Patent Publication Number: US-2021173050-A1

Title: Photodetection element and lidar device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims the benefit of priority from earlier Japanese Patent Application No. 2018-139607 filed Jul. 25, 2018 and earlier Japanese Patent Application No. 2018-139608 filed Jul. 25, 2018, the descriptions of which are incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a photodetection element that detects light and a lidar device that includes the photodetection element. 
     Related Art 
     A lidar device measures distances to objects around a vehicle by applying light toward an area around the vehicle and receiving reflected light of the applied light returning from the objects. 
     SUMMARY 
     As an aspect of the present disclosure, a photodetection element includes: at least one pixel region formed in a semiconductor substrate and configured to internally generate an electron and a hole in accordance with incident light; a first absorption region formed in the pixel region and configured to absorb a first discharge carrier, the first discharge carrier being either of the electron and the hole generated in the pixel region; a first discharge electrode formed on the semiconductor substrate and configured to discharge, from the first absorption region, the first discharge carrier absorbed in the first absorption region; a pixel neighboring region formed so as to be adjacent to the pixel region in the semiconductor substrate and configured to internally generate an electron and a hole in accordance with incident light; a second absorption region formed in the pixel neighboring region and configured to absorb a second discharge carrier, the second discharge carrier being a carrier that is either of the electron and the hole generated in the pixel neighboring region and equal to the first discharge carrier; and a second discharge electrode formed on the semiconductor substrate and configured to discharge, from the second absorption region, the second discharge carrier absorbed in the second absorption region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a perspective view of a lidar device; 
         FIG. 2  is a perspective view of a photodetection module; 
         FIG. 3  is a front view of the photodetection module with a part of its frame not shown; 
         FIG. 4  is a plan view of the lidar device with its housing not shown; 
         FIG. 5  shows a configuration of a mirror module; 
         FIG. 6  shows a configuration of a light source; 
         FIG. 7  shows a configuration of a light receiving element; 
         FIG. 8  shows a path of emitted light; 
         FIG. 9  shows a path of received light; 
         FIG. 10  illustrates positional adjustment of light sources and the light receiving element; 
         FIG. 11  shows an illumination range of light beams emitted from a deflection mirror; 
         FIG. 12  shows a correspondence between light emitting areas of the light sources and light receiving area of the light receiving element; 
         FIG. 13  is a plan view of an APD array according to a first embodiment; 
         FIG. 14  is a cross-sectional view of an APD; 
         FIG. 15  is a cross-sectional view of an APD, showing a depletion layer; 
         FIG. 16  is a plan view of a light receiving element according to the first embodiment; 
         FIG. 17  is a cross-sectional view of the light receiving element according to the first embodiment; 
         FIG. 18  is a plan view of an APD array according to a second embodiment; 
         FIG. 19  is a plan view of an APD array according to a third embodiment; 
         FIG. 20  is a plan view of an APD array according to a fourth embodiment; 
         FIG. 21  is a plan view of an APD array according to a fifth embodiment; 
         FIG. 22  is a cross-sectional view of an APD according to the fifth embodiment; 
         FIG. 23  is a plan view of the APD array, showing the arrangement of light shielding films. 
         FIG. 24  is a plan view of an APD array according to a sixth embodiment; 
         FIG. 25  is a cross-sectional view of an APD according to the sixth embodiment; 
         FIG. 26  is a plan view of an APD array according to a seventh embodiment; and 
         FIG. 27  is a plan view of a light receiving element according to the seventh embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     JP 2016-17904 A describes a lidar device that measures distances to objects around a vehicle by applying light toward an area around the vehicle and receiving reflected light of the applied light returning from the objects. 
     The inventors have conducted detailed research, and as a result, found that the lidar device described in JP 2016-17904 A has measurement accuracy lower for the distance to an object near to the lidar device than for the distance to an object far from the lidar device. 
     The present disclosure improves measurement accuracy of a lidar device. 
     First Embodiment 
     A first embodiment of the present disclosure will now be described with reference to the drawings. 
     A lidar device  1  according to the present embodiment is installed in a vehicle and used to detect various objects around the vehicle. Lidar is also written as LIDAR. LIDAR is an abbreviation for light detection and ranging. 
     The lidar device  1 , as shown in  FIG. 1 , includes a housing  100  and an optical window  200 . 
     The housing  100  is a rectangular resin box with an opening in one of its six surfaces and contains a photodetection module  2  described later. 
     The optical window  200  is a resin lid fixed to the housing  100  to cover the opening in the housing  100 . The photodetection module  2  installed in the housing  100  emits a laser beam, which passes through the optical window  200 . 
     Hereinafter, the direction along the length of the substantially rectangular opening is referred to as the X axis direction, the direction along the width of the opening is referred to as the Y axis direction, and the direction orthogonal to the X axis direction and the Y axis direction is referred to as the Z axis direction. Right and left in the X axis direction, and up and down in the Y axis direction are defined as viewed from the opening in the housing  100 . In the Z axis direction, forward is defined as a direction from the depth toward the opening in the housing  100 , and rearward is defined as a direction toward the depth. 
     The photodetection module  2 , as shown in  FIGS. 2, 3, and 4 , includes a light emitting unit  10 , a scanning unit  20 , a light receiving unit  30 , and a frame  40 . The photodetection module  2  is installed in the housing  100  via the frame  40 . 
     The scanning unit  20  includes a mirror module  21 , a partition  22 , and a motor  23 . 
     The mirror module  21 , as shown in  FIG. 5 , includes a pair of deflection mirrors  211  and  212  and a mirror frame  213 . 
     The pair of deflection mirrors  211  and  212  are flat members each having a reflective surface that reflects light. The mirror frame  213  includes a circular plate  213   a  and a support  213   b.  The circular plate  213   a  is a disc-shaped portion and fixed to the rotational shaft of the motor  23  at the center of the circle. The support  213   b  is a plate member with the deflection mirrors  211  and  212  fixed on both sides. The support  213   b  protrudes from the circular surface of the circular plate  213   a  in a direction perpendicular to the circular surface of the circular plate  213   a.    
     The deflection mirrors  211  and  212  and the support  213   b  each have an integrated shape of two rectangles with different lengths. More specifically, the deflection mirrors  211  and  212  and the support  213   b  have a shape of two integrated rectangles arranged along their central axes extending in the width direction, with the axes aligned with each other. Hereinafter, in the integrated portion of the deflection mirrors  211  and  212  and the support  213   b  of the mirror module  21 , the rectangular part smaller in a longitudinal direction is referred to as a short portion, and the rectangular part larger in a longitudinal direction is referred to as a long portion. 
     The pair of deflection mirrors  211  and  212  integrated via the mirror frame  213  are placed with the long portion being under the short portion, with the central axis being aligned with the center of the circle of the circular plate  213   a,  and to protrude from the circular surface of the circular plate  213   a  in a direction perpendicular to the circular surface of the circular plate  213   a.  This arrangement allows the deflection mirrors  211  and  212  to rotate about the rotational shaft of the motor  23  in accordance with the motor driving. The reflective surfaces of the deflection mirrors  211  and  212  are parallel with the rotational shaft of the motor  23  irrespective of the rotational position of the motor  23 . 
     The partition  22  is a disc-shaped member having a diameter equal to the length of the long portion of the mirror module  21 . The partition  22  is divided into two semicircular portions. The two semicircular portions hold the short portion of the mirror module  21  from both sides and fixed in contact with a step formed by the long portion and the short portion of the mirror module  21 . 
     Hereinafter, in the deflection mirrors  211  and  212 , the part above the partition  22  (i.e., the part of the short portion) is referred to as an emitted light deflection section  20   a,  and the part below the partition  22  (i.e., the part of the long portion) is referred to as a received light deflection section  20   b.    
     The light emitting unit  10 , as shown in  FIGS. 2 to 4 , includes a pair of light sources  11  and  12 , a pair of emitter lenses  13  and  14 , and an emitted light turning mirror  15 . 
     The light sources  11  and  12  have the same structure, and the structure of the light source  11  will be described herein. The light source  11  is, as shown in  FIG. 6 , a so-called multi-stripe semiconductor laser including a plurality of light emitting areas A 1  and A 2 . The light emitting areas A 1  and A 2  are formed as rectangles arranged in their longitudinal directions. The light emitting areas A 1  and A 2  have an area length L in the arrangement direction equal to or greater than the area interspace S between the light emitting area A 1  and the light emitting area A 2 . The light emitting areas A 1  and A 2  emit light beams having optical axes parallel with each other. 
     Hereinafter, in the emitted light deflection section  20   a,  the point on which light beams are incident from the light sources  11  and  12  is referred to as a reflection point. Furthermore, the plane orthogonal to the rotation axis and including the reflection point is referred to as a reference plane. 
     As shown in  FIGS. 2 to 4 , the light source  11  is positioned to the left along the X axis away from the reflection point, with the light emitting surface facing the emitted light deflection section  20   a.  The light source  12  is positioned to the rear along the Z axis away from the turning point at or near the middle of the path from the reflection point to the light source  11 , with the light emitting surface facing forward along the Z axis. Regarding the positions of the light sources  11  and  12  in the Y axis direction, the light source  11  is placed below the reference plane, and the light source  12  is placed above the reference plane. The light sources  11  and  12  are placed so that the light emitting areas A 1  and A 2  are arranged in the Y axis direction. 
     The emitter lens  13  is placed opposite the light emitting surface of the light source  11 . Similarly, the emitter lens  14  is placed opposite the light emitting surface of the light source  12 . The light sources  11  and  12  are placed near the focuses of the emitter lenses  13  and  14 , respectively. 
     The emitted light turning mirror  15 , which is placed at the turning point described above, reflects and guides light emitted from the light source  12  to the reflection point described above. The emitted light turning mirror  15  is, as shown in  FIG. 8 , placed above the path of light emitted from the light source  11  to the reflection point so as not to obstruct the path. The optical path from the light source  11  to the reflection point has the same length as the optical path from the light source  12  to the reflection point via the emitted light turning mirror  15 . The light source  11  has an optical axis inclined 1 to 2 degrees upward from the reference plane, and the light source  12  has an optical axis inclined 1 to 2 degrees downward from the reference plane. In other words, the optical axes of the light sources  11  and  12  are oriented symmetrically about the reference plane. The angles are not limited to 1 to 2 degrees, but may be determined as appropriate depending on the intended light beam emission angle in the sub-scanning direction. 
     The light receiving unit  30 , as shown in  FIGS. 2 to 4 , includes a light receiving element  31 , a light receiving lens  32 , and a received light turning mirror  33 . 
     The light receiving element  31 , as shown in  FIG. 7 , includes an avalanche photodiode array  311  (hereinafter, APD array  311 ) and a lens array  312 . APD is an abbreviation for avalanche photodiode. The APD array  311  includes 12 avalanche photodiodes (hereinafter, APDs) arranged in a row. The lens array  312  includes 12 lenses facing the 12 APDs of the APD array  311  on a one-to-one basis, and narrows and guides light incident on the light receiving element  31  to each APD. 
     The light receiving element  31  is, as shown in  FIGS. 3 and 9 , placed below the received light turning mirror  33 , with the light receiving surface facing upward along the Y axis and the APDs of the APD array  311  aligned with the X axis direction. In  FIG. 3 , a part of the frame  40  is not shown to increase the visibility of each arranged component. 
     The received light turning mirror  33  is positioned to the left along the X axis from the received light deflection section  20   b.  The received light turning mirror  33  bends the optical path substantially 90 degrees downward in the Y axis direction so that light incident from the received light deflection section  20   b  via the light receiving lens  32  reaches the light receiving element  31 . 
     The light receiving lens  32  is placed between the received light deflection section  20   b  and the received light turning mirror  33 . The light receiving lens  32  narrows the light beam incident on the light receiving element  31  so that its width in the Z axis direction becomes substantially equal to the APD element width. 
     The frame  40  is a member that integrates each component included in the light emitting unit  10 , the scanning unit  20 , and the light receiving unit  30 . That is, the components included in the light emitting unit  10 , the scanning unit  20 , and the light receiving unit  30  are installed in the housing  100  with the positional relationship between the components established. 
     The frame  40 , as shown in  FIGS. 2 to 4 , includes a frame bottom  41 , a frame side  42 , a frame back  43 , and a partition  44 . 
     The frame bottom  41  is underlain by a light receiver substrate  51  to which the light receiving element  31  is fixed and a motor substrate  52  to which the scanning unit  20  is fixed. Thus, the frame bottom  41  has holes at a site through which light passes from the received light turning mirror  33  to the light receiving element  31 , and a site at which the motor  23  of the scanning unit  20  is placed. 
     The frame side  42  has a front surface, which is the surface facing the scanning unit  20 , and a cylindrical holder  421  is installed on the front surface. The holder  421  has the emitter lens  13  fitted in the opening in its front end (i.e., the right end in the X axis direction). The back surface of the frame side  42  is fitted with a light emitter substrate  53  on which the light source  11  is installed. When the light emitter substrate  53  is attached to the frame side  42 , the light source  11  is placed at the back end of the holder  421  (i.e., the left end in the X axis direction). 
     In the same manner as for the frame side  42 , a holder  431  is installed on the frame back  43 . The holder  431  has the emitter lens  14  fitted in its front end (i.e., the forward end in the Z axis direction). The back surface of the frame back  43  is fitted with a light emitter substrate  54  on which the light source  12  is installed. When the light emitter substrate  54  is attached to the frame back  43 , the light source  12  is placed at the back end of the holder  431  (i.e., the rearward end in the Z axis direction). 
     The partition  44  is positioned in a manner to define a space in which the components of the light emitting unit  10  are placed and a space in which components of the light receiving unit  30  are placed. The partition  44  is fitted with the emitted light turning mirror  15 , the received light turning mirror  33 , and the light receiving lens  32 . 
     The light receiver substrate  51  and the light emitter substrates  53  and  54  are each screwed to the frame  40 . The lidar device  1  allows three-dimensional fine adjustments to the installation position and the angle of each of the light receiving element  31  and the light sources  11  and  12  by modifying the installation positions and the angles of the light receiver substrate  51  and the light emitter substrates  53  and  54 . In the present embodiment, the holders  421  and  431  are integrated with the frame side  42  and the frame back  43 , respectively. However, the holders  421  and  431  may be integrated with the light emitter substrate  53  and the light emitter substrate  54 . 
     A controller (not shown) is fitted to, for example, the housing  100 . The controller controls the timing of light emission from the light sources  11  and  12  in synchronization with the rotation of the mirror module  21  of the scanning unit  20 . More specifically, the controller controls the light beam from the light source  11  to be incident on the deflection mirror  211  and the light beam from the light source  12  to be incident on the deflection mirror  212 . 
     As shown in  FIG. 8 , the light emitted from the light source  11  is incident on the reflection point P on the emitted light deflection section  20   a  through the emitter lens  13 . The light emitted from the light source  12  passes through the emitter lens  14 . After that, its traveling direction is deflected substantially 90 degrees by the emitted light turning mirror  15 . The light is then incident on the reflection point P on the emitted light deflection section  20   a.  It is noted that the light source  11  and the light source  12  use different surfaces of the emitted light deflection section  20   a.  The light incident on the reflection point P is directed in accordance with the rotational position of the mirror module  21 . 
     As shown in  FIG. 9 , the light reflected from a subject positioned in the predetermined direction in accordance with the rotational position of the mirror module  21  (i.e., the direction in which light is emitted from the emitted light deflection section  20   a ) is reflected by the received light deflection section  20   b  and detected in the light receiving element  31  through the light receiving lens  32  and the received light turning mirror  33 . Note that subjects are various targets to be detected by the lidar device  1 . 
     More specifically, in the lidar device  1 , horizontal scanning in the X axis direction (hereinafter, main scanning) is mechanically achieved by the rotation of the mirror module  21 . Additionally, vertical scanning in the Y axis direction (hereinafter, sub-scanning) is electronically achieved by the light sources  11  and  12  for outputting four beams that are adjacent to each other in the vertical direction and the APD array  311  for receiving the four beams. 
     As shown in  FIGS. 8 to 10 , the light sources  11  and  12  are placed so that their optical paths to the reflection point P on the emitted light deflection section  20   a  have an equal length, and their optical axes intersect at reflection point P. The light receiving element  31  is placed near the focus of the light receiving lens  32 . 
     Light beams from the light emitting areas A 1  and A 2  of the light source  11  are denoted by B 11  and B 12 , and light beams from the light emitting areas A 1  and A 2  of the light source  12  are denoted by B 21  and B 22 . As shown in  FIG. 11 , the light beams emitted from the reflection point P on the emitted light deflection section  20   a  are the light beam B 11 , the light beam B 21 , the light beam B 12 , and the light beam B 22  in this order from top to bottom along the Y axis. Additionally, the positions of the light sources  11  and  12  are finely adjusted so as not to form a gap between the light beams B 11 , B 21 , B 12 , and B 22 . As shown in  FIG. 12 , the positions of the light sources  11  and  12  are also finely adjusted so that the APD array  311  of the light receiving element  31  receives reflected light (hereinafter, returning light beams) from the subject irradiated with the light beams B 11 , B 21 , B 12 , and B 22 , and the returning light beams are applied to the center in the Z axis direction of each APD with each beam hitting three different elements. 
     The reflective surface of the emitted light deflection section  20   a  is parallel with the rotation axis of the mirror module  21 , and thus the inclination angle of the reflective surface in vertical plane including the path of light incident on the emitted light deflection section  20   a  is unaffected by the rotational position of the mirror module  21 . The vertical planes refer to planes along the Y axis. More specifically, as indicated in the graph of  FIG. 11 , irrespective of the emission angle in the X axis direction (i.e., the horizontal angle), which is the main scanning direction of the light emitted from the emitted light deflection section  20   a,  the emission angle in the Y axis direction (i.e., the vertical angle), which is the sub-scanning direction, is constant. Thus, the light beams are applied to a two-dimensionally defined scan range entirely. 
     The APD array  311 , as shown in  FIG. 13 , is formed by arranging 12 APDs  502  in a row on the front surface of a substrate  501  of a p-type semiconductor (in the present embodiment, silicon). However, the substrate  501  may be formed of an i-type semiconductor. 
     The APD array  311  includes the 12 APDs  502 , one unnecessary-carrier discharge electrode  503 , and two bonding pads  504  on the front surface of the substrate  501 . 
     An APD  502 , as shown in  FIG. 14 , includes an n+ region  511 , an n region  512 , an n region  513 , a p+ region  514 , an antireflection film  515 , and a signal extraction electrode  516 . 
     The n+ region  511  is an n-type region formed in the front surface of the substrate  501 . The n+ region  511  contains a higher concentration of n-type impurities than the n regions  512  and  513 . 
     The n region  512  is an n-type region formed in the front surface of the substrate  501 , and around and in contact with the n+ region  511 . 
     The n region  513  is an n-type region formed in the front surface of the substrate  501  and around, but not in contact with, the n region  512 . 
     The p+ region  514  is a p-type region formed in the back surface of the substrate  501 . The p+ region  514  contains a higher concentration of p-type impurities than the substrate  501 . 
     The antireflection film  515  is a film formed from, for example, silicon nitride and placed on the n+ region  511 . The antireflection film  515  reduces the surface reflection of light incident on the antireflection film  515 . 
     The signal extraction electrode  516  is an electrode formed from, for example, Al or Cu, placed on the n region  512 , and electrically connected to the n region  512 . The signal extraction electrode  516  receives a high voltage (e.g., 300 V). 
     In the substrate  501 , the n+ region  511 , the n region  512 , and the region under the n+ region  511  and the n region  512  are hereinafter referred to as a pixel region  521 . In the substrate  501 , the region other than the pixel region  521  is hereinafter referred to as a pixel neighboring region  522 . 
     The unnecessary-carrier discharge electrode  503  is an electrode formed from, for example, Al or Cu, placed on the n region  513 , and electrically connected to the n region  513 . 
     The APD array  311  also includes, on the back surface of the substrate  501 , a back-side electrode  505  formed from, for example, AlSiCu. 
     As shown in  FIG. 13 , the antireflection film  515  is rectangular. The signal extraction electrode  516  includes a frame portion  531  and a linear portion  532 . The frame portion  531  is shaped as a rectangular frame surrounding the antireflection film  515 . The linear portion  532  is shaped as a straight line extending from the frame portion  531  in a direction perpendicular to the arrangement direction Da of the 12 APDs  502 . 
     The unnecessary-carrier discharge electrode  503  is formed around, but not in contact with, the 12 signal extraction electrodes  516  arranged in a row. The unnecessary-carrier discharge electrode  503  is also formed between every two adjacent frame portions  531  to surround each of the 12 signal extraction electrodes  516 . 
     The bonding pads  504  are formed from, for example, Al or Cu. The two bonding pads  504  are positioned on the rectangular substrate  501  at one end and the other end in the arrangement direction Da of the 12 APDs  502 . The two bonding pads  504  are electrically connected to the unnecessary-carrier discharge electrode  503 . 
     As shown in  FIG. 15 , the pixel region  521  includes a depletion layer DL 1  immediately below the n+ region  511  and the n region  512 . In addition, the pixel neighboring region  522  includes a depletion layer DL 2  immediately below the region between the n region  512  and the n region  513 . 
     The depletion layer DL 1  has a length L 1  along a thickness direction D 1  of the substrate  501 , and the depletion layer DL 2  has a length L 2  along a direction D 2  from the n region  512  to the n region  513 , with the length L 1  being smaller than the length L 2 . The depletion layer DL 1  has an electric field intensity in the thickness direction D 1  greater than the electric field intensity of the depletion layer DL 2  in the direction D 2 . 
     For an APD  502  having the structure described above, as shown in  FIG. 14 , when a returning light beam Br passes through the antireflection film  515  and enters the pixel region  521 , an electron-hole pair is generated in the pixel region  521 . Then, as indicated by an arrow ALL the electron of the electron-hole pair generated in the pixel region  521  is absorbed by the n region  512  and then discharged through the signal extraction electrode  516 . 
     The lens array  312  is, as shown in  FIGS. 16 and 17 , formed by arranging  12  convex lenses  551  in a row in the arrangement direction Da of the APDs  502 . The convex lenses  551  are formed from, for example, glass or silicone resin. As shown in  FIG. 17 , the lens array  312  is fixed to the APD array  311  via an adhesive layer  313  to cover the APDs  502 . The adhesive layer  313  is formed from a material that cures when exposed to ultraviolet radiation. The adhesive layer  313  is also formed from a material having substantially the same transmittance as the transmittance of the convex lenses  551  and the antireflection film  515 . As shown in  FIG. 16 , the  12  convex lenses  551  are placed opposite the antireflection films  515  of the  12  APDs  502 . Each of the  12  convex lenses  551  narrows light incident on the convex lens  551  and guides the light to the corresponding APD  502 . 
     The APD array  311  having the structure described above includes the 12 pixel regions  521 , the n regions  512 , the signal extraction electrodes  516 , the pixel neighboring regions  522 , the n regions  513 , and the unnecessary-carrier discharge electrode  503 . 
     The pixel regions  521  are formed in the substrate  501  and internally generate electrons and holes in accordance with the incident light. The n regions  512  are formed in the pixel regions  521  and absorb the electrons generated in the pixel regions  521  (hereinafter, first discharge carriers). The signal extraction electrodes  516  are formed on the substrate  501  and discharge, from the n regions  512 , the first discharge carriers absorbed in the n regions  512 . 
     The pixel neighboring regions  522  are formed so as to be adjacent to the pixel regions  521  in the substrate  501  and internally generate electrons and holes in accordance with the incident light. The n regions  513  are formed in the pixel neighboring regions  522  and absorb the electrons generated in the pixel neighboring regions  522  (hereinafter, second discharge carriers). The unnecessary-carrier discharge electrode  503  is formed on the substrate  501  and allows the second discharge carriers absorbed in the n regions  513  to be discharged from the n regions  513 . 
     In this manner, the APD array  311  allows the signal extraction electrodes  516  to discharge carriers generated in the pixel regions  521  by light entry into the pixel regions  521  (hereinafter, necessary carriers), and the unnecessary-carrier discharge electrode  503  to discharge carriers generated in the pixel neighboring regions  522  by light entry into the pixel neighboring regions  522  (hereinafter, unnecessary carriers). Thus, the APD array  311  reduces the discharge of unnecessary carriers through the signal extraction electrodes  516 . More specifically, the APD array  311  reduces the inclusion of unnecessary-carrier-caused signals in signals output through the signal extraction electrodes  516 . 
     If a pixel region  521  and a pixel neighboring region  522  receive light at the same time, the necessary carrier and the unnecessary carrier will reach the signal extraction electrode  516  at different times. Thus, the inclusion of unnecessary-carrier-caused signals in signals output through the signal extraction electrode  516  would reduce the accuracy in distance measurement. In the lidar device  1 , as shown in  FIG. 13 , when light reflected from an object near to the lidar device  1  and received by the APD array  311  is called reflected light SP 1 , and light reflected from an object far from the lidar device  1  and received by the APD array  311  is called reflected light SP 2 , the reflected light SP 1  has a spot size greater than the spot size of the reflected light SP 2 . For this reason, in the APD array  311 , reflected light received from an object near to the APD array  311  would tend to enter a pixel neighboring region  522 , generating an unnecessary carrier. 
     However, even when receiving light reflected from an object near to the lidar device  1 , the APD array  311  reduces the inclusion of unnecessary-carrier-caused signals in signals output through the signal extraction electrodes  516  as described above. The APD array  311  thus improves the measurement accuracy of the lidar device  1 . 
     The substrate  501  is p-type. The n region  512  and the n region  513  in the front surface of the substrate  501  are n-type, which is a conductivity type different from the p-type. The length L 1  of the depletion layer DL 1  in the pixel region  521  along the thickness direction D 1  of the substrate  501  is smaller than the length L 2  of the depletion layer DL 2  between the n region  512  and the n region  513  along the direction D 2  from the n region  512  to the n region  513 . The dimensions can prevent electrons generated in the pixel region  521  from drifting toward the n region  513 . As a result, the APD array  311  further reduces the inclusion of unnecessary-carrier-caused signals in signals output through the signal extraction electrode  516 , thus further improving the measurement accuracy of the lidar device  1 . 
     The electric field intensity of the depletion layer DL 1  in the thickness direction D 1  is greater than the electric field intensity of the depletion layer DL 2  in the direction D 2 . This can prevent electrons generated in the pixel region  521  from drifting toward the n region  513 . As a result, the APD array  311  further reduces the inclusion of unnecessary-carrier-caused signals in signals output through the signal extraction electrode  516 , thus further improving the measurement accuracy of the lidar device  1 . 
     The APD array  311  includes the convex lenses  551  placed opposite the pixel regions  521  over the substrate  501 . This arrangement enables light incident on the convex lenses  551  to be narrowed and guided to the pixel regions  521 , so that the APD array  311  can have a higher effective aperture ratio to incident light and also reduce light incident on the pixel neighboring regions  522 . 
     The substrate  501  is shaped as a rectangular plate, and the APD array  311  includes the bonding pads  504 . The bonding pads  504  are positioned on the substrate  501  at both longitudinal ends of the substrate  501  and electrically connected to the unnecessary-carrier discharge electrode  503 . As a result, the APD array  311  does not easily cause the unnecessary-carrier discharge electrode  503  to have a potential difference along the longitudinal direction of the substrate  501 . 
     The lidar device  1  includes the light emitting unit  10 , the light receiving unit  30 , and the scanning unit  20 . 
     The light emitting unit  10  includes the light sources  11  and  12  that output light. The light receiving unit  30  includes the APD array  311  that receives light coming in a predetermined direction. The scanning unit  20  includes the reflective surface that reflects light incident from the light emitting unit  10 . By rotating the reflective surface about the predefined rotation axis, the scanning unit  20  changes the emission direction of the light incident from the light emitting unit  10  along the main scanning direction orthogonal to the rotation axis direction, and reflects and guides the light reflected from a subject within the scan range to the light receiving unit  30 . 
     The lidar device  1 , which includes the APD array  311 , achieves the same advantageous effects as the APD array  311 . 
     In the embodiment described above, the APD array  311  corresponds to a photodetection element, the substrate  501  corresponds to a semiconductor substrate, the n region  512  corresponds to a first absorption region, the signal extraction electrode  516  corresponds to a first discharge electrode, the n region  513  corresponds to a second absorption region, and the unnecessary-carrier discharge electrode  503  corresponds to a second discharge electrode. Although the APDs described above are produced by n-doping a p substrate, an n substrate may be p-doped. Although described based on the cross section of reach-through devices, which may have a thicker depletion layer, the above embodiment may be applied to reverse-type devices, which cannot have a thick depletion layer compared with reach-through devices. The above embodiment may be applied not only to APDs, but also to photodiodes with no multiplication effect or single-photon avalanche diodes with quite a high multiplication factor. 
     Second Embodiment 
     A second embodiment of the present disclosure will now be described with reference to the drawings. In the second embodiment, differences from the first embodiment will be described. Common components will be given the same reference numerals. 
     A lidar device  1  according to the second embodiment is different from the lidar device according to the first embodiment in that an APD array  311  includes a different number of bonding pads  504 . 
     The APD array  311  according to the second embodiment, as shown in  FIG. 18 , includes three bonding pads  504 . The three bonding pads  504  are positioned on a rectangular substrate  501 : one at one end in the arrangement direction Da, another at the other end, and the other at the center. 
     The APD array  311  has higher performance of discharging unnecessary carriers owing to the additional bonding pad  504 . 
     Third Embodiment 
     A third embodiment of the present disclosure will now be described with reference to the drawings. In the third embodiment, differences from the first embodiment will be described. Common components will be given the same reference numerals. 
     A lidar device  1  according to the third embodiment is different from the lidar device according to the first embodiment in that an APD array  311  includes no lens array  312  and a different number of bonding pads  504 . 
     The APD array  311  according to the third embodiment, as shown in  FIG. 19 , includes four bonding pads  504 . The four bonding pads  504  are respectively positioned at the four corners of a rectangular substrate  501 . 
     The APD array  311  has higher performance of discharging unnecessary carriers owing to the additional bonding pads  504 . 
     Fourth Embodiment 
     A fourth embodiment of the present disclosure will now be described with reference to the drawings. In the fourth embodiment, differences from the first embodiment will be described. Common components will be given the same reference numerals. 
     A lidar device  1  according to the fourth embodiment is, as shown in  FIG. 20 , different from the lidar device according to the first embodiment in that an APD array  311  includes an unnecessary-carrier discharge electrode  503  of a different shape. 
     The unnecessary-carrier discharge electrode  503  of the APD array  311  according to the fourth embodiment is, as shown in  FIG. 20 , formed around, but not in contact with,  12  signal extraction electrodes  516  arranged in a row. However, the unnecessary-carrier discharge electrode  503  is not formed between any two adjacent frame portions  531 . 
     In the APD array  311 , the unnecessary-carrier discharge electrode  503  is not placed between any two adjacent pixel regions  521 . This arrangement allows the space between every two adjacent pixel regions  521  to be narrowed, and the APD array  311  may have a smaller size. 
     Fifth Embodiment 
     A fifth embodiment of the present disclosure will now be described with reference to the drawings. In the fifth embodiment, differences from the first embodiment will be described. Common components will be given the same reference numerals. 
     An APD array  311  according to the fifth embodiment is, as shown in  FIG. 21 , formed by arranging 12 APDs  502  in a row on the front surface of a substrate  501  of a p-type semiconductor (in the present embodiment, silicon). However, the substrate  501  may be formed of an i-type semiconductor. 
     The APD array  311  includes the 12 APDs  502  and one light shielding electrode  503  on the front surface of the substrate  501 . 
     An APD  502 , as shown in  FIG. 22 , includes an n+ region  511 , an n region  512 , a p+ region  514 , an antireflection film  515 , and a signal extraction electrode  516 . 
     The n+ region  511  is an n-type region formed in the front surface of the substrate  501 . The n+ region  511  contains a higher concentration of n-type impurities than the n region  512 . 
     The n region  512  is an n-type region formed in the front surface of the substrate  501 , and around and in contact with the n+ region  511 . 
     The p+ region  514  is a p-type region formed in the back surface of the substrate  501 . The p+ region  514  contains a higher concentration of p-type impurities than the substrate  501 . 
     The antireflection film  515  is a film formed from, for example, silicon nitride and placed on the n+ region  511 . The antireflection film  515  reduces the surface reflection of light incident on the antireflection film  515 . 
     The signal extraction electrode  516  is an electrode formed from, for example, Al or Cu, placed on the n region  512 , and electrically connected to the n region  512 . The signal extraction electrode  516  receives a high voltage (e.g., 300 V). 
     In the substrate  501 , the n+ region  511 , the n region  512 , and the region under the n+ region  511  and the n region  512  are hereinafter referred to as a pixel region  521 . In the substrate  501 , the region other than the pixel region  521  is hereinafter referred to as a pixel neighboring region  522 . 
     The light shielding electrode  503  is an electrode formed from, for example, Al or Cu and placed opposite the pixel neighboring regions  522  with a dielectric film (not shown) between them. 
     The APD array  311  also includes, on the back surface of the substrate  501 , a back-side electrode  505  formed from, for example, AlSiCu. 
     As shown in  FIG. 21 , the antireflection film  515  is rectangular. The signal extraction electrode  516  includes a frame portion  531  and a linear portion  532 . The frame portion  531  is shaped as a rectangular frame surrounding the antireflection film  515 . The linear portion  532  is shaped as a straight line extending from the frame portion  531  in a direction perpendicular to the arrangement direction Da of the 12 APDs  502 . 
     The light shielding electrode  503  is formed around, but not in contact with, the 12 signal extraction electrodes  516  arranged in a row. The light shielding electrode  503  is also formed between every two adjacent frame portions  531  to surround each of the 12 signal extraction electrodes  516 . 
     The linear portion  532  of the signal extraction electrode  516  is connected with a bonding wire  315  used to apply a high voltage to the signal extraction electrode  516  from outside the APD array  311 . 
     For an APD  502  having the structure described above, as shown in  FIG. 22 , when a returning light beam Br passes through the antireflection film  515  and enters the pixel region  521 , an electron-hole pair is generated in the pixel region  521 . Then, as indicated by an arrow AL 1 , the electron of the electron-hole pair generated in the pixel region  521  is absorbed by the n region  512  and discharged through the signal extraction electrode  516 . 
     The APD array  311 , as shown in  FIG. 23 , also includes light shielding films  507  and  508 . The light shielding films  507  and  508  are formed by applying a member (e.g., made of black potting resin) capable of attenuating, absorbing, or reflecting light within the sensitive wavelength range of the APDs  502  (e.g., a wavelength of 1.2 μm or less). 
     The light shielding film  507  is placed on the front surface and the side surfaces of the substrate  501  to cover the four sides of the rectangular substrate  501 . The light shielding film  508  is placed on the front surface of the substrate  501  to cover the areas of the 12 linear portions  532  arranged in a row, from one end to the other end in the arrangement direction Da. 
     The light shielding films  507  and  508  are formed by application with the bonding wires  315  connected to the linear portions  532  of the signal extraction electrodes  516 . The light shielding films  507  and  508  thus cover substantially the entire part of each bonding wire  315  on the front surface of the substrate  501 . 
     The APD array  311  having the structure described above includes the  12  pixel regions  521 , the n regions  512 , the signal extraction electrodes  516 , the pixel neighboring regions  522 , the light shielding electrode  503 , and the light shielding films  507  and  508 . 
     The pixel regions  521  are formed in the substrate  501  and internally generate electrons and holes in accordance with the incident light. The n regions  512  are formed in the pixel regions  521  and absorb the electrons generated in the pixel regions  521  (hereinafter, discharge carriers). The signal extraction electrodes  516  are formed on the substrate  501  and allow the discharge carriers absorbed in the n regions  512  to be discharged from the n regions  512 . 
     The pixel neighboring regions  522  are formed so as to be adjacent to the pixel regions  521  in the substrate  501  and internally generate electrons and holes in accordance with the incident light. The light shielding electrode  503  and the light shielding films  507  and  508  are formed from a member capable of blocking light with a wavelength that allows the generation of electrons and holes in the pixel neighboring regions  522 , and placed so as to cover the pixel neighboring regions  522 . 
     In this manner, the APD array  311  allows the signal extraction electrodes  516  to discharge carriers generated in the pixel regions  521  by light entry into the pixel regions  521  (hereinafter, necessary carriers). In the APD array  311 , the light shielding electrode  503  and the light shielding films  507  and  508  placed so as to cover the pixel neighboring regions  522  reduce carrier generation in the pixel neighboring regions  522  caused by light entry into the pixel neighboring regions  522 . Thus, the APD array  311  reduces the discharge of carriers generated in the pixel neighboring regions  522  (hereinafter, unnecessary carriers) through the signal extraction electrodes  516 . More specifically, the APD array  311  reduces the inclusion of unnecessary-carrier-caused signals in signals output through the signal extraction electrodes  516 . 
     If a pixel region  521  and a pixel neighboring region  522  receive light at the same time, the necessary carrier and the unnecessary carrier will reach the signal extraction electrode  516  at different times. Thus, the inclusion of unnecessary-carrier-caused signals in signals output through the signal extraction electrode  516  would reduce the accuracy in distance measurement. In the lidar device  1 , as shown in  FIG. 21 , when light reflected from an object near to the lidar device  1  and received by the APD array  311  is called reflected light SP 1 , and light reflected from an object far from the lidar device  1  and received by the APD array  311  is called reflected light SP 2 , the reflected light SP 1  has a spot size greater than the spot size of the reflected light SP 2 . For this reason, in the APD array  311 , reflected light received from an object near to the APD array  311  would tend to enter a pixel neighboring region  522 , generating an unnecessary carrier. 
     However, even when receiving light reflected from an object near to the lidar device  1 , the APD array  311  reduces the inclusion of unnecessary-carrier-caused signals in signals output through the signal extraction electrodes  516  as described above. The APD array  311  thus improves the measurement accuracy of the lidar device  1 . 
     The light shielding film  507  is an insulator member. The light shielding film  507  covers at least a portion of each pixel neighboring region  522  in the substrate  501 , the portion formed in an edge of the substrate  501 . 
     Applying high voltage to the APDs  502  generates high voltage in the edges of the substrate  501 . Thus, with the edges of the substrate  501  exposed, the package for the APD array  311  would be larger in order to satisfy the creepage distance requirement defined in IEC 60950 (i.e., to ensure a predefined creepage distance). IEC 60950 is an international standard for the safety of information technology equipment. 
     For the APD array  311 , however, the light shielding film  507  that is an insulator member covers the edges of the substrate  501 . Thus, the above creepage distance requirement may be relaxed for the APD array  311 , allowing the package for the APD array  311  to be smaller. 
     The signal extraction electrodes  516  are each connected with the bonding wires  315 . The light shielding film  507  covers a portion of each bonding wire  315  in addition to the pixel neighboring regions  522 . This covering in the APD array  311  reduces the exposure of the high-voltage bonding wires  315  on an edge of the substrate  501 . Thus, the above creepage distance requirement may be relaxed for the APD array  311 , allowing the package for the APD array  311  to be smaller. 
     The lidar device  1  includes the light emitting unit  10 , the light receiving unit  30 , and the scanning unit  20 . 
     The light emitting unit  10  includes the light sources  11  and  12  that output light. The light receiving unit  30  includes the APD array  311  that receives light coming in a predetermined direction. The scanning unit  20  includes the reflective surface that reflects light incident from the light emitting unit  10 . By rotating the reflective surface about the predefined rotation axis, the scanning unit  20  changes the emission direction of the light incident from the light emitting unit  10  along the main scanning direction orthogonal to the rotation axis direction, and reflects and guides the light reflected from a subject within a scan range to the light receiving unit  30 . 
     The lidar device  1 , which includes the APD array  311 , achieves the same advantageous effects as the APD array  311 . 
     In the embodiment described above, the APD array  311  corresponds to a photodetection element, the substrate  501  corresponds to a semiconductor substrate, the n region  512  corresponds to an absorption region, the signal extraction electrode  516  corresponds to a discharge electrode, and the light shielding electrode  503  and the light shielding films  507  and  508  correspond to light shields. Although the APDs described above are produced by n-doping a p substrate, an n substrate may be p-doped. Although described based on the cross section of reach-through devices, which may have a thicker depletion layer, the above embodiment may be applied to reverse-type devices, which cannot have a thick depletion layer compared with reach-through devices. The above embodiment may be applied not only to APDs, but also to photodiodes with no multiplication effect or single-photon avalanche diodes with quite a high multiplication factor. 
     Sixth Embodiment 
     A sixth embodiment of the present disclosure will now be described with reference to the drawings. In the sixth embodiment, differences from the fifth embodiment will be described. Common components will be given the same reference numerals. 
     A lidar device  1  according to the sixth embodiment, as shown in 
       FIGS. 24 and 25 , is different from the lidar device according to the fifth embodiment in that an APD array  311  additionally includes a light shielding electrode  509 . 
     The light shielding electrode  509  is an electrode formed from, for example, A 1  or Cu and, as shown in  FIG. 24 , and is placed on the front surface of the substrate  501  to cover the areas of  12  frame portions  531  arranged in a row, from one end to the other end in the arrangement direction Da. However, the light shielding electrode  509  has openings opposite the  12  antireflection films  515  arranged in a row. The light shielding electrode  509  is, as shown in  FIG. 25 , placed opposite the light shielding electrode  503  and the signal extraction electrode  516  with a dielectric film (not shown) between them. 
     The APD array  311  having the structure described above includes the light shielding electrode  509 . The light shielding electrode  509  is formed of metal capable of blocking light with a wavelength that allows the generation of electrons and holes in the pixel neighboring regions  522 . The light shielding electrode  509  is electrically isolated from the signal extraction electrodes  516  of every two adjacent pixel regions  521  and is placed so as to cover the gap between the signal extraction electrodes  516  of every two adjacent pixel regions  521 . 
     This arrangement enables the APD array  311  to reduce light incident on the pixel neighboring region  522  formed between every two adjacent pixel regions  521 . As a result, the APD array  311  further reduces the inclusion of unnecessary-carrier-caused signals in signals output through the signal extraction electrode  516 , thus further improving the measurement accuracy of the lidar device  1 . 
     In the embodiment described above, the light shielding electrode  509  corresponds to light shielding metal. 
     Seventh Embodiment 
     A seventh embodiment of the present disclosure will now be described with reference to the drawings. In the seventh embodiment, differences from the fifth embodiment will be described. Common components will be given the same reference numerals. 
     A lidar device  1  according to the seventh embodiment is, as shown in  FIG. 26 , different from the fifth embodiment in that the light shielding films  507  and  508  are removed, and a lens array  312 , a light shield  317 , and a light shielding film  560  are added. 
     A light receiving element  31  according to the seventh embodiment further includes the lens array  312  and the light shield  317 . The lens array  312  includes  12  lenses facing  12  APDs  502  for the APD array  311  on a one-to-one basis, and narrows light incident on the light receiving element  31  and guides the light to each APD  502 . The lens array  312  is, as shown in  FIGS. 26 and 27 , formed by arranging  12  convex lenses  551  in a row in the arrangement direction Da of the APDs  502 . The convex lenses  551  are formed from, for example, glass or silicone resin. As shown in  FIG. 27 , the lens array  312  is fixed to the APD array  311  via an adhesive layer  313  to cover the APDs  502 . The adhesive layer  313  is formed from a material that cures when exposed to ultraviolet radiation. The adhesive layer  313  is also formed from a material having substantially the same transmittance as the transmittance of the convex lenses  551  and antireflection films  515 . As shown in  FIG. 26 , the  12  convex lenses  551  are placed opposite the antireflection films  515  of the  12  APDs  502 . Each of the  12  convex lenses  551  narrows light incident on the convex lens  551  and guides the light to the corresponding APD  502 . 
     The light shield  317  is, for example, formed from silicone resin, epoxy resin, or acrylic resin mixed or painted with a member capable of absorbing light within the sensitive wavelength range of the APDs  502 . The light shield  317  is, as shown in  FIGS. 26 and 27 , placed on the side surfaces of the substrate  501  to cover the four sides of the rectangular substrate  501 . As a result, the light shield  317  is placed on side surfaces of the lens array  312  and covers three of the four sides of the rectangular lens array  312  that are also included in the rectangle of the substrate  501 . 
     The APD array  311  according to the seventh embodiment, as shown in  FIG. 26 , further includes the light shielding film  560 . The light shielding film  560  is formed, in the same manner as the light shielding films  507  and  508 , by applying a member capable of attenuating, absorbing, or reflecting light within the sensitive wavelength range of the APDs  502 . The light shielding film  560  is placed on the front surface of the substrate  501  to cover an area other than the lens array  312 . 
     The APD array  311  having the structure described above includes the convex lenses  551  placed opposite the pixel regions  521  over the substrate  501 . This arrangement enables light incident on the convex lenses  551  to be narrowed and guided to the pixel regions  521 , so that the APD array  311  can have a higher effective aperture ratio to incident light and also reduce light incident on the pixel neighboring regions  522 . 
     The light incident surface of the convex lenses  551  for receiving light includes a light incident surface incapable of guiding incident light to the pixel regions  521  (in the present embodiment, the side surfaces of the lens array  312 ), and at least a portion of the surface incapable of guiding incident light is covered with the light shielding film  560  capable of blocking light with a wavelength that allows the generation of electrons and holes in the pixel neighboring regions  522 . 
     This arrangement enables the APD array  311  to reduce light incident on the pixel neighboring regions  522 . As a result, the APD array  311  further reduces the inclusion of unnecessary-carrier-caused signals in signals output through the signal extraction electrode  516 , thus further improving the measurement accuracy of the lidar device  1 . 
     In the embodiment described above, the light shielding film  560  corresponds to a lens light shield. 
     The embodiments of the present disclosure have been described. However, the present disclosure is not limited to the above embodiments, but may be implemented in various modified embodiments. 
     [First Modification] 
     Although the substrate  501  is p-type, and the n region  512  and the n region  513  are n-type in the above embodiments, for example, the substrate  501  may be n-type or i-type, and the n region  512  and the n region  513  may be p-type. In this case, carriers discharged through the signal extraction electrode  516  are holes. 
     [Second Modification] 
     Although the substrate  501  in the above embodiments is formed from silicon, the material for the substrate  501  is not limited to silicon as long as it is a semiconductor. The substrate  501  may be formed from, for example, InGaAs. 
     [Third Modification] 
     Although the substrate  501  is p-type, and the n region  512  is n-type in the above embodiments, for example, the substrate  501  may be n-type or i-type, and the n region  512  may be p-type. In this case, carriers discharged through the signal extraction electrode  516  are holes. 
     [Fourth Modification] 
     Although the substrate  501  in the above embodiments is formed from silicon, the material for the substrate  501  is not limited to silicon as long as it is a semiconductor. The substrate  501  may be formed from, for example, InGaAs. In this case, the APDs have a sensitive wavelength range of 3 μm or less (i.e., wavelengths from visible light to mid-infrared light). 
     In some cases, a plurality of components may share the functions of one component in the above embodiments. In other cases, one component may implement functions of a plurality of components. Some of the components in the above embodiments may be omitted. At least some components in one of the above embodiments may be added to or substituted for components in another of the above embodiments. 
     An aspect of the present disclosure is a photodetection element ( 311 ) including at least one pixel region ( 521 ), a first absorption region ( 512 ), a first discharge electrode ( 516 ), a pixel neighboring region ( 522 ), a second absorption region ( 513 ), and a second discharge electrode ( 503 ). 
     The pixel region is formed in a semiconductor substrate ( 501 ) and internally generates an electron and a hole in accordance with the incident light. 
     The first absorption region is formed in the pixel region and absorbs a first discharge carrier that is either of the electron and the hole generated in the pixel region. 
     The first discharge electrode is formed on the semiconductor substrate and discharges, from the first absorption region, the first discharge carrier absorbed in the first absorption region. 
     The pixel neighboring region is formed so as to be adjacent to the pixel region in the semiconductor substrate and internally generates an electron and a hole in accordance with the incident light. 
     The second absorption region is formed in the pixel neighboring region and absorbs a second discharge carrier that is the same carrier of either of the electron and the hole generated in the pixel neighboring region, as the first discharge carrier. 
     The second discharge electrode is formed on the semiconductor substrate and discharges, from the second absorption region, the second discharge carrier absorbed in the second absorption region. 
     The photodetection element according to the present disclosure having the above configuration allows the first discharge electrode to discharge carriers generated in the pixel region by light entry into the pixel region (hereinafter, necessary carriers), and the second discharge electrode to discharge carriers generated in the pixel neighboring region by light entry into the pixel neighboring region (hereinafter, unnecessary carriers). Thus, the photodetection element according to the present disclosure reduces the discharge of unnecessary carriers through the first discharge electrode. More specifically, the photodetection element according to the present disclosure reduces the inclusion of unnecessary-carrier-caused signals in signals output from through the first discharge electrode. 
     If the pixel region and the pixel neighboring region receive light at the same time, the necessary carrier and the unnecessary carrier will reach the first discharge electrode at different times. Thus, the inclusion of unnecessary-carrier-caused signals in signals output from the first discharge electrode would reduce the accuracy in distance measurement. In the lidar device, light reflected from an object near to the lidar device and received by the photodetection element according to the present disclosure has a spot size greater than the spot size of light reflected from an object far from the lidar device and received by the photodetection element according to the present disclosure. For this reason, in the photodetection element according to the present disclosure, reflected light received from an object near to the lidar device would tend to enter the pixel neighboring region, generating an unnecessary carrier. 
     However, even when receiving light reflected from an object near to the lidar device, the photodetection element according to the present disclosure reduces the inclusion of unnecessary-carrier-caused signals in signals output through the first discharge electrode as described above. The photodetection element according to the present disclosure thus improves the measurement accuracy of the lidar device. 
     Another aspect of the present disclosure is a lidar device ( 1 ) including a light emitting unit ( 10 ), a light receiving unit ( 30 ), and a scanning unit ( 20 ). The light emitting unit includes a light source that outputs light. The light receiving unit includes a photodetection element that receives light arriving from a predetermined direction. The scanning unit includes a reflective surface for reflecting light incident from the light emitting unit. By rotating the reflective surface about a predefined rotation axis, the scanning unit changes the emission direction of light incident from the light emitting unit along the main scanning direction orthogonal to the rotation axis direction, and reflects and guides light reflected from a subject within a scan range to the light receiving unit. 
     The photodetection element includes at least one pixel region, a first absorption region, a first discharge electrode, a pixel neighboring region, a second absorption region, and a second discharge electrode. 
     The lidar device according to the present disclosure having the above configuration, which includes the photodetection element according to the aspect of the present disclosure, achieves the same advantageous effects as the photodetection element according to the present disclosure. 
     Still another aspect of the present disclosure is a photodetection element including at least one pixel region ( 521 ), an absorption region ( 512 ), a discharge electrode ( 516 ), a pixel neighboring region ( 522 ), and a light shield ( 503 ,  507 ,  508 ). 
     The pixel region is formed in a semiconductor substrate ( 501 ) and internally generates an electron and a hole in accordance with the incident light. 
     The absorption region is formed in the pixel region and absorbs a discharge carrier that is either of the electron and the hole generated in the pixel region. 
     The discharge electrode is formed on the semiconductor substrate and discharges, from the absorption region, the discharge carrier absorbed in the absorption region. 
     The pixel neighboring region is formed so as to be adjacent to the pixel region in the semiconductor substrate and internally generates an electron and a hole in accordance with the incident light. 
     The light shield is formed from a member capable of blocking light with a wavelength allowing the generation of the electron and the hole in the pixel neighboring region, and is placed so as to cover the pixel neighboring region. 
     The photodetection element according to the present disclosure having the above configuration allows the discharge electrode to discharge carriers generated in the pixel region by light entry into the pixel region (hereinafter, necessary carriers). In the photodetection element of the present disclosure, the light shield placed so as to cover the pixel neighboring region reduces carrier generation in the pixel neighboring region caused by light entry into the pixel neighboring region. Thus, the photodetection element according to the present disclosure reduces the discharge of carriers generated in the pixel neighboring region (hereinafter, unnecessary carriers) through the discharge electrode. More specifically, the photodetection element according to the present disclosure reduces the inclusion of unnecessary-carrier-caused signals in signals output through the discharge electrode. 
     If the pixel region and the pixel neighboring region were to receive light at the same time, the necessary carrier and the unnecessary carrier would reach the discharge electrode at different times. Thus, the inclusion of unnecessary-carrier-caused signals in signals output from the discharge electrode would reduce the accuracy in distance measurement. In the lidar device, light reflected from an object near to the lidar device and received by the photodetection element according to the present disclosure has a spot size greater than the spot size of light reflected from an object far from the lidar device and received by the photodetection element according to the present disclosure. For this reason, in the photodetection element according to the present disclosure, reflected light received from an object near to the lidar device would tend to enter the pixel neighboring region, generating an unnecessary carrier. 
     However, even when receiving light reflected from an object near to the lidar device, the photodetection element according to the present disclosure reduces the inclusion of unnecessary-carrier-caused signals in signals output through the discharge electrode as described above. The photodetection element according to the present disclosure thus improves the measurement accuracy of the lidar device. 
     Still another aspect of the present disclosure is a lidar device ( 1 ) including a light emitting unit ( 10 ), a light receiving unit ( 30 ), and a scanning unit ( 20 ). The light emitting unit includes a light source that outputs light. The light receiving unit includes a photodetection element that receives light coming in a predetermined direction. The scanning unit includes a reflective surface for reflecting light incident from the light emitting unit. By rotating the reflective surface about a predefined rotation axis, the scanning unit changes the emission direction of light incident from the light emitting unit along the main scanning direction orthogonal to the rotation axis direction, and reflects and guides light reflected from a subject within a scan range to the light receiving unit. 
     The photodetection element includes at least one pixel region, an absorption region, a discharge electrode, a pixel neighboring region, and a light shield. 
     The lidar device according to the present disclosure having the above configuration, which includes the photodetection element according to the above aspect of the present disclosure, achieves the same advantageous effects as the photodetection element according to the present disclosure.