelectric conversion element according to the present embodiment that includes the p-type semiconductor region  138 . 
       FIG.  8 A  is a schematic cross-sectional view illustrating a photoelectric conversion element of a reference example. The photoelectric conversion element of the reference example has a structure similar to that of the photoelectric conversion element  22  according to the present embodiment except that the p-type semiconductor region  138  is not provided.  FIG.  8 B  is a diagram illustrating a result obtained by simulation of a potential distribution in a region enclosed by a broken line in  FIG.  8 A . 
     In the photoelectric conversion element of the reference example, it is assumed that charge e c  and charge e d  are generated by incidence of light in the semiconductor region  136  on the side of the p-type semiconductor region  134  with respect to the n-type semiconductor region  128 . Here, the charge e c  is a charge generated in the center portion (region C) of the photoelectric conversion element in a plan view, and the charge e d  is a charge generated in the peripheral portion (region D) of the photoelectric conversion element in the plan view. 
     When the photoelectric conversion element is driven, a predetermined reverse bias voltage is applied between the p-type semiconductor regions  132  and  134  and the n-type semiconductor region  126 . Thereby, a potential distribution as illustrated in  FIG.  8 B  is formed in the n-type semiconductor region  128  and the semiconductor region  136 . The potential gradient formed in the n-type semiconductor region  128  and the semiconductor region  136  in this manner is lower in the direction from the peripheral portion of the semiconductor region  136  toward the avalanche multiplication region  140  with respect to the signal charges. Therefore, each of the charge e c  generated in the region C and the charge e d  generated in the region D is drawn toward the avalanche multiplication region  140  along, for example, a path indicated by a broken arrow in  FIG.  8 A  and  FIG.  8 B . 
       FIG.  9 A  and  FIG.  9 B  illustrate the frequency distribution (probability distribution) of the time required for the charge e c  generated in the region C and the charge e d  generated in the region D to reach the avalanche multiplication region  140 .  FIG.  9 A  separately illustrates a frequency distribution of time required for the charge e c  generated in the region C to reach the avalanche multiplication region  140  and a frequency distribution of time required for the charge e d  generated in the region D to reach the avalanche multiplication region  140 .  FIG.  9 B  illustrates one frequency distribution of the time required for the charge e c  generated in the region C and the charge e d  generated in the region D to reach the avalanche multiplication region  140 . In  FIG.  9 A , T3 is an average value of the time required for the charge e c  to reach the avalanche multiplication region  140 , and T4 is an average value of the time required for the charge e d  to reach the avalanche multiplication region  140 . The spread in the horizontal axis direction in the graphs of  FIG.  9 A  and  FIG.  9 B  represents a variation in time required for charges to reach the avalanche multiplication region  140 .  FIG.  9 A  and  FIG.  9 B  illustrate the full width at half maximum (FWHM) as an index of the time variation. The cause of the time variation is, for example, lattice scattering or ionized impurity scattering. Here, it is assumed that the time variation (the full width at half maximum FWHM (e c )) until the charge e c  reaches the avalanche multiplication region  140  and the time variation (the full width at half maximum FWHM (e d )) until the charge e d  reaches the avalanche multiplication region  140  are equivalent to each other. 
     As indicated by the broken arrows in  FIG.  8 A  and  FIG.  8 B , under the potential distribution of  FIG.  8 B , the movement distance until the charge e d  reaches the avalanche multiplication region  140  from the region D is longer than the movement distance until the charge e c  reaches the avalanche multiplication region  140  from the region C. In other words, the time T4 required for the charge e d  to reach the avalanche multiplication region  140  is longer than the time T3 required for the charge e c  to reach the avalanche multiplication region  140 . Therefore, the frequency distribution of the time required for the charge e d  to reach the avalanche multiplication region  140  is shifted in a longer time direction than the frequency distribution of the time required for the charge e c  to reach the avalanche multiplication region  140 . 
     Therefore, when the time required for the charge e c  generated in the region C and the charge e d  generated in the region D to reach the avalanche multiplication region  140  is represented by one frequency distribution, the full width at half maximum FWHM (e c +e d ) of the frequency distribution widens as illustrated in  FIG.  9 B . As described above, when there is a time difference between the time T3 and the time T4, the full width at half maximum FWHM (e c +e d ) in the frequency distribution after addition is larger than the full width at half maximum FWHM (e c ) and FWHM (e d ) in each frequency distribution before addition. Further, as the time difference between the time T3 and the time T4 increases, the full width at half maximum FWHM (e c +e d ) in the frequency distribution after addition increases. The time variation represented by the full width at half maximum FWHM (e c +e d ) of the frequency distribution is one of the factors relating to the response performance of the sensor, e.g., time resolution in ranging applications, and is desired to be smaller. 
       FIG.  10 A  is a schematic cross-sectional view illustrating the photoelectric conversion element  22  according to the present embodiment. As described above, the photoelectric conversion element  22  according to the present embodiment further includes the p-type semiconductor region  138  in comparison with the photoelectric conversion element of the reference example.  FIG.  10 B  is a diagram illustrating a result obtained by simulation of a potential distribution in a region enclosed by a broken line in  FIG.  10 A . 
     In the photoelectric conversion element  22  according to the present embodiment, a region A and a region B corresponding to the region C and the region D assumed in the photoelectric conversion element of the reference example are assumed. Then, it is assumed that charge e a  is generated in the region A and charge e b  is generated in the region B by incidence of light. In the photoelectric conversion element  22  according to the present embodiment, the region A where the charge e a  is generated overlaps with the region where the p-type semiconductor region  138  is provided. 
     When the photoelectric conversion element  22  is driven, a predetermined reverse bias voltage is applied between the p-type semiconductor regions  132  and  134  and the n-type semiconductor region  126 . Thereby, a potential distribution as illustrated in  FIG.  10 B  is formed in the n-type semiconductor region  128  and the semiconductor region  136 . The potential gradient formed in the n-type semiconductor region  128  and the semiconductor region  136  in this manner is lower in the direction from the peripheral portion of the semiconductor region  136  toward the avalanche multiplication region  140  with respect to the signal charges as a whole. 
     However, in the photoelectric conversion element  22  according to the present embodiment, at the same depth where the n-type semiconductor region  128  and the p-type semiconductor region  138  are arranged, the potential of the p-type semiconductor region  138  with respect to the signal charge is higher than the potential of the n-type semiconductor region  128  with respect to the signal charge. In other words, in the photoelectric conversion element  22  according to the present embodiment, since the p-type semiconductor region  138  is provided, the potential gradient of this portion is smoother than the potential gradient of the corresponding portion in the photoelectric conversion element of the reference example. Since the signal charge is guided to a portion where the potential gradient is steeper, the charge e a  generated in the region A is drawn in the direction of the avalanche multiplication region  140  by bypassing the center portion of the gentle potential gradient and passing through the peripheral portion, for example, as indicated by the broken arrows in  FIG.  10 A  and  FIG.  10 B . That is, in the photoelectric conversion element  22  according to the present embodiment, the time required for the charge e a  generated in the region A to reach the avalanche multiplication region  140  is longer than the time required for the charge e c  generated in the region C to reach the avalanche multiplication region  140  in the photoelectric conversion element of the reference example. 
     As a result, as illustrated in  FIG.  11 A , the difference between the time T1 required for the charge e a  to reach the avalanche multiplication region  140  and the time T2 required for the charge e b  to reach the avalanche multiplication region  140  is smaller than the difference between the time T3 and the time T4. As illustrated in  FIG.  11 B , the full width at half maximum FWHM (e a +e b ) when the time required for the charges e a  and e b  to reach the avalanche multiplication region  140  is represented by one frequency distribution is narrower than the full width at half maximum FWHM (e c +e d ). 
     Therefore, according to the photoelectric conversion element  22  according to the present embodiment, as compared with the photoelectric conversion element of the reference example, it is possible to reduce variation in time required for charges generated according to incident light to reach the avalanche multiplication region  140 . 
     As described above, the photoelectric conversion element  22  according to the present embodiment may include the microlens  196 . When the photoelectric conversion element  22  includes the microlens  196 , the incident light is focused on the center portion of the photoelectric conversion element  22  by the microlens  196 . Although the description of  FIG.  8 A  to  FIG.  11 B  is based on the assumption that the frequency of generation of charges in the center portion and the peripheral portion of the photoelectric conversion element  22  is the same, the configuration of the present embodiment is also effective in the case where the photoelectric conversion element  22  includes the microlens  196 . 
       FIG.  12 A  and  FIG.  12 B  are frequency distributions (probability distributions) of time required for charges generated in the photoelectric conversion element to reach the avalanche multiplication region when the photoelectric conversion element includes the microlens.  FIG.  12 A  illustrates the case of the photoelectric conversion element according to the reference example, and  FIG.  12 B  illustrates the case of the photoelectric conversion element  22  according to the present embodiment. 
       FIG.  12 A  illustrates a frequency distribution (a thin solid line) of the time required for the charge e c  generated in the region C to reach the avalanche multiplication region  140 , and a frequency distribution (a broken line) of the time required for the charge e d  generated in the region D to reach the avalanche multiplication region  140 .  FIG.  12 A  also illustrates a frequency distribution (thick solid line) obtained by summing these. In  FIG.  12 A , it is assumed that the ratio of the generation frequency of the charge e c  in the region C to the generation frequency of the charge e d  in the region D is 8:2, and the full width at half maximum FWHM (e c ) and the full width at half maximum FWHM (e d ) are equal to each other.  FIG.  12 B  illustrates a frequency distribution (a thin solid line) of the time required for the charge e a  generated in the region A to reach the avalanche multiplication region  140  and a frequency distribution (a broken line) of the time required for the charge e b  generated in the region B to reach the avalanche multiplication region  140 .  FIG.  12 B  also illustrates a frequency distribution (thick solid line) obtained by summing these. In  FIG.  12 B , it is assumed that the ratio of the frequency of generation of the charge e a  in the region A to the frequency of generation of the charge e b  in the region D is 8:2, and the full width at half maximum FWHM (e a ) and the full width at half maximum FWHM (e b ) are equal to each other. 
     In the case of the photoelectric conversion element according to the reference example, between the peak position of the frequency distribution of the charge e c  and the peak position of the frequency distribution of the charge e d , as described above, there is a time difference (T4-T3) corresponding to the difference in the moving distance of the signal charges to the avalanche multiplication region  140 . Therefore, the frequency distribution (e c +e d ) obtained by summing the frequency distribution of the charge e c  and the frequency distribution of the charge e d  becomes a broader distribution than the frequency distribution of the charge e c  by a time difference corresponding to the difference in the moving distance between the charge e c  and the charge e d , as illustrated by a thick solid line in  FIG.  12 A . 
     On the other hand, in the case of the photoelectric conversion element  22  according to the present embodiment, the difference between the peak position of the frequency distribution of the charge e a  and the peak position of the frequency distribution of the charge e b  is smaller than that in the case of the photoelectric conversion element of the reference example as described above. Therefore, the full width at half maximum FWHM (e a +e b ) of the frequency distribution (e a +e b ) obtained by adding the frequency distribution of the charge e a  and the frequency distribution of the charge e b  becomes narrower than the full width at half maximum FWHM (e c +e d ) of the frequency distribution (e c +e d ). 
     Therefore, in the photoelectric conversion element  22  according to the present embodiment, even in the case where the photoelectric conversion element  22  includes the microlens  196 , it is possible to reduce variation in time required for charges generated according to incident light to reach the avalanche multiplication region. 
     As described above, according to the present embodiment, it is possible to realize a photoelectric conversion element and a photoelectric conversion device excellent in high-speed response. 
     Second Embodiment 
     A photoelectric conversion device according to a second embodiment of the present invention will be described with reference to  FIG.  13   . Components similar to those of the photoelectric conversion device according to the first embodiment are denoted by the same reference numerals, and description thereof will be omitted or simplified.  FIG.  13    is a schematic cross-sectional view illustrating a configuration example of a photoelectric conversion element in the photoelectric conversion device according to the present embodiment. 
     The photoelectric conversion device according to the present embodiment is the same as the photoelectric conversion device according to the first embodiment except that the configuration of the photoelectric conversion element  22  is different. In the present embodiment, portions of the photoelectric conversion element  22  according to the present embodiment which are different from the photoelectric conversion element  22  of the first embodiment will be mainly described, and a description of portions common to the photoelectric conversion element  22  according to the first embodiment will be appropriately omitted. 
     Also in the photoelectric conversion device according to the present embodiment, one photoelectric conversion element  22  includes n-type semiconductor regions  126  and  128 , p-type semiconductor regions  130 ,  132 ,  134  and  138 , and a semiconductor region  136 . Note that in the present embodiment, the conductivity type of the semiconductor region  136  is n-type. As illustrated in  FIG.  13   , the photoelectric conversion element  22  according to the present embodiment differs from the photoelectric conversion element  22  according to the first embodiment in the configurations of the n-type semiconductor region  126  and the p-type semiconductor region  130 . That is, in the photoelectric conversion element  22  according to the present embodiment, the area of the n-type semiconductor region  126  in the plan view is smaller than the area of the p-type semiconductor region  130  in the plan view, and the n-type semiconductor region  126  is disposed at the center of the p-type semiconductor region  130  in the plan view. The outer peripheral portion of the p-type semiconductor region  130  in the plan view is in contact with the p-type semiconductor region  132 . 
     By configuring the photoelectric conversion element  22  in this manner, more charges may be collected in the avalanche multiplication region at the center portion where the n-type semiconductor region  126  and the p-type semiconductor region  130  are in contact with each other, and the sensitivity may be improved as compared with the photoelectric conversion element  22  according to the first embodiment. According to this configuration, since the avalanche multiplication region may be reduced, it is possible to reduce noise caused by the strong electric field. 
     On the other hand, in the photoelectric conversion element  22  according to the present embodiment, as the avalanche multiplication region becomes smaller, a difference is likely to occur between the time until the charge generated in the center portion reaches the avalanche multiplication region and the time until the charge generated in the outer peripheral portion reaches the avalanche multiplication region. However, in the photoelectric conversion element  22  according to the present embodiment, similarly to the photoelectric conversion element  22  according to the first embodiment, since the p-type semiconductor region  138  is disposed in the center portion, charges may be propagated to the avalanche multiplication region so as to pass through the peripheral portion by bypassing the center portion. This makes it possible to reduce the time variation between the time required for the charge generated in the center portion to reach the avalanche multiplication region and the time required for the charge generated in the peripheral portion to reach the avalanche multiplication region to approximately the same level as the photoelectric conversion element  22  of the first embodiment. 
     That is, according to the configuration of the present embodiment, it is possible to reduce variation in time required for charges generated in response to incidence of light to reach the avalanche multiplication region while improving sensitivity and reducing noise with respect to the photoelectric conversion element according to the first embodiment. 
     As described above, according to the present embodiment, it is possible to realize a photoelectric conversion element and a photoelectric conversion device excellent in high-speed response. 
     Third Embodiment 
     A photoelectric conversion device according to a third embodiment of the present invention will be described with reference to  FIG.  14   . Components similar to those of the photoelectric conversion devices according to the first and second embodiments are denoted by the same reference numerals, and description thereof will be omitted or simplified.  FIG.  14    is a plan view illustrating a configuration example of a photoelectric conversion element in the photoelectric conversion device according to the present embodiment. 
     The photoelectric conversion device according to the present embodiment is the same as the photoelectric conversion device according to the first embodiment except that the configuration of the photoelectric conversion element  22  is different. In the present embodiment, portions of the photoelectric conversion element  22  according to the present embodiment which are different from the photoelectric conversion element  22  according to the first embodiment will be mainly described, and a description of portions common to the photoelectric conversion element  22  according to the first embodiment will be appropriately omitted. 
     The photoelectric conversion element  22  according to the present embodiment differs from the photoelectric conversion element  22  according to the first embodiment in that a plurality of p-type semiconductor regions  138  is provided as illustrated in  FIG.  14   . That is, in the photoelectric conversion element  22  according to the first embodiment, one p-type semiconductor region  138  is disposed at the center portion of the photoelectric conversion element  22  in the plan view. On the other hand, in the photoelectric conversion element  22  according to the present embodiment, four p-type semiconductor regions  138  arranged in two diagonal directions are further arranged so as to sandwich the p-type semiconductor regions  138  arranged in the center portion in the plan view. The four p-type semiconductor regions  138  are disposed between the p-type semiconductor region  138  and the p-type semiconductor region  132  at the center in the plan view. The area of the p-type semiconductor regions  138  arranged in the four corners in the plan view is smaller than the area of the p-type semiconductor region  138   arranged in the center portion in the plan view. 
     By disposing the p-type semiconductor regions  138  also in portions other than the center portion in the plan view, it is possible to more appropriately control the time required for the signal charges generated in each portion of the photoelectric conversion element  22  to reach the avalanche multiplication region. For example, by reducing the area and density of the p-type semiconductor regions  138  in the plan view from the center portion toward the peripheral portion, the bypass path may be made longer as the signal charge generated near the center portion becomes larger. In particular, when the arrangement interval (pitch) of the photoelectric conversion elements  22  (or the pixels  12 ) is larger than the thickness of the semiconductor layer  120 , it is effective to arrange a plurality of p-type semiconductor regions  138  or to increase the area of the p-type semiconductor region  138  at the center portion. 
     The arrangement of the p-type semiconductor regions  138  in the plan view is preferably appropriately selected according to the size, shape, and the like of the photoelectric conversion element  22  so that the variation in the time required for the signal charge generated in each portion of the photoelectric conversion element  22  to reach the avalanche multiplication region is reduced. 
     As described above, according to the present embodiment, it is possible to realize a photoelectric conversion element and a photoelectric conversion device excellent in high-speed response. 
     Fourth Embodiment 
     A photoelectric conversion device according to a fourth embodiment of the present invention will be described with reference to  FIG.  15   . Components similar to those of the photoelectric conversion devices according to the first to third embodiments are denoted by the same reference numerals, and description thereof will be omitted or simplified.  FIG.  15    is a schematic cross-sectional view illustrating a configuration example of a photoelectric conversion element in the photoelectric conversion device according to the present embodiment. 
     The photoelectric conversion device according to the present embodiment is the same as the photoelectric conversion device according to the first embodiment except that the configuration of the photoelectric conversion element  22  is different. In the present embodiment, portions of the photoelectric conversion element  22  according to the present embodiment which are different from the photoelectric conversion element  22  according to the first embodiment will be mainly described, and a description of portions common to the photoelectric conversion element  22  of the first embodiment will be appropriately omitted. 
     As illustrated in  FIG.  15   , the photoelectric conversion element  22  according to the present embodiment differs from the photoelectric conversion element  22  according to the first embodiment in the arrangement of the p-type semiconductor region  138  in the semiconductor layer  120 . That is, in the photoelectric conversion element  22  according to the first embodiment, the p-type semiconductor region  138  is disposed such that the long side direction in the cross-sectional view is parallel to the depth direction. On the other hand, in the photoelectric conversion element  22  according to the present embodiment, the p-type semiconductor region  138  is disposed such that the long side direction in the cross-sectional view is perpendicular to the depth direction. 
     Depending on the size and shape of the photoelectric conversion element  22 , by disposing the p-type semiconductor region  138  such that the long side direction in the cross-sectional view is perpendicular to the depth direction, the propagation path of charges generated in the center portion may be made longer. In such a case, by applying the configuration of the present embodiment, it is possible to further reduce the variation in time until the signal charges generated in the respective portions of the photoelectric conversion element  22  reach the avalanche multiplication region. 
     Similarly to the planar layout of the p-type semiconductor regions  138  described in the third embodiment, the cross-sectional shape of the p-type semiconductor region  138  is preferably appropriately selected according to the size, shape, and the like of the photoelectric conversion element  22 . When a plurality of p-type semiconductor regions  138  are arranged as described in the third embodiment, the plurality of p-type semiconductor regions  138  may have different cross-sectional shapes. The p-type semiconductor region  138  is not necessarily in contact with the p-type semiconductor region  134 , and may be isolated from the p-type semiconductor region  134  as illustrated in  FIG.  15   . 
     As described above, according to the present embodiment, it is possible to realize a photoelectric conversion element and a photoelectric conversion device excellent in high-speed response. 
     Fifth Embodiment 
     A photoelectric conversion device according to a fifth embodiment of the present invention will be described with reference to  FIG.  16    to  FIG.  19   . Components similar to those of the photoelectric conversion devices according to the first to fourth embodiments are denoted by the same reference numerals, and description thereof will be omitted or simplified.  FIG.  16    and  FIG.  17    are plan views illustrating a configuration example of a photoelectric conversion elements in the photoelectric conversion device according to the present embodiment.  FIG.  18    is a schematic cross-sectional view illustrating the configuration example of the photoelectric conversion element in the photoelectric conversion device according to the present embodiment.  FIG.  19    is a schematic cross-sectional view illustrating a configuration example of a photoelectric conversion element in a photoelectric conversion device according to a modified example of the present embodiment. 
     The photoelectric conversion device according to the present embodiment is the same as the photoelectric conversion device according to the first embodiment except that the configuration of the photoelectric conversion element  22  is different. In the present embodiment, portions of the photoelectric conversion element  22  according to the present embodiment which are different from the photoelectric conversion element  22  according to the first embodiment will be mainly described, and a description of portions common to the photoelectric conversion element  22  according to the first embodiment will be appropriately omitted. 
       FIG.  16    and  FIG.  17    illustrate two photoelectric conversion elements  22  arranged adjacent to each other among the plurality of pixels  12  constituting the pixel region  10 .  FIG.  16    is a plan view of the semiconductor layer  120  viewed from the side of the first surface  122 , and  FIG.  17    is a plan view of the semiconductor layer  120  viewed from the side of the second surface  124 .  FIG.  18    is a cross-sectional view taken along the line A-A′ of  FIG.  16    and  FIG.  17   . Broken lines illustrated in  FIG.  16    to  FIG.  18    indicate boundaries between adjacent pixels  12  (photoelectric conversion elements  22 ). 
     As illustrated in  FIG.  16    to  FIG.  18   , the photoelectric conversion element  22  according to the present embodiment is different from the photoelectric conversion element  22  according to the first embodiment in that the photoelectric conversion element  22  further includes an isolation structure  142  and a concave-convex structure  144 . 
     The isolation structure  142  is provided in a region between the pixel  12  (photoelectric conversion element  22 ) and the pixel  12  (photoelectric conversion element  22 ) in the plan view so as to extend from the first surface  122  to the second surface  124  of the semiconductor layer  120 . For example, the isolation structure  142  may be provided inside a region where the p-type semiconductor region  132  is disposed, as illustrated in  FIG.  16    and  FIG.  17   . The isolation structure  142  has a role of preventing light from leaking into the adjacent photoelectric conversion elements  22 , and is preferably a wallshaped body surrounding each of the regions where the photoelectric conversion element  22  is arranged. The isolation structure  142  may be formed, for example, by filling an insulating member or a metal member in a groove formed in the semiconductor layer  120 . Although the isolation structure  142  is provided so as to extend from the first surface  122  to the second surface  124  of the semiconductor layer  120  in the configuration examples of  FIG.  16    and  FIG.  17   , the isolation structure  142  may not necessarily extend from the first surface  122  to the second surface  124 . 
     The concave-convex structure  144  is provided on the second surface  124  of the semiconductor layer  120 .  FIG.  17    and  FIG.  18    illustrate an example in which a lattice-shaped groove is formed on the second surface  124  of the semiconductor layer  120  as an example of the concave-convex structure  144 . However, the concave-convex structure  144  has a role of scattering light incident from the side of the second surface  124  of the semiconductor layer  120 , and the pattern of the concave-convex structure  144  is not particularly limited as long as the concave-convex structure  144  has a function of scattering light incident from the side of the second surface  124 . The concave-convex structure  144  may be formed, for example, by filling an insulating member in a groove formed on the second surface  124  of the semiconductor layer  120 . 
     By providing the isolation structure  142  and the concave-convex structure  144  in the semiconductor layer  120 , light incident on the semiconductor layer  120  is scattered by the concave-convex structure  144  and confined in one pixel by the isolation structure  142 . Thereby, the effective optical path length is extended, and the sensitivity may be improved. On the other hand, in comparison with the configuration in which light is condensed to the center of the pixel by the microlens  196 , in the present embodiment, the probability of generation of charges in the vicinity of the boundary portion of the pixel  12  is relatively high. The increased probability of generation of charges in the vicinity of the boundary portion of the pixel  12  is likely to cause a time difference until the charges reach the avalanche multiplication region  140  with respect to charges generated in the vicinity of the center portion of the pixel  12 . 
     However, also in the configuration of the present embodiment, by providing the p-type semiconductor region  138 , charges may be propagated to the avalanche multiplication region  140  so as to pass through the peripheral portion by bypassing the center portion. Thus, the time variation until the charge reaches the avalanche multiplication region  140  may be reduced to the same level as in the first embodiment. 
     That is, according to the configuration of the present embodiment, the sensitivity may be improved with respect to the photoelectric conversion element according to the first embodiment, and variation in time required for charges generated in response to light incidence to reach the avalanche multiplication region may be reduced. 
       FIG.  19    is a schematic cross-sectional view illustrating a structure of a photoelectric conversion element according to a modified example of the present embodiment. In this modified example, a part of the concave-convex structure  144  (the concave-convex structure  146 ) is arranged to extend to the p-type semiconductor region  138 . By arranging the concave-convex structures  144  and  146  in this manner, it is possible to reduce variation in time until the signal charges reach the avalanche multiplication region, similarly to the above description. 
     As described above, according to the present embodiment, it is possible to realize a photoelectric conversion element and a photoelectric conversion device excellent in high-speed response. 
     Sixth Embodiment 
     A photodetection system according to a sixth embodiment of the present invention will be described with reference to  FIG.  20   .  FIG.  20    is a block diagram illustrating a schematic configuration of the photodetection system according to the present embodiment. In the present embodiment, a light detection sensor to which the photoelectric conversion device  100  described in any one of the first to fifth embodiments is applied will be described. 
     The photoelectric conversion device  100  described in the first to fifth embodiments may be applied to various photodetection systems. Examples of applicable photodetection systems include imaging systems such as digital still cameras, digital camcorders, surveillance cameras, copying machines, facsimiles, mobile phones, on-vehicle cameras, observation satellites, and the like. A camera module including an optical system such as a lens and an imaging device is also included in the photodetection system.  FIG.  20    is a block diagram of a digital still camera as an example of these. 
     The photodetection system  200  illustrated in  FIG.  20    includes a photoelectric conversion device  201 , a lens  202  for forming an optical image of an object on the photoelectric conversion device  201 , an aperture  204  for varying the amount of light passing through the lens  202 , and a barrier  206  for protecting the lens  202 . The lens  202  and the aperture  204  are optical systems for focusing light on the photoelectric conversion device  201 . The photoelectric conversion device  201  is the photoelectric conversion device  100  described in any of the first to fifth embodiments, and converts an optical image formed by the lens  202  into image data. 
     The photodetection system  200  also includes a signal processing unit  208  that processes an output signal output from the photoelectric conversion device  201 . The signal processing unit  208  generates image data from the digital signal output from the photoelectric conversion device  201 . The signal processing unit  208  performs various corrections and compressions as necessary to output image data. The photoelectric conversion device  201  may include an AD conversion unit that generates a digital signal to be processed by the signal processing unit  208 . The AD conversion unit may be formed on a semiconductor layer (semiconductor substrate) on which the photoelectric conversion element of the photoelectric conversion device  201  is formed, or may be formed on a semiconductor substrate different from the semiconductor layer on which the photoelectric conversion element of the photoelectric conversion device  201  is formed. The signal processing unit  208  may be formed on the same semiconductor substrate as the photoelectric conversion device  201 . 
     The photodetection system  200  further includes a buffer memory unit  210  for temporarily storing image data, and an external interface unit (external I/F unit)  212  for communicating with an external computer or the like. Further, the photodetection system  200  includes a storage medium  214  such as a semiconductor memory for storing or reading out captured image data, and a storage medium control interface unit (storage medium control I/F unit)  216  for storing or reading out image data on or from the storage medium  214 . The storage medium  214  may be built in the photodetection system  200 , or may be detachable. Further, communication between the storage medium control I/F unit  216  and the storage medium  214  and communication from the external I/F unit  212  may be performed wirelessly. 
     Further, the photodetection system  200  includes a general control/operation unit  218  that controls various calculations and the entire digital still camera, and a timing generation unit  220  that outputs various timing signals to the photoelectric conversion device  201  and the signal processing unit  208 . Here, the timing signal or the like may be input from the outside, and the photodetection system  200  may include at least the photoelectric conversion device  201  and a signal processing unit  208  that processes an output signal output from the photoelectric conversion device  201 . The timing generation unit  220  may be mounted on the photoelectric conversion device  201 . Further, the general control/operation unit  218  and the timing generation unit  220  may be configured to implement some or all of the control functions of the photoelectric conversion device  201 . 
     The photoelectric conversion device  201  outputs an imaging signal to the signal processing unit  208 . The signal processing unit  208  performs predetermined signal processing on the imaging signal output from the photoelectric conversion device  201 , and outputs image data. The signal processing unit  208  generates an image using the imaging signal. The signal processing unit  208  may be configured to perform a distance measurement operation on a signal output from the photoelectric conversion device  201 . 
     As described above, according to the present embodiment, by configuring the photodetection system using the photoelectric conversion device according to the first to the fifth embodiments, it is possible to realize a photodetection system capable of obtaining a higher quality image. 
     Seventh Embodiment 
     A range image sensor according to a seventh embodiment of the present invention will be described with reference to  FIG.  21   .  FIG.  21    is a block diagram illustrating a schematic configuration of the range image sensor according to the present embodiment. In the present embodiment, a range image sensor will be described as an example of a photodetection system to which the photoelectric conversion device  100  described in any one of the first to fifth embodiments is applied. 
     As illustrated in  FIG.  21   , the range image sensor  300  according to the present embodiment may include an optical system  302 , a photoelectric conversion device  304 , an image processing circuit  306 , a monitor  308 , and a memory  310 . The range image sensor  300  receives light (modulated light or pulse light) emitted from the light source device  320  toward the object  330  and reflected by the surface of the object  330 , and acquires a distance image corresponding to the distance to the object  330 . 
     The optical system  302  includes one or a plurality of lenses, and has a role of forming an image of image light (incident light) from the object  330  on a light receiving surface (sensor unit) of the photoelectric conversion device  304 . 
     The photoelectric conversion device  304  is the photoelectric conversion device  100  described in any of the first to fifth embodiments, and has a function of generating a distance signal indicating the distance to the object  330  based on the image light from the object  330  and supplying the generated distance signal to the image processing circuit  306 . 
     The image processing circuit  306  has a function of performing image processing for constructing a distance image based on the distance signal supplied from the photoelectric conversion device  304 . 
     The monitor  308  has a function of displaying a distance image (image data) obtained by image processing in the image processing circuit  306 . The memory  310  has a function of storing (recording) a distance image (image data) obtained by image processing in the image processing circuit  306 . 
     As described above, according to the present embodiment, by configuring the range image sensor using the photoelectric conversion devices of the first to fifth embodiments, it is possible to realize a range image sensor capable of acquiring a distance image including more accurate distance information in conjunction with improvement in characteristics of the pixels  12 . 
     Eighth Embodiment 
     An endoscopic surgical system according to an eighth embodiment of the present invention will be described with reference to  FIG.  22   .  FIG.  22    is a schematic diagram illustrating a configuration example of the endoscopic surgical system according to the present embodiment. In the present embodiment, an endoscopic surgical system will be described as an example of a photodetection system to which the photoelectric conversion device  100  described in any one of the first to fifth embodiments is applied. 
       FIG.  22    illustrates a state in which an operator (surgeon)  460  performs an operation on a patient  472  on a patient bed  470  using an endoscopic surgical system  400 . 
     As illustrated in  FIG.  22   , the endoscopic surgical system  400  according to the present embodiment may include an endoscope  410 , a surgical tool  420 , and a cart  430  on which various devices for endoscopic surgery are mounted. The cart  430  may include a CCU (Camera Control Unit)  432 , a light source device  434 , an input device  436 , a processing tool control device  438 , a display device  440 , and the like. 
     The endoscope  410  includes a lens barrel  412  in which an area of a predetermined length from the tip is inserted into the body cavity of the patient  472 , and a camera head  414  connected to the base end of the lens barrel  412 . Although  FIG.  22    illustrates an endoscope  410  configured as a rigid mirror having a rigid lens barrel  412 , the endoscope  410  may be configured as a flexible mirror having a flexible lens barrel. The endoscope  410  is held in a movable state by an arm  416 . 
     An opening into which the objective lens is fitted is provided at the tip of the lens barrel  412 . A light source device  434  is connected to the endoscope  410 , and light generated by the light source device  434  is guided to the tip of the lens barrel  412  by a light guide extended inside the lens barrel  412 , and is irradiated to an observation target in the body cavity of the patient  472  via an objective lens. The endoscope  410  may be a direct-viewing mirror, an oblique-viewing mirror, or a side-viewing mirror. 
     An optical system and a photoelectric conversion device (not illustrated) are provided inside the camera head  414 , and reflected light (observation light) from the observation target is focused on the photoelectric conversion device by the optical system. The photoelectric conversion device photoelectrically converts the observation light and generates an electric signal corresponding to the observation light, i.e., an image signal corresponding to the observation image. As the photoelectric conversion device, the photoelectric conversion device  100  described in any of the first to fifth embodiments may be used. The image signal is transmitted to the CCU  432  as RAW data. 
     The CCU  432  is configured by a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), and the like, and integrally controls the operation of the endoscope  410  and the display device  440 . Further, the CCU  432  receives an image signal from the camera head  414 , and performs various types of image processing for displaying an image based on the image signal, such as development processing (demosaic processing), on the image signal. 
     The display device  440  displays an image based on the image signal subjected to the image processing by the CCU  432  under the control of the CCU  432 . 
     The light source device  434  is configured by, for example, a light source such as an LED (Light Emitting Diode), and supplies irradiation light to the endoscope  410  when capturing an image of a surgical part or the like. 
     Input device  436  is an input interface for the endoscopic surgical system  400 . The user may input various kinds of information and instructions to the endoscopic surgical system  400  via the input device  436 . 
     The processing tool control device  438  controls the actuation of the energy processing tool  450  for tissue ablation, incision, blood vessel sealing, etc. 
     The light source device  434  for supplying the irradiation light to the endoscope  410  when capturing an image of the surgical part may be composed of a white light source composed of, for example, an LED, a laser light source, or a combination thereof. When a white light source is constituted by a combination of RGB laser light sources, since the output intensity and output timing of each color (each wavelength) may be controlled with high accuracy, the white balance of the captured image may be adjusted in the light source device  434 . In this case, the observation object is irradiated with the laser light from each of the RGB laser light sources in a time division manner, and the driving of the imaging element of the camera head  414  is controlled in synchronization with the irradiation timing, whereby the images corresponding to the RGB light sources may be captured in a time division manner. According to this method, a color image may be obtained without providing a color filter in the imaging element. 
     Further, the driving of the light source device  434  may be controlled so as to change the intensity of the output light every predetermined time. By controlling the driving of the imaging element of the camera head  414  in synchronization with the timing of changing the intensity of the light to acquire images in a time-division manner and compositing the images, it is possible to generate an image in a high dynamic range without so-called blocked up shadows and blown out highlights. 
     The light source device  434  may be configured to be capable of supplying light in a predetermined wavelength band corresponding to the special light observation. In special light observation, for example, wavelength dependency of light absorption in body tissue is utilized. Specifically, a predetermined tissue such as a blood vessel in the surface layer of the mucosa is imaged with high contrast by irradiating light in a narrower band compared to the irradiation light (i.e., white light) during normal observation. Alternatively, in special light observation, fluorescence observation for obtaining an image by fluorescence generated by irradiation with excitation light may be performed. In the fluorescence observation, the body tissue may be irradiated with excitation light to observe fluorescence from the body tissue, or a reagent such as indocyanine green (ICG) may be locally poured into to the body tissue, and the body tissue may be irradiated with excitation light corresponding to the fluorescence wavelength of the reagent to obtain a fluorescence image. The light source device  434  may be configured to supply narrowband light and/or excitation light corresponding to such special light observation. 
     As described above, according to the present embodiment, by configuring the endoscopic surgical system using the photoelectric conversion devices of the first to fifth embodiments, it is possible to realize an endoscopic surgical system capable of acquiring images of better quality. 
     Ninth Embodiment 
     A photodetection system and a movable object according to a ninth embodiment of the present invention will be described with reference to  FIG.  23 A  to  FIG.  25   .  FIG.  23 A  to  FIG.  23 C  are schematic diagrams illustrating a configuration example of a movable object according to the present embodiment.  FIG.  24    is a block diagram illustrating a schematic configuration of a photodetection system according to the present embodiment.  FIG.  25    is a flowchart illustrating the operation of the photodetection system according to the present embodiment. In the present embodiment, an application example to an on-vehicle camera will be described as a photodetection system to which the photoelectric conversion device  100  described in any one of the first to fifth embodiments is applied. 
       FIG.  23 A  to  FIG.  23 C  are schematic diagrams illustrating a configuration example of a movable object (a vehicle system) according to the present embodiment.  FIG.  23 A  to  FIG.  23 C  illustrate a configuration of a vehicle  500  (an automobile) as an example of a vehicle system incorporating a photodetection system to which the photoelectric conversion device according to any one of the first to fifth embodiments is applied.  FIG.  23 A  is a schematic front view of the vehicle  500 ,  FIG.  23 B  is a schematic plan view of the vehicle  500 , and  FIG.  23 C  is a schematic rear view of the vehicle  500 . The vehicle  500  includes a pair of photoelectric conversion devices  502  on the front side thereof. Here, the photoelectric conversion devices  502  are the photoelectric conversion device  100  described in any of the first to fifth embodiments. The vehicle  500  includes an integrated circuit  503 , an alert device  512 , and a main control unit  513 . 
       FIG.  24    is a block diagram illustrating a configuration example of a photodetection system  501  mounted on the vehicle  500 . The photodetection system  501  includes a photoelectric conversion device  502 , an image preprocessing unit  515 , an integrated circuit  503 , and an optical system  514 . The photoelectric conversion device  502  is the photoelectric conversion device  100  described in any of the first to fifth embodiments. The optical system  514  forms an optical image of an object on the photoelectric conversion device  502 . The photoelectric conversion device  502  converts the optical image of the object formed by the optical system  514  into an electric signal. The image preprocessing unit  515  performs predetermined signal processing on the signal output from the photoelectric conversion device  502 . The function of the image preprocessing unit  515  may be incorporated in the photoelectric conversion device  502 . The photodetection system  501  is provided with at least two sets of the optical system  514 , the photoelectric conversion device  502 , and the image preprocessing unit  515 , and outputs from the image preprocessing units  515  of each set are input to the integrated circuit  503 . 
     The integrated circuit  503  is an integrated circuit for use in an imaging system, and includes an image processing unit  504 , an optical ranging unit  506 , a parallax calculation unit  507 , an object recognition unit  508 , and an abnormality detection unit  509 . The image processing unit  504  processes the image signal output from the image preprocessing unit  515 . For example, the image processing unit  504  performs image processing such as development processing and defect correction on the output signal of the image preprocessing unit  515 . The image processing unit  504  includes a memory  505   for temporarily storing image signals. The memory  505  may store, for example, the position of a known defective pixel in the photoelectric conversion device  502 . 
     The optical ranging unit  506  performs focusing and distance measurement of the object. The parallax calculation unit  507  calculates distance measurement information (distance information) from a plurality of image data (parallax images) acquired by a plurality of photoelectric conversion devices  502 . Each of the photoelectric conversion devices  502  may have a configuration capable of acquiring various kinds of information such as distance information. The object recognition unit  508  recognizes an object such as a vehicle, a road, a sign, or a person. When the abnormality detection unit  509  detects an abnormality of the photoelectric conversion device  502 , the abnormality detection unit  509  notifies the main control unit  513  of the abnormality. 
     The integrated circuit  503  may be implemented by dedicated hardware, software modules, or a combination thereof. Further, it may be implemented by FPGA (Field Programmable Gate Array), ASIC (Application Specific Integrated Circuit), or the like, or may be implemented by a combination of these. 
     The main control unit  513  collectively controls the operations of the photodetection system  501 , the vehicle sensor  510 , the control unit  520 , and the like. The vehicle  500  may not include the main control unit  513 . In this case, the photoelectric conversion device  502 , the vehicle sensor  510 , and the control unit  520  transmit and receive control signals via a communication network. For example, the CAN (Controller Area Network) standard may be applied to transmit and receive the control signals. 
     The integrated circuit  503  has a function of receiving a control signal from the main control unit  513  or transmitting a control signal and a setting value to the photoelectric conversion device  502  by its own control unit. 
     The photodetection system  501  is connected to the vehicle sensor  510 , and may detect a traveling state of the own vehicle such as a vehicle speed, a yaw rate, a steering angle, and the like, an environment outside the own vehicle, and states of other vehicles and obstacles. The vehicle sensor  510  is also a distance information acquisition means for acquiring distance information to an object. The photodetection system  501  is connected to a driving support control unit  511  that performs various driving support functions such as an automatic steering function, an automatic cruising function, and a collision prevention function. In particular, with regard to the collision determination function, based on the detection results of the photodetection system  501  and the vehicle sensor  510 , it is determined whether or not there is a collision with another vehicle or an obstacle. Thus, avoidance control when a collision is estimated and activation of the safety device at the time of collision are performed. 
     The photodetection system  501  is also connected to an alert device  512  that issues an alert to the driver based on the determination result of the collision determination unit. For example, when the collision possibility is high as the determination result of the collision determination unit, the main control unit  513  performs vehicle control to avoid collision and reduce damage by braking, returning an accelerator, suppressing engine output, or the like. The alert device  512  alerts a user by sounding an alarm such as a sound, displaying alert information on a display screen of a car navigation system or a meter panel, or applying vibration to a seat belt or a steering wheel. 
     In the present embodiment, the photodetection system  501  images the periphery of the vehicle, for example, the front side or the rear side.  FIG.  23 B  illustrates an example of the arrangement of the photodetection system  501  when the photodetection system  501  captures an image in front of the vehicle. 
     As described above, the photoelectric conversion device  502  is disposed in front of the vehicle  500 . More specifically, when a center line with respect to a forward/backward direction of the vehicle  500  or an outer shape (e.g., a vehicle width) is regarded as a symmetry axis, and two photoelectric conversion devices  502  are disposed axisymmetrically with respect to the symmetry axis, it is preferable to acquire distance information between the vehicle  500  and an object to be imaged and to determine a collision possibility. Further, it is preferable that the photoelectric conversion device  502  is disposed so as not to obstruct the field of view of the driver when the driver sees a situation outside the vehicle  500  from the driver’s seat. The alert device  512  is preferably arranged to be easy to enter the field of view of the driver. 
     Next, a failure detection operation of the photoelectric conversion device  502  in the photodetection system  501  will be described with reference to  FIG.  25   . The failure detection operation of the photoelectric conversion device  502  may be performed according to steps S 110  to S 180  illustrated in  FIG.  25   . 
     Step S 110  is a step of performing setting at the time of startup of the photoelectric conversion device  502 . That is, a setting for the operation of the photoelectric conversion device  502  is transmitted from the outside of the photodetection system  501  (for example, the main control unit  513 ) or from the inside of the photodetection system  501 , and the imaging operation and the failure detection operation of the photoelectric conversion device  502  are started. 
     Next, in step S 120 , pixel signals are acquired from the effective pixels. In step S 130 , an output value from the failure detection pixel provided for failure detection is acquired. The failure detection pixel includes a photoelectric conversion element as in the case of the effective pixels. A predetermined voltage is written to the photoelectric conversion element. The failure detection pixel outputs a signal corresponding to the voltage written to the photoelectric conversion element. Step S 120  and step S 130  may be reversed. 
     Next, in step S 140 , a classification of the output expected value of the failure detection pixel and the actual output value from the failure detection pixel is performed. As a result of the classification in step S 140 , when the output expected value matches the actual output value, the process proceeds to step S 150 , it is determined that the imaging operation is normally performed, and the process proceeds to step S 160 . In step S 160 , the pixel signals of the scanning row are transmitted to the memory  505  to temporarily store them. After that, the process returns to step S 120  to continue the failure detection operation. On the other hand, as a result of the classification in step S 140 , when the output expected value does not match the actual output value, the processing step proceeds to step S 170 . In step S 170 , it is determined that there is an abnormality in the imaging operation, and an alert is notified to the main control unit  513  or the alert device  512 . The alert device  512  causes the display unit to display that an abnormality has been detected. Thereafter, in step S 180 , the photoelectric conversion device  502  is stopped, and the operation of the photodetection system  501  is terminated. 
     Although the present embodiment exemplifies the example in which the flowchart is looped for each row, the flowchart may be looped for each plurality of rows, or the failure detection operation may be performed for each frame. The alert of step S 170  may be notified to the outside of the vehicle via the wireless network. 
     Further, in the present embodiment, the control in which the own vehicle does not collide with other vehicles has been described, but the present invention is also applicable to a control in which the own vehicle is automatically driven following another vehicle, a control in which the own vehicle is automatically driven so as not to go out of the lane, and the like. Further, the photodetection system  501  may be applied not only to a vehicle such as an own vehicle, but also to, for example, other movable objects (moving devices) such as a ship, an aircraft, or an industrial robot. In addition, the present invention may be applied not only to a movable object but also to equipment using object recognition in a wide range such as an intelligent transport system (ITS). 
     Tenth Embodiment 
     A photodetection system according to a tenth embodiment of the present invention will be described with reference to  FIG.  26 A  and  FIG.  26 B .  FIG.  26 A  and  FIG.  26 B  are schematic diagrams illustrating a configuration example of a photodetection system according to the present embodiment. In the present embodiment, an application example to eyeglasses (smartglasses) will be described as a photodetection system to which the photoelectric conversion device  100  described in any one of the first to fifth embodiments is applied. 
       FIG.  26 A  illustrates eyeglasses  600  (smartglasses) according to one application example. The eyeglasses  600  include lenses  601 , a photoelectric conversion device  602 , and a control device  603 . 
     The photoelectric conversion device  602  is the photoelectric conversion device  100  described in any of the first to fifth embodiments, and is provided on the lens  601 . One photoelectric conversion device  602  or a plurality of photoelectric conversion devices  602  may be provided on the lens  601 . When a plurality of photoelectric conversion devices  602  is used, a plurality of types of photoelectric conversion devices  602  may be used in combination. The arrangement position of the photoelectric conversion device  602  is not limited to that illustrated in  FIG.  26 A . A display device (not illustrated) including a light emitting device such as an OLED or an LED may be provided on the rear surface side of the lens  601 . 
     The control device  603  functions as a power supply for supplying power to the photoelectric conversion device  602  and the display device. The control device  603  has a function of controlling the operations of the photoelectric conversion device  602  and the display device. The lens  601  is provided with an optical system for focusing light on the photoelectric conversion device  602 . 
       FIG.  26 B  illustrates eyeglasses  610  (smartglasses) according to another application example. The eyeglasses  610  include lenses  611  and a control device  612 . A photoelectric conversion device corresponding to the photoelectric conversion device  602  and a display device (not illustrated) may be mounted on the control device  612 . 
     The lens  611  is provided with a photoelectric conversion device in the control device  612  and an optical system for projecting light from the display device, and an image is projected thereon. The control device  612  functions as a power supply for supplying power to the photoelectric conversion device and the display device, and has a function of controlling the operations of the photoelectric conversion device and the display device. 
     The control device  612  may further include a line-of-sight detection unit that detects the line of sight of the wearer. In this case, an infrared light emitting unit is provided in the control device  612 , and infrared light emitted from the infrared light emitting unit may be used for detection of a line of sight. Specifically, the infrared light emitting unit emits infrared light to the eyeball of the user who is watching the display image. The reflected light of the emitted infrared light from the eyeball is detected by the imaging unit having the light receiving element, whereby a captured image of the eyeball is obtained. By providing a reduction unit that reduces light from the infrared light emitting unit to the display unit in a plan view, a decrease in image quality may be reduced. 
     The line of sight of the user with respect to the display image may be detected from the captured image of the eyeball obtained by capturing the infrared light. Any known method may be applied to the line-of-sight detection using the captured image of the eyeball. As an example, a line-of-sight detection method based on a Purkinje image caused by reflection of irradiation light on the cornea may be used. More specifically, a line-of-sight detection processing based on the pupil cornea reflection method is performed. By using the pupil cornea reflection method, a line-of-sight vector representing the direction (rotation angle) of the eyeball is calculated based on the image of the pupil image and the Purkinje image included in the captured image of the eyeball, whereby the line-of-sight of the user is detected. 
     The display device according to the present embodiment may include a photoelectric conversion device having a light receiving element, and may be configured to control a display image based on line-of-sight information of a user from the photoelectric conversion device. Specifically, the display device determines a first viewing area to be gazed by the user and a second viewing area other than the first viewing area based on the line-of-sight information. The first viewing area and the second viewing area may be determined by a control device of the display device, or may be determined by an external control device. When an external control device determines, the determination result is transmitted to a display device via communication. In the display region of the display device, the display resolution of the first viewing area may be controlled to be higher than the display resolution of the second viewing area. That is, the resolution of the second viewing area may be lower than the resolution of the first viewing area. 
     Further, the display area may have a first display area and a second display area different from the first display area, and may be configured to determine an area having a high priority from the first display area and the second display area based on the line-of-sight information. The first display area and the second display area may be determined by a control device of the display device, or may be determined by an external control device. When an external control device determines, the determination result is transmitted to a display device via communication. The resolution of the area with high priority may be controlled to be higher than the resolution of the area other than the area with high priority. That is, the resolution of the area having a relatively low priority may be reduced. 
     An AI (Artificial Intelligence) may be used to determine the first viewing area or the area with high priority. The AI may be a model configured to estimate an angle of a line of sight and a distance to a target object ahead of the line of sight from an image of an eyeball, using an image of the eyeball and a direction in which the eyeball of the image is actually viewed as teacher data. The AI program may be held by a display device, a photoelectric conversion device, or an external device. When the external device has, the information is transmitted to the display device via communication. 
     When the display control is performed based on the visual recognition detection, the present invention may be preferably applied to a smartglasses which further include a photoelectric conversion device for capturing an image of the outside. The smartglasses may display captured external information in real time. 
     Modified Embodiments 
     The present invention is not limited to the above embodiment, and various modifications are possible. 
     For example, an example in which some of the configurations of any of the embodiments are added to other embodiments or an example in which some of the configurations of any of the embodiments are substituted with some of the configurations of the other embodiments is also an embodiment of the present invention. 
     The circuit configuration of the pixel  12  is not limited to the above-described embodiments. For example, a switch such as a transistor may be provided between the photoelectric conversion element  22  and the quenching element  24  or between the photoelectric conversion unit  20  and the pixel signal processing unit  30  to control an electrical connection state therebetween. Further, a switch such as a transistor may be provided between the node to which the voltage VH is supplied and the quenching element  24  and/or between the node to which the voltage VL is supplied and the photoelectric conversion element  22  to control an electrical connection state therebetween. 
     Although the counter  34  is used as the pixel signal processing unit  30  in the above-described embodiments, a TDC (Time to Digital Converter) and a memory may be used instead of the counter  34 . In this case, the generation timing of the pulse signal output from the signal processing circuit  32  is converted into a digital signal by the TDC. A control pulse pREF (reference signal) is supplied from the vertical scanning circuit unit  40  to the TDC via the control line  14  when the timing of the pulse signal is measured. The TDC acquires a signal corresponding to a relative time of the input timing of the signal output from each pixel  12  with respect to the control pulse pREF as a digital signal. 
     In the above-described embodiments, one pixel  12  includes one photoelectric conversion element  22 , but one pixel  12  may include a plurality of photoelectric conversion elements  22 . Although one photoelectric conversion element  22  is disposed in one semiconductor region  136  surrounded by the p-type semiconductor regions  132  and  134  in the above-described embodiments, a plurality of photoelectric conversion elements  22  may be disposed in one semiconductor region  136 . 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2021 -191170, filed Nov. 25, 2021, which is hereby incorporated by reference herein in its entirety.