Patent Publication Number: US-10325953-B2

Title: Range sensor and solid-state imaging device

Description:
TECHNICAL FIELD 
     The present invention relates to a range sensor having a lock-in pixel function for detecting photoelectrons in synchronization with a light pulse, and a solid-state imaging device including a plurality of range sensors arranged as pixels. 
     BACKGROUND ART 
     In recent years, development of time-of-flight range image sensors using semiconductors is actively being carried out. For example, a range image sensor includes a plurality of lock-in pixels arrayed and connected in parallel for transferring carriers generated in photodiodes in synchronization with a light source to a plurality of charge accumulators, while each lock-in pixel element is made into a smaller pixel so as to transfer charges at high speed as disclosed in Patent Literature (PTL) 1. 
     In the image sensor disclosed in PTL 1, load capacitances of a plurality of transfer gates connected in parallel increase, and the entire power consumption increases as the number of pixels increases. Further, connecting a plurality of lock-in pixel elements increases the area of a diffusion layer of a signal detector, which may cause a dark current. 
     CITATION LIST 
     Patent Literature 
     PTL 1: U.S. Pat. No. 6,794,214 
     SUMMARY INVENTION 
     An objective of the present invention is to provide a range sensor having a large light receiving area and capable of high-speed modulation with high sensitivity at a low dark current, and a solid-state imaging device including a plurality of range sensors as pixels arranged on a semiconductor chip. 
     Solution to Problem 
     In order to achieve the objective, a first aspect of the present invention inheres in a range sensor encompassing (a) a pixel layer made of a semiconductor of a first conductivity type, (b) a shielding plate having an aperture and provided above the pixel layer, the aperture defines a light-receiving area in the pixel layer, (c) a surface-buried region of a second conductivity type selectively buried in an upper portion of the pixel layer to implement a photodiode in the light-receiving area by forming a junction with the pixel layer, and extending from the light-receiving area toward plural portions shielded by the shielding plate along the upper portion of the pixel layer, delineating a configuration with a plurality of branches, (d) a plurality of charge-accumulation regions of the second conductivity type having a higher impurity concentration than the surface-buried region connected to tips of the respective branches, (e) a plurality of transfer-control mechanisms provided at the respective branches adjacent to the charge-accumulation regions to control a transfer of signal charges to the charge-accumulation regions, and (f) a guide region of a second conductivity type having a higher impurity concentration than the surface-buried region, and a lower impurity concentration than the charge-accumulation regions, and buried in a part of an upper portion of the surface-buried region with one end provided below a part of the aperture and other ends extending to at least a part of the respective transfer-control mechanisms. 
     A second aspect of the present invention inheres in a range sensor encompassing (a) a pixel layer made of a semiconductor of a first conductivity type, (b) a shielding plate having a plurality of apertures and provided above the pixel layer to define a plurality of light-receiving areas on the pixel layer below the respective apertures, (c) a surface-buried region of a second conductivity type buried in an upper portion of the pixel layer, configured to assign a plurality of photodiodes in the respective light-receiving areas, by forming a plurality of junctions with the pixel layer, the surface-buried region including a charge-modulation arrangement-region having: a plurality of branches disposed at a position shielded by the shielding plate, and a plurality of light-receiving terminal-portions, a number of the light-receiving terminal-portions corresponding to the number of the apertures, extending toward the respective apertures at both ends of the charge-modulation arrangement-region, each of occupying areas of the light-receiving terminal-portions having an area sufficient to cover substantially the entire area of corresponding aperture; (d) a plurality of charge-accumulation regions of the second conductivity type having a higher impurity concentration than the surface-buried region connected to tips of the respective branches; (e) a plurality of transfer-control mechanisms provided at the respective branches adjacent to the charge-accumulation regions to control a transfer of signal charges to the charge-accumulation regions; and (f) a guide region of the second conductivity type having a higher impurity concentration than the surface-buried region, and a lower impurity concentration than the charge-accumulation regions, the guide region including: a plurality of protruding-edges, a number of protruding-edges corresponding to the number of respective apertures, extending from an area shielded by the shielding plate on the charge-modulation arrangement-region to the respective apertures so that tips of the protruding-edges are provided below the respective apertures. 
     A third aspect of the present invention inheres in a range sensor encompassing (a) a pixel layer made of a semiconductor of a first conductivity type, (b) a shielding plate having a plurality of apertures around a pixel region defined by the pixel layer and provided above the pixel layer to define a plurality of light-receiving areas on the pixel layer below the respective apertures, (c) a surface-buried region of a second conductivity type buried in an upper portion of the pixel layer, configured to assign a plurality of photodiodes in the respective light-receiving areas, by forming a plurality of junctions with the pixel layer, the surface-buried region including a charge-modulation arrangement-region having: a plurality of branches disposed in a central portion of the pixel region shielded by the shielding plate, and a plurality of light-receiving terminal-portions, a number of the light-receiving terminal-portions corresponding to the number of the apertures, the light-receiving terminal-portions radially extending toward the respective apertures, each of occupying areas of the light-receiving terminal-portions having an area sufficient to cover substantially the entire area of corresponding aperture; (d) a plurality of charge-accumulation regions of the second conductivity type having a higher impurity concentration than the surface-buried region connected to tips of the respective branches; (e) a plurality of transfer-control mechanisms provided at the respective branches adjacent to the charge-accumulation regions to control a transfer of signal charges to the charge-accumulation regions; and (f) a guide region of the second conductivity type having a higher impurity concentration than the surface-buried region, and a lower impurity concentration than the charge-accumulation regions, the guide region including: a plurality of protruding-edges, a number of the protruding-edges corresponding to the number of respective apertures, tips of the respective protruding-edges radially extending below the respective apertures from an area shielded by the shielding plate on the charge-modulation arrangement-region. 
     A fourth aspect of the present invention inheres in a solid-state imaging device including a plurality of pixels arranged on a semiconductor chip, each of the pixels being the range sensor according to the first aspect. 
     A fifth aspect of the present invention inheres in a solid-state imaging device including a plurality of pixels arranged on a semiconductor chip, each of the pixels being the range sensor according to the second aspect. 
     A sixth aspect of the present invention inheres in a solid-state imaging device including a plurality of pixels arranged on a semiconductor chip, each of the pixels being the range sensor according to the third aspect. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to achieve a range sensor having a large light receiving area and capable of high-speed modulation with high sensitivity at a low dark current, and a solid-state imaging device including a plurality of range sensors as pixels arranged on a semiconductor chip. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic plan view from above of a principal part of a range sensor according to a first embodiment of the present invention; 
         FIG. 2A  is a cross-sectional view of the range sensor according to the first embodiment as viewed from direction II-II in  FIG. 1 ; 
         FIG. 2B  is a view illustrating a potential profile for the structure illustrated in  FIG. 2A ; 
         FIG. 3A  is a cross-sectional view of the range sensor according to the first embodiment as viewed from direction III-III in  FIG. 1 ; 
         FIG. 3B  is a view illustrating a potential profile for the structure illustrated in  FIG. 3A ; 
         FIG. 4A  is a cross-sectional view of the range sensor according to the first embodiment as viewed from direction IV-IV in  FIG. 1 ; 
         FIG. 4B  is a view illustrating a potential profile for the structure illustrated in  FIG. 4A ; 
         FIG. 5A  is a cross-sectional view of the range sensor according to the first embodiment as viewed from direction V-V in  FIG. 1 ; 
         FIG. 5B  is a view illustrating a potential profile for the structure illustrated in  FIG. 5A ; 
         FIG. 6  is a diagram illustrating a potential profile and a transport path of signal charges on equipotential lines in the principal part of the range sensor according to the first embodiment when a gate signal of an intermediate potential (M) is applied to a first transfer gate electrode; 
         FIG. 7  is a diagram illustrating a potential profile and a transport path of signal charges on equipotential lines in the principal part of the range sensor according to the first embodiment when a gate signal of an intermediate potential (M) is applied to a second transfer gate electrode; 
         FIG. 8  is a diagram illustrating a potential profile and a transport path of signal charges on equipotential lines in the principal part of the range sensor according to the first embodiment when a gate signal of an intermediate potential (M) is applied to a third transfer gate electrode; 
         FIG. 9  is a diagram illustrating a potential profile and an extraction path of charges on equipotential lines in the principal part of the range sensor according to the first embodiment when a gate signal of an intermediate potential (M) is applied to a first extraction gate electrode; 
         FIG. 10  is a circuit diagram of the principal part of the range sensor according to the first embodiment illustrated with an equivalent circuit; 
         FIG. 11  is another circuit diagram of the principal part of the range sensor according to the first embodiment illustrated with an equivalent circuit; 
         FIG. 12  is a timing diagram when the range sensor according to the first embodiment is operated as a pixel of a range image sensor; 
         FIG. 13  is a schematic plan view from above of a principal part of a range sensor according to a modified example of the first embodiment of the present invention; 
         FIG. 14  is a cross-sectional view of the range sensor according to the modified example of the first embodiment as viewed from direction XIV-XIV in  FIG. 13 ; 
         FIG. 15  is a schematic plan view from above of a principal part of a range sensor according to a second embodiment of the present invention; 
         FIG. 16  is a schematic plan view from above of a principal part of a range sensor according to a third embodiment of the present invention; 
         FIG. 17  is a circuit diagram of the principal part of the range sensor according to the third embodiment illustrated with an equivalent circuit; 
         FIG. 18  is a schematic plan view from above of a principal part of a range sensor according to a fourth embodiment of the present invention; 
         FIG. 19  is a circuit diagram of the principal part of the range sensor according to the fourth embodiment illustrated with an equivalent circuit. 
         FIG. 20  is a schematic plan view from above of a principal part of a range sensor according to a fifth embodiment of the present invention; 
         FIG. 21  is an enlarged plan view of the central portion of the range sensor according to the fifth embodiment shown in  FIG. 20 ; 
         FIG. 22  is a circuit diagram of the principal part of the range sensor according to the fifth embodiment of the present invention illustrated with an equivalent circuit; 
         FIG. 23  is a schematic plan view from above of a principal part of a range sensor according to Modified Example  1  of the fifth embodiment of the present invention; 
         FIG. 24  is a schematic plan view from above of a principal part of a range sensor according to Modified Example  2  of the fifth embodiment of the present invention; 
         FIG. 25  is a schematic plan view from above of a principal part of a range sensor according to Modified Example  3  of the fifth embodiment of the present invention; 
         FIG. 26  is a schematic plan view from above of a principal part of a range sensor according to a sixth embodiment of the present invention; 
         FIG. 27  is a circuit diagram of the principal part of the range sensor according to the sixth embodiment of the present invention illustrated with an equivalent circuit; 
         FIG. 28  is a schematic plan view from above of a principal part of a range sensor according to a seventh embodiment of the present invention; 
         FIG. 29A  is a cross-sectional view of the range sensor according to the seventh embodiment as viewed from direction XXIX-XXIX in  FIG. 28 ; 
         FIG. 29B  is a view illustrating potential profiles for the structures illustrated in  FIG. 29A  and  FIG. 29C ; 
         FIG. 29C  is a cross-sectional view of the range sensor according to the first embodiment as viewed from direction V-V in  FIG. 1 ; 
         FIG. 30A  is a cross-sectional view of the range sensor according to the seventh embodiment as viewed from direction XXX-XXX in  FIG. 28 ; 
         FIG. 30B  is a view illustrating potential profiles for the structures illustrated in  FIG. 30A  and  FIG. 30C  when a gate signal of an intermediate potential (M) is applied to a first transfer gate electrode; 
         FIG. 30C  is a cross-sectional view of the range sensor according to the first embodiment as viewed from the direction opposite to direction III-III in  FIG. 1 ; 
         FIG. 31A  is a cross-sectional view of the range sensor according to the seventh embodiment as viewed from direction XXX-XXX in  FIG. 28 ; 
         FIG. 31B  is a view illustrating potential profiles for the structures illustrated in  FIG. 31A  and  FIG. 31C  when a gate signal of an intermediate potential (M) is applied to a third transfer gate electrode; 
         FIG. 31C  is a cross-sectional view of the range sensor according to the first embodiment as viewed from the direction opposite to direction III-III in  FIG. 1 ; 
         FIG. 32  is a diagram illustrating a potential profile and a transport path of signal charges on equipotential lines in the principal part of the range sensor according to the seventh embodiment when a gate signal of an intermediate potential (M) is applied to the first transfer gate electrode; 
         FIG. 33  is a diagram illustrating a potential profile and a transport path of signal charges on equipotential lines in the principal part of the range sensor according to the seventh embodiment when a gate signal of an intermediate potential (M) is applied to the second transfer gate electrode; 
         FIG. 34  is a diagram illustrating a potential profile and a transport path of signal charges on equipotential lines in the principal part of the range sensor according to the seventh embodiment when a gate signal of an intermediate potential (M) is applied to the third transfer gate electrode; 
         FIG. 35  is a diagram illustrating a potential profile and an extraction path of charges on equipotential lines in the principal part of the range sensor according to the seventh embodiment when a gate signal of an intermediate potential (M) is applied to the first extraction gate electrode; 
         FIG. 36A  is a cross-sectional view of a range sensor according to another embodiment at a position corresponding to the cross section as viewed from direction II-II in  FIG. 1  for an n-type semiconductor substrate; 
         FIG. 36B  is a diagram illustrating a potential profile for the structure illustrated in  FIG. 36A ; 
         FIG. 37A  is a cross-sectional view of the range sensor according to the other embodiment at a position corresponding to the cross section as viewed from direction III-III in  FIG. 1  for the n-type semiconductor substrate; 
         FIG. 37B  is a diagram illustrating a potential profile for the structure illustrated in  FIG. 37A ; 
         FIG. 38A  is a cross-sectional view of the range sensor according to the other embodiment at a position corresponding to the cross section as viewed from direction IV-IV in  FIG. 1  for the n-type semiconductor substrate; 
         FIG. 38B  is a diagram illustrating a potential profile for the structure illustrated in  FIG. 38A ; 
         FIG. 39  is a schematic plan view from above of a principal part of a range sensor according to still another embodiment of the present invention; 
         FIG. 40A  is a cross-sectional view as viewed from direction XXXXIV-XXXXIV in  FIG. 39 ; 
         FIG. 40B  is a view illustrating a potential profile for the structure illustrated in FIG. 
         40 A; 
         FIG. 41A  is a cross-sectional view of a range sensor according to still another embodiment at a position corresponding to the cross section in  FIG. 40A ; 
         FIG. 41B  is a view illustrating a potential profile for the structure illustrated in  FIG. 41A ; 
         FIG. 42  is a schematic plan view from above of a principal part of a range sensor according to still another embodiment of the present invention; 
         FIG. 43  is a schematic plan view from above of a principal part of a range sensor according to still another embodiment of the present invention; 
         FIG. 44  is a cross-sectional view as viewed from direction II-II in  FIG. 43 ; and 
         FIG. 45  is a schematic plan view from above of a principal part of a range sensor according to still another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First to seventh embodiments of the present invention will be described below. In the drawings, the same or similar elements are indicated by the same or similar reference numerals. It should be understood that the drawings are illustrated schematically; and relationships of thicknesses and planar dimensions, and proportions of the respective layers are not drawn to scale. The specific thicknesses and dimensions should be considered in accordance with the following description. It should also be understood that the respective drawings are illustrated with the dimensional relationships and proportions different from each other. 
     Although the first to seventh embodiments of the present invention described below will be exemplified with the case in which a first conductivity type is “p-type” and a second conductivity type is “n-type”, the first and second conductivity types may be assigned in the inverted manner such that the first conductivity type is “n-type” and the second conductivity type is “p-type”. It is noted that carriers as signal charges serve as electrons when the first conductivity type is “p-type” and the second conductivity type is “n-type”, and carriers as signal charges serve as holes when the first conductivity type is “n-type” and the second conductivity type is “p-type”. In the description, directions “left and right” or “up and down” are merely defined for the convenience of description, and thus, this definition does not limit the technical spirit of the present invention. Therefore, for example, if the paper plane is rotated by 90 degrees, the “left and right” and the “up and down” are read in exchange. If the paper plane is rotated by 180 degrees, the “left” is changed to the “right”, and the “right” is changed to the “left”. 
     (First Embodiment) 
     A range sensor according to the first embodiment of the present invention is a lock-in pixel including a shielding plate  51  having an aperture indicated by the dashed-dotted line in  FIG. 1  defining a light-receiving area. As shown in the schematic plan view from above of the principal part of the range sensor in  FIG. 1 , the range sensor according to the first embodiment is provided with the shielding plate  51  including the aperture opened in a region corresponding to a photodiode of the light-receiving area, and further provided with a pixel region, excluding the aperture, covered with the shielding plate  51  so as to be shielded from light. 
     The range sensor according to the first embodiment serves as a lock-in pixel. As understood from the cross-sectional views of  FIG. 2  to  FIG. 4 , the lock-in pixel includes a pixel layer  22  made of a semiconductor of a first conductivity type (p-type), the shielding plate  51  having the aperture and provided above the pixel layer  22  to define the light-receiving area on the pixel layer  22  below the aperture, and a surface-buried region  25  of a second conductivity type (n-type) selectively buried in the upper portion of the pixel layer  22 . The surface-buried region  25  implements a photodiode in the light-receiving area, by forming a junction with the pixel layer  22 . The surface-buried region  25  extends from the light-receiving area toward plural portions shielded by the shielding plate  51  along the upper portion of the pixel layer  22  so as to delineate a plurality of branches, implementing a T-shaped configuration on the end side. A first charge-accumulation region  24   b , a second charge-accumulation region  24   d  and a third charge-accumulation region  24   c  of n-type conductivity are connected to the tips of the branches. The first charge-accumulation region  24   b , the second charge-accumulation region  24   d  and the third charge-accumulation region  24   c  have a higher impurity concentration than the surface-buried region  25 . A first transfer-control mechanism ( 31 ,  42 ), a second transfer-control mechanism ( 31 ,  44 ) and a third transfer-control mechanism ( 31 ,  43 ) are provided at the branches adjacent to the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c , respectively, so as to control a transfer of signal charges to the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c . A guide region  26   a  of n-type conductivity is buried in a part of the upper portion of the surface-buried region  25 . One end of the guide region  26   a  is provided below a part of the aperture, and other protruding-edges of the guide region  26   a  extend to at least a part of the respective transfer-control mechanisms. The guide region  26   a  has a higher impurity concentration than the surface-buried region  25  and a lower impurity concentration than the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c . As used herein, the term “branch” denotes each of protruding portions defining a branched configuration in a planar pattern. Each of the branches may be a convex portion having a rectangular shape in a mask level in photolithography. 
     As understood from the cross-sectional views of  FIG. 2  to  FIG. 4 , the range sensor according to the first embodiment further includes a p-type pinning layer  27 , which is in contact with the surface of the surface-buried region  25 . The pixel layer  22  is laminated on a p-type semiconductor substrate  21 . 
     In the range sensor according to the first embodiment shown in  FIG. 1 , the surface-buried region  25  of n-type conductivity; which implements a photodiode, has a comb-like (fork-like) shape in a planar pattern. The surface-buried region  25  collects photoelectrons as signal charges in the central portion at the base of the fork-like shape, and rapidly transfers the photoelectrons generated in the light-receiving area to a charge-modulation portion. The light-receiving area is defined in a large area of pixel. Here, the pixel size is five micrometers square or greater. The charge-modulation portion includes the first transfer-control mechanism ( 31 ,  42 ), the second transfer-control mechanism ( 31 ,  44 ) and the third transfer-control mechanism ( 31 ,  43 ). Each of prongs of the fork-like shape has a stepped shape gradually increased in width from upper-side to lower-side in the drawing of  FIG. 1 . The range sensor according to the first embodiment generates a high drift field in the entire depleted prongs of the surface-buried region  25  having a fork-like shape gradually increased in width in the planar pattern in the light-receiving area, so as to rapidly transfer the photoelectrons as signal charges in the longitudinal direction of the prongs of the fork-like shape even when the light-receiving area has a large area of which a pixel size is five micrometers square or greater. 
     The guide region  26   a  in the range sensor according to the first embodiment is a semiconductor region for guiding the photoelectrons collected in the central portion at the base of the fork-like shape to the respective narrow transfer channels in the charge-modulation portion. The guide region  26   a  has a stepped shape of which the width in the direction perpendicular to the longitudinal direction of the guide region  26   a  (in the vertical direction in  FIG. 1 ) is gradually increased from upper-side to lower-side in the drawing of  FIG. 1 . The range sensor according to the first embodiment generates a high drift field in the entire depleted guide region  26   a  gradually increased in width in the planar pattern. Accordingly, the photoelectrons as signal charges can rapidly be transferred in the longitudinal direction of the guide region  26   a . As understood from the plan view of  FIG. 1  and the potential profile of  FIG. 5B , the narrow tip of the guide region  26   a  is in contact with the surface-buried region  25  at the bottom of the potential profile, because carriers generated are transferred to a position at the bottom of the potential profile in the surface-buried region  25 . For example, the guide region  26   a  is formed as a semiconductor region having a higher impurity concentration than the surface-buried region  25  such that a part of an ion-implanted area for forming the surface-buried region  25  is doubly ion-implanted in the planar pattern shown in  FIG. 1 . 
     As shown in.  FIG. 3A  and  FIG. 4A , the first transfer-control mechanism ( 31 ,  42 ), the second transfer-control mechanism ( 31 ,  44 ) and the third transfer-control mechanism ( 31 ,  43 ) implementing the range sensor according to the first embodiment include an insulating film  31  provided on the respective branches, and further include a first transfer gate electrode  42 ,  a  second transfer gate electrode  44  and a third transfer gate electrode  43 , respectively, provided on the insulating film  31 . As shown in the cross-sectional views of  FIG. 3A  and  FIG. 4A , the thickness of the insulating film  31  immediately below the respective first transfer gate electrode  42 , second transfer gate electrode  44  and third transfer gate electrode  43  is thinner than the other sites, and the thinner parts of the insulating film  31  each serve as a “gate insulating film”. The voltage applied to each of the first transfer gate electrode  42 , the second transfer gate electrode  44  and the third transfer gate electrode  43  controls potentials in the transfer channels defined by the respective branches, so as to control the signal charge transferred to the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c.    
     As understood from  FIG. 3A ,  FIG. 4A  and  FIG. 5A , the p-type pinning layer  27  is buried in a part of the surface-buried region  25 , and a part of the insulating film  31  having a greater thickness is provided in contact with the pinning layer  27 . Although the guide region  26   a  cannot be viewed from above because of the upper elements such as the insulating film  31  and the pinning layer  27 , the guide region  26   a  has a planar pattern in which the end portion of the guide region  26   a  toward the photodiode is exposed to the aperture of the shielding plate  51 , and other portions are shielded by the shielding plate  51 , in the plan view of the shielding plate  51  as viewed from above. The guide region  26   a  shown in  FIG. 3A  either has a constant depth or varies in depth, but preferably has a constant depth because of easiness of manufacture. 
     The insulating film  31 , when serving as a gate insulating film, is preferably a silicon oxide film (SiO 2  film), but may use various kinds of film other than the SiO 2  film to have an insulating gate structure for an insulated gate transistor (MIS transistor). For example, the insulating film  31  may be an ONO film having a triple-layered structure including a silicon oxide film (SiO 2  film), a silicon nitride film (Si 3 N 4  film), and a silicon oxide film (SiO 2  film). Further, a gate insulating film may be used including an oxide containing at least one element selected from strontium (Sr), aluminum (Al), magnesium (Mg), yttrium (Y), hafnium (Hf), zirconium (Zr), tantalum (Ta), and bismuth (Bi), or silicon nitride containing at least one element selected from the above-listed elements. 
     The actual structure of the insulating film  31  may be a double-layered structure having a stepped shape obtained such that an interlayer insulating film is selectively deposited on a thin insulating film serving as a gate insulating film to surround the first transfer gate electrode  42 , the second transfer gate electrode  44  and the third transfer gate electrode  43 . Alternatively, the insulating film  31  having a stepped shape may be obtained such that an interlayer insulating film or a field insulating film having a thickness different from that of a gate insulating film is selectively deposited on a region other than the gate insulating film to surround the first transfer gate electrode  42 , the second transfer gate electrode  44  and the third transfer gate electrode  43 . 
     The material used for the interlayer insulating film or the field insulating film may be the same as that used for the gate insulating film or may be a different dielectric. For example, the interlayer insulating film may be a dielectric of which relative dielectric constant is smaller than the gate insulating film. Reference numeral  32  shown in  FIG. 1  denotes an edge of the field insulating film, and the edge  32  defines an active area. As shown in  FIG. 3A ,  FIG. 4A  and  FIG. 5A , a p-type well region  23  is buried below the field insulating film. 
     As understood from  FIG. 3A  and  FIG. 4A , the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c  are surrounded by the well region  23  to serve as floating diffusion layers provided above the pixel layer  22 . Although the range sensor according to the first embodiment has a structure for illustration purposes in which the number of the floating diffusion layers in which signal charges are accumulated is three, the number of the floating diffusion layers may be two or more than three.  FIG. 3A ,  FIG. 4A  and  FIG. 5A  merely illustrate the case in which the well region  23  is provided to cover regions other than the photodiode per pixel. Since the well region  23  is preferably restrictedly buried only in transistors (see  FIG. 10 ) of the “charge-modulation portion” implemented by the first transfer-control mechanism ( 31 ,  42 ), the second transfer-control mechanism ( 31 ,  44 ) and the third transfer-control mechanism ( 31 ,  43 ) so as not to have an influence on the potential profile, the present invention is not intended to be limited to the arrangement pattern of the well region  23  shown in  FIG. 3A ,  FIG. 4A  and  FIG. 5A  (the arrangement pattern of the well region  23  in the second to seventh embodiments is also the same, and overlapping explanations will not be repeated below). 
     A portion extending to the middle bar of the T shape of the surface-buried region  25  in the top view of  FIG. 1  and corresponding to a straight portion provided toward the light-receiving area is defined as a “trunk path” in the present invention. The range sensor according to the first embodiment is further provided with additional branches extending adjacent to the light-receiving area in the direction perpendicular to the longitudinal direction of the trunk path of the surface-buried region  25 . The branches provided in the trunk path adjacent to the light-receiving area have a planar topology; as the entire shape of the surface-buried region  25  provided below the shielding plate  51 , having a fishbone shape in which the trunk path in the extending direction corresponds to a backbone part. The respective tips of the additional branches formed into the fishbone shape are connected with a first extraction region  24   a  and a second extraction region  24   e  of n-type conductivity having a higher impurity concentration than the surface-buried region  25 . 
     As shown in  FIG. 1 , the range sensor according to the first embodiment further includes a first extraction control mechanism ( 31 ,  41   a ) provided adjacent to the first extraction region  24   a  and configured to control extraction of charges toward the first extraction region  24   a  via one of the additional branches extending in the left direction of the surface-buried region  25 , and a second extraction control mechanism ( 31 ,  41   b ) provided adjacent to the second extraction region  24   e  and configured to control extraction of charges toward the second extraction region  24   e  via the other branch extending in the right direction of the surface-buried region  25 . 
     As shown in  FIG. 2A , the first extraction control mechanism ( 31 ,  41   a ) and the second extraction control mechanism ( 31 ,  41   b ) include the insulating film  31  provided on the respective additional branches, and further include a first extraction gate electrode  41   a  and a second extraction gate electrode  41   b , respectively, provided on the insulating film  31 . As shown in the cross-sectional view of  FIG. 2A , the thickness of the insulating film  31  immediately below the respective first extraction gate electrode  41   a  and second extraction gate electrode  41   b  is thinner than the other sites, and the thinner parts of the insulating film  31  each serve as a gate insulating film. As understood from  FIG. 2A , the p-type pinning layer  27  is buried in a part of the surface-buried region  25 , and a part of the insulating film  31  having a greater thickness is provided in contact with the pinning layer  27 . As shown in  FIG. 2A , the p-type well region  23  is provided below the thicker portion of the insulating film  31  serving as a field insulating film. 
     In other words, as shown in  FIG. 1 ., the range sensor according to the first embodiment is provided with the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  symmetrically arranged to interpose the trunk path in front of the branch portions of the T shape between the photodiode and the charge-modulation portion. Accordingly, the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  can switch the operation between the extraction of photoelectrons toward the first extraction region  24   a  and the second extraction region  24   e  and the transport of the photoelectrons toward the charge-modulation portion including the first transfer-control mechanism ( 31 ,  42 ), the second transfer-control mechanism ( 31 ,  44 ) and the third transfer-control mechanism ( 31 ,  43 ). 
       FIG. 2A  is a cross-sectional view as viewed from direction II-II in  FIG. 1 , and  FIG. 2B  is a view illustrating a potential profile corresponding to the cross section of  FIG. 2A .  FIG. 3A  is a cross-sectional view as viewed from direction in  FIG. 1 , and  FIG. 3B  is a view illustrating a potential profile corresponding to the cross section of  FIG. 3A . When an intermediate potential (M) is applied to the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  arranged symmetrically, a U-shaped potential channel is thrilled in a static induction channel of the trunk path extending to the middle bar of the T shape, while potential barriers remain in the cross-sectional direction as viewed from direction II-II as shown in  FIG. 2B . As shown in the potential profile in  FIG. 3B , no potential barrier appears in a static induction channel in the cross-sectional view as viewed from direction III-III from the trunk path toward the third charge-accumulation region  24   c  of the charge-modulation portion. The photoelectrons generated in the photodiode are rapidly transferred to the charge-modulation portion as signal charges via the U-shaped potential channel formed in the static induction channel. 
     When gate signal G D  of a high potential (H) higher than the intermediate potential (M) is applied to the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  in the range sensor according to the first embodiment, the potential barriers on both sides disappear, as indicated by the broken line in the potential profile in the cross-sectional view as viewed from direction II-II as shown in  FIG. 2B . Namely, when the gate signal G D  of the high potential (H) higher than the intermediate potential (M) is applied to the first extraction gate electrode  41   a , a potential profile indicated by equipotential lines in the plane as shown in  FIG. 9  appears. Electrons reached the static induction channel formed in the trunk path are directed to the first extraction region  24   a  to be extracted along the path as indicated by the very thick solid line shown in  FIG. 9 . The thick solid line in  FIG. 9  indicates an equipotential line in which a potential is −0.2 V (indicated by “−0.2 V” in  FIG. 9 ). The dashed-dotted line in  FIG. 9  indicates an equipotential line in which a potential is 0 V or greater and less than 1 V (indicated by “0 V” in  FIG. 9 ) (at 0.25 V intervals from 0 V). The thin solid line in  FIG. 9  indicates an equipotential line in which a potential is 1 V or greater and less than 2 V (indicated by “1 V” in  FIG. 9 ) (at 0.25 V intervals from 1 V). The fine broken line in  FIG. 9  indicates an equipotential line in which a potential is 2 V or greater and less than 3 V (indicated by “2 V” in  FIG. 9 ) (at 0.25 V intervals from 2 V). The long broken line in  FIG. 9  indicates an equipotential line in which a potential is 3 V or greater and less than 4 V (indicated by “3 V” in  FIG. 9 ) (at 0.25 V intervals from 3 V). When the gate signal G D  of the high potential (H) is applied to the second extraction gate electrode  41   b , although the potential profile is not shown in the drawing, electrons reached the static induction channel formed in the trunk path are directed to the second extraction region  24   e  to be extracted. 
     When the high potential (H) is applied to the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b , the potential profile toward the third charge-accumulation region  24   c  of the charge-modulation portion shows a topology in which the potential is high in the static induction channel adjacent to a portion immediately below the third transfer gate electrode  43 , and a dip is provided at the guide region  26   a  as indicated by the broken line in  FIG. 3B . Thus, the photoelectrons generated in the photodiode cannot reach the third charge-accumulation region  24   c  of the charge-modulation portion. 
     When the potential applied to the respective first extraction gate electrode  41   a  and second extraction gate electrode  41   b  is set at the intermediate potential (M), the dip at the guide region  26   a  disappears, as indicated by the solid line in  FIG. 3B , so that the photoelectrons generated in the photodiode as signal charges reach the third charge-accumulation region  24   c  of the charge-modulation portion. The trunk path adjacent to the photodiode, the static induction channel from the trunk path to the charge-modulation portion, and the first transfer-control mechanism ( 31 ,  42 ), the second transfer-control mechanism ( 31 ,  44 ) and the third transfer-control mechanism ( 31 ,  43 ) implementing the charge-modulation portion are shielded from light by the shielding plate  51  so as to prevent direct incident light. 
     It is characteristic of the range sensor according to the first embodiment to provide the static induction channel with a sufficient length from the trunk path adjacent to the light-receiving area to the charge-modulation portion, and shield the charge-modulation portion including the static induction channel by the shielding plate  51 . These characteristics achieve remarkable effects upon reducing the influence of background light while using a pulse at short intervals. 
     As shown in  FIG. 1 , the charge-modulation portion in the range sensor according to the first embodiment is provided with three transfer gate electrodes: the first transfer gate electrode  42 , the second transfer gate electrode  44 , and the third transfer gate electrode  43 . When gate signal G 1  of the intermediate potential (M) is applied to the first transfer gate electrode  42 , the potential profile as shown in  FIG. 6  appears. When gate signal G 2  of the intermediate potential (M) is applied to the second transfer gate electrode  44 , the potential profile as shown in  FIG. 7  appears. When gate signal G 3  of the intermediate potential (M) is applied to the third transfer gate electrode  43 , the potential profile as shown in  FIG. 8  appears. Thus, the gate signal of the intermediate potential (M) is applied to each of the first transfer gate electrode  42 , the second transfer gate electrode  44  and the third transfer gate electrode  43 , so as to rapidly transfer the signal charges to the corresponding first charge-accumulation region  24   b , second charge-accumulation regions  24   d  and third charge-accumulation region  24   c  along the respective paths as indicated by the thick solid lines in  FIG. 6  to  FIG. 8 . This is the principal operation of the range sensor for detecting photoelectrons in synchronization with a light pulse. In  FIG. 6  to  FIG. 8 , the thick solid line indicates an equipotential line in which a potential is −0.2 V (indicated by “−0.2 V” in  FIG. 6  to  FIG. 8 ). The dashed-dotted line indicates an equipotential line in which a potential is 0 V or greater and less than 1 V (indicated by “0 V” in  FIG. 6  to  FIG. 8 ) (at 0.25 V intervals from 0 V). The thin solid line indicates an equipotential line in which a potential is 1 V or greater and less than 2 V (indicated by “1 V” in  FIG. 6  to  FIG. 8 ) (at 0.25 V intervals from  1  V). The fine broken line indicates an equipotential line in which a potential is 2 V or greater and less than 3 V (indicated by “2 V” in  FIG. 6  to  FIG. 8 ) (at 0.25 V intervals from 2 V). The long broken line indicates an equipotential line in which a potential is 3 V or greater and less than 4 V (indicated by “3 V” in  FIG. 6  to  FIG. 8 ) (at 0.25 V intervals from 3 V). 
     The number of the transfer gate electrodes provided in the charge-modulation portion may be two or four or more. However, the area of the charge-modulation portion is preferably reduced as much as possible. For example, the area of the charge-modulation portion is preferably set at less than or equal to a quarter of the area of the photodiode. 
     When the range sensor according to the first embodiment is used as a range image sensor for implementing time-of-flight measurement while canceling the influence (offset) of background light, the use of three outputs is effective.  FIG. 3B  and  FIG. 4B  show a voltage level of the gate signal G 1  applied to the first transfer gate electrode  42 ,  a  voltage level of the gate signal G 3  applied to the third transfer gate electrode  43 ,  a  voltage level of the gate signal G 2  applied to the second transfer gate electrode  44 , and a change in the potential profile of the corresponding charge-modulation portion when the range sensor according to the first embodiment is provided with three outputs. 
     While the intermediate potential (M) is applied to each of the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b , the voltage level G 1  of the gate signal applied to the first transfer gate electrode  42  is set at the low potential (L), the voltage level G 2  of the gate signal applied to the second transfer gate electrode  44  is set at the low potential (L), and the voltage level G 3  of the gate signal applied to the third transfer gate electrode  43  is set at the intermediate potential (M) or the low potential (L) lower than the intermediate potential (M), so as to control whether to transfer the photoelectrons to the third charge-accumulation region  24   c  implemented as a floating diffusion layer, as shown in  FIG. 3 . 
     In addition, the voltage level G 3  of the gate signal applied to the third transfer gate electrode  43  is set at the low potential (L), and the voltage level G 1  of the gate signal applied to the first transfer gate electrode  42  and the voltage level G 2  of the gate signal applied to the second transfer gate electrode  44  are set at either the low potential (L) or the intermediate potential (M) differently from each other, so as to transfer the photoelectrons to either the first charge-accumulation region  24   b  or the second charge-accumulation region  24   d , as shown in  FIG. 4 . 
       FIG. 10  illustrates a circuit diagram of an actual one pixel in the range sensor according to the first embodiment. The “charge-modulation portion” in the range sensor according to the first embodiment is implemented by three transistors connected in parallel in the middle in  FIG. 10 : a first transfer transistor Q 1T  as the first transfer-control mechanism ( 31 ,  42 ), a second transfer transistor Q 2T  as the second transfer-control mechanism ( 31 ,  44 ), and a third transfer transistor Q 3T  as the third transfer-control mechanism ( 31 ,  43 ). 
     In  FIG. 10 , the static induction channel from the photodiode to the charge-modulation portion is indicated by junction field-effect transistors Q P1  and Q P2  of which own gates are grounded. A source terminal of a MOS transistor Q D  for charge extraction is connected to a center tap of the two junction field-effect transistors Q P1  and Q P2  connected in series, and a drain terminal of the MOS transistor Q D  is connected to a power source V DD  having a high potential. 
     The broken lines in  FIG. 10  indicate paths in which electrons flow rapidly due to a depletion electric field because of the connection in the semiconductor region in  FIG. 10 , photoelectrons generated in the photodiode D ij  immediately reach the charge-modulation portion when the voltage G D  applied to each of the two first extraction gate electrode  41   a  and second extraction gate electrode  41   b  implementing the MOS transistor Q D  is set at the low potential (L). When the voltage at the intermediate potential (M) is applied to one of the first transfer gate electrode  42 , the second transfer gate electrode  44  and the third transfer gate electrode  43 , and the voltage at the low potential (L) is applied to the other two electrodes, the photoelectrons are transferred to one of the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c . The equivalent circuit is illustrated with the first transfer transistor Q 1T , the second transfer transistor Q 2T  and the third transfer transistor Q 3T  implementing the charge-modulation portion, each of which one end is connected to the junction field-effect transistor Q P2  in the T-shaped manner. 
     In the circuit diagram, the other terminals of the first transfer transistor Q 1T , the second transfer transistor Q 2T  and the third transfer transistor Q 3T  are connected to the first charge-accumulation region  24   b  serving as node D 1 , the second charge-accumulation region  24   d  serving as node D 2  and the third charge-accumulation region  24   c  serving as node D 3 , respectively. 
     The first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c  serving as the three nodes D 1 , D 3  and D 2  shown in the circuit diagram in  FIG. 10  are connected to capacitors C 1 , C 3  and C 2 , for charge-accumulation. The capacitors C 1 , C 3  and C 2  are preferably depletion-mode MOS capacitors in which a threshold voltage is set at a negative voltage so as to reduce voltage dependence. The first node D 1  is connected to a gate terminal of a first amplification transistor Q 1A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the first node D 1 . The first node D 1  is further connected to a first reset transistor Q 1R  for initializing signal charges after signal readout. A source terminal of the first amplification transistor Q 1A  is connected to a first selection transistor Q 1s , serving as a switch for readout image selection. An output of the first selection transistor Q 1s  is connected to a signal readout line extending in the vertical direction. 
     The second node D 2  is connected to a gate terminal of a second amplification transistor Q 2A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the second node D 2 . The second node D 2  is further connected to a second reset transistor Q 2R  for initializing signal charges after signal readout. A source terminal of the second amplification transistor Q 2A  is connected to a second selection transistor Q 2s , serving as a switch for readout image selection. An output of the second selection transistor Q 2s  is connected to a signal readout line extending in the vertical direction. 
     The third node D 3  is connected to a gate terminal of a third amplification transistor Q 3A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the third node D 3 . The third node D 3  is further connected to a third reset transistor Q 3R  for initializing signal charges after signal readout. A source terminal of the third amplification transistor Q 3A  is connected to a third selection transistor Q 3S , serving as a switch for readout image selection. An output of the third selection transistor Q 3S  is connected to a signal readout line extending in the vertical direction. 
     The signal readout method in the range sensor according to the first embodiment may be either a method of reading out a signal from each of three signal readout lines arranged in parallel as shown in  FIG. 10 , or a method of turning on a switch sequentially by selection signals SL 1 , SL 2  and SL 3  to read out each signal as a time-series signal from a single signal readout line as shown in  FIG. 11 . 
       FIG. 12  is a timing diagram when the range sensor according to the first embodiment is used as a range image sensor. A pulse width T 0  of outgoing light is the same as a pulse width of each of the gate signal G 1  applied to the first transfer gate electrode  42 , the gate signal G 3  applied to the third transfer gate electrode  43  and the gate signal G 2  applied to the second transfer gate electrode  44 . When a delay time (time-of-flight) of a pulse of first incoming light (received light) is T d1  with respect to the pulse of the outgoing light in  FIG. 12 , a first stored charges Q 1  accumulated in the first charge-accumulation region  24   b , a second stored charges Q 2  accumulated in the second charge-accumulation region  24   d , and a third stored charges Q 3  accumulated in the third charge-accumulation region  24   c  are each obtained according to the following Eq. (a photocurrent generated is defined as I ph ):
 
 Q 1= I   ph ( T   0   −T   d )+1 a   T   0   (1)
 
 Q 2 =I   ph   T   d +1 a   T   0   (2)
 
Q3=1 a   T   0   (3)
 
     The delay time T d1  of the first incoming light is obtained according to the following Eq. with reference to the Eq. (1) to (3):
 
 T   d1   =T   0 ( Q 2 −Q 3)/( Q 1 +Q 2−2 Q 3)  (4)
 
     As shown in  FIG. 12 , when a delay time (time-of-flight) of a pulse of second incoming light is T d2  with respect to the pulse of the outgoing light, the first stored charges Q 1  accumulated in the first charge-accumulation region  24   b , the second stored charges Q 2  accumulated in the second charge-accumulation region  24   d , and the third stored charges Q 3  accumulated in the third charge-accumulation region  24   c  are each obtained according to the following Eq.:
 
Q1=1 a   T   0   (5)
 
 Q 2 =I   ph (2 T   0   −T   d2 )+1 a   T   0   (6)
 
 Q 3 =I   ph ( T   d2   −T   0 )+1 a   T   0   (7)
 
     The delay time T d2  of the second incoming light is obtained according to the following Eq. with reference to the Eq. (5) to (7):
 
 T   d2   =T   0   +T   0 ( Q 3 −Q 1)/( Q 2 +Q 3−2 Q 1)   (8)
 
     Whether or not the time-of-flight of the light pulse is greater than the pulse width T 0  of the outgoing light can be confirmed by the comparison between the first stored charges Q 1  and the third stored charges Q 3 . If Q 1 &gt;Q 3 , the time-of-flight of the light pulse is calculated according to the Eq. (4). If Q 1 ≤Q 3 , the time-of-flight of the light pulse is calculated according to the Eq. (8). 
     The range sensor according to the first embodiment has the function suitable for the time-of-flight measurement by use of the Eq. (4) or the Eq. (8). The range sensor according to the first embodiment thus can serve as a pixel (lock-in pixel) of a solid-state imaging device to achieve a time-of-flight range image sensor capable of rapid transfer of signal charges with high sensitivity at a low dark current even when a plurality of pixels each having a large area of which a pixel size is five micrometers square or greater are provided on a single semiconductor chip. 
     &lt;Modified Example of First Embodiment&gt; 
     As shown in the schematic plan view from above of the principal part in  FIG. 13 , a range sensor according to a modified example of the first embodiment is a lock-in pixel including a shielding plate  51  having an aperture defining a light-receiving area. Although not shown in the drawings, the range sensor according to the modified example of the first embodiment has the same cross sections as shown in  FIG. 2  to  FIG. 4  and includes: a pixel layer  22  made of a semiconductor of a first conductivity type (p-type); the shielding plate  51  having the aperture and provided above the pixel layer  22  to define the light-receiving area on the pixel layer  22  below the aperture; a surface-buried region  25  of a second conductivity type (n-type) selectively buried in the upper portion of the pixel layer  22  to implement a photodiode in the light-receiving area by forming a junction with the pixel layer  22 , and extending from the light-receiving area toward plural portions shielded by the shielding plate  51  along the upper portion of the pixel layer  22  so as to delineate a plurality of branches, implementing a T-shaped configuration on the end side; a first charge-accumulation region  24   b , a second charge-accumulation region  24   d  and a third charge-accumulation region  24   c  of n-type conductivity; which is connected to the tips of the branches and having a higher impurity concentration than the surface-buried region  25 ; and a first transfer-control mechanism ( 31 ,  42 ), a second transfer-control mechanism ( 31 ,  44 ) and a third transfer-control mechanism ( 31 ,  43 ) provided at the branches adjacent to the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c , respectively, to control a transfer of signal charges to the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c.    
     The range sensor according to the modified example of the first embodiment is also provided with additional branches extending adjacent to the light-receiving area in the direction perpendicular to the longitudinal direction of the trunk path of the surface-buried region  25 , in the same manner as the topology shown in  FIG. 1 . The respective tips of the branches are connected with a first extraction region  24   a  and a second extraction region  24   e  of n-type conductivity having a higher impurity concentration than the surface-buried region  25 . 
     As understood from the cross-sectional view of  FIG. 14 , a guide region  26   b  buried in a part of the upper portion of the surface-buried region  25  and having a higher impurity concentration than the surface-buried region  25  and a lower impurity concentration than the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c , differs in the topology from the guide region  26   a  shown in  FIG. 1 . In the range sensor according to the modified example of the first embodiment, the guide region  26   b  differs from the guide region  26   a  shown in  FIG. 1  in the topology including a path from one end provided below a part of the aperture of the shielding plate  51  to other ends extending to at least part of the respective transfer-control mechanisms, and a path extending to the first extraction region  24   a  and the second extraction region  24   e  along the additional branches extending in the direction perpendicular to the longitudinal direction of the trunk path. 
     As shown in  FIG. 13 , the range sensor according to the modified example of the first embodiment includes a first extraction control mechanism ( 31 ,  41   a ) provided in the guide region  26   b  provided at the branch extending in the left direction of the surface-buried region  25  adjacent to the first extraction region  24   a  and configured to control extraction of charges toward the first extraction region  24   a  via one of the additional branches on the left side, and a second extraction control mechanism ( 31 ,  41   b ) provided in the guide region  26   b  provided at the branch extending in the right direction of the surface-buried region  25  adjacent to the second extraction region  24   e  and configured to control extraction of charges toward the second extraction region  24   e  via the other branch on the right side. 
     Namely, as shown in  FIG. 14 , the guide region  26   b  is buried below a first extraction gate electrode  41   a  implementing the first extraction control mechanism ( 31 ,  41   a ) and below a second extraction gate electrode  41   b  implementing the second extraction control mechanism ( 31 ,  41   b ). As shown in  FIG. 13 , the guide region  26   b  extends below the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  to reach the first extraction region  24   a  and the second extraction region  24   e , so as to control extraction of photoelectrons at relatively low extraction gate voltage G D . 
     The range sensor according to the modified example of the first embodiment has the same structure as shown in  FIG. 3A  and  FIG. 4A , in which a p-type pinning layer  27  is provided in contact with the surface of the surface-buried region  25 , and the pixel layer  22  is laminated on a p-type semiconductor substrate  21 . The other elements, including the surface-buried region  25  of n-type conductivity, which implements a photodiode having a comb-like (fork-like) shape in a plan view to collect photoelectrons in the central portion at the base of the fork-like shape so as to rapidly transfer the photoelectrons generated in the light-receiving area having a large area of which the pixel size is five micrometers square or greater to the charge-modulation portion including the first transfer-control mechanism ( 31 ,  42 ), the second transfer-control mechanism ( 31 ,  44 ) and the third transfer-control mechanism ( 31 ,  43 ), are also the same as in the range sensor according to the first embodiment shown in  FIG. 1 , and overlapping explanations are not repeated below. 
     Since the range sensor according to the modified example of the first embodiment includes the guide region  26   b  having the planar topology as shown in  FIG. 13 , the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  can switch the operation between the extraction of photoelectrons toward the first extraction region  24   a  and the second extraction region  24   e  and the transport of the photoelectrons toward the charge-modulation portion including the first transfer-control mechanism ( 31 ,  42 ), the second transfer-control mechanism ( 31 ,  44 ) and the third transfer-control mechanism ( 31 ,  43 ). 
     (Second Embodiment) 
     A range sensor according to the second embodiment of the present invention is a lock-in pixel including a shielding plate  51  having an aperture indicated by the dashed-dotted line in  FIG. 15  to define a light-receiving area. The range sensor according to the second embodiment fundamentally has the same structure as the range sensor according to the first embodiment, as shown in the cross-sectional views of  FIG. 2  to  FIG. 4 , and serves as a lock-in pixel including: a pixel layer  22  made of a semiconductor of a first conductivity type (p-type); the shielding plate  51  having the aperture and provided above the pixel layer  22  to define the light-receiving area on the pixel layer  22  below the aperture; a surface-buried region  25  of a second conductivity type (n-type) selectively buried in the upper portion of the pixel layer  22  to implement a photodiode in the light-receiving area by forming a junction with the pixel layer  22 , and extending from the light-receiving area toward plural portions shielded by the shielding plate  51  along the upper portion of the pixel layer  22  so as to delineate a plurality of branches, implementing a T-shaped configuration on the end side; a first charge-accumulation region  24   b , a second charge-accumulation region  24   d  and a third charge-accumulation region  24   c  of n-type conductivity, which is connected to the tips of the branches and having a higher impurity concentration than the surface-buried region  25 ;  a  first transfer-control mechanism ( 31 ,  42   p ,  42   q ), a second transfer-control mechanism ( 31 ,  44   p ,  44   q ) and a third transfer-control mechanism ( 31 ,  43   p ,  43   q ) provided at the branches adjacent to the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c , respectively, to control a transfer of signal charges to the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c;  and an n-type guide region  26   a  buried in a part of the upper portion of the surface-buried region  25  with one end provided below a part of the aperture and other protruding-edges extending to at least a part of the respective transfer-control mechanisms, and having a higher impurity concentration than the surface-buried region  25  and a lower impurity concentration than the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c.    
     The first transfer-control mechanism ( 31 ,  42   p ,  42   q ) implementing the range sensor according to the second embodiment differs from the transfer-control mechanism in the range sensor according to the first embodiment in that the first transfer-control mechanism ( 31 ,  42   p ,  42   q ) includes a pair of first field-control electrodes  42   p  and  42   q  deposited on the pixel layer  22  via an insulating film while interposing the branch extending on the left side of the T-shaped topology. The insulating film is a “gate insulating film” of which the thickness immediately below the pair of the first field-control electrodes  42   p  and  42   q  is thinner than the other sites, as in the case of the insulating film  31  provided immediately below the first transfer gate electrode  42 , the second transfer gate electrode  44  and the third transfer gate electrode  43  as shown in  FIG. 3A  and  FIG. 4A . The insulating film in the range sensor according to the second embodiment may use the same material as the insulating film  31  in the range sensor according to the first embodiment. 
     The first field-control electrodes  42   p  and  42   q  are opposed to each other while interposing the branch extending on the left side and provided in the direction perpendicular to the transfer direction of signal charges along the branch on the left side. The operation mechanism of the first field-control electrodes  42   p  and  42   q  differs from the transfer-control mechanism in the range sensor according to the first embodiment in exhibiting a “lateral electric field control effect” of controlling the movement of signal charge transferred in the branch extending on the left side by applying an field-control voltage to the respective first field-control electrodes  42   p  and  42   q  to change a depletion potential in the branch extending on the left side. 
     The second transfer-control mechanism ( 31 ,  44   p ,  44   q ) implementing the range sensor according to the second embodiment includes a pair of second field-control electrodes  44   p  and  44   q  deposited on the pixel layer  22  via an insulating film while interposing the branch extending on the right side of the T-shaped topology. The insulating film is a “gate insulating film” of which the thickness immediately below the second field-control electrodes  44   p  and  44   q  is thinner than the other sites. The second field-control electrodes  44   p  and  44   q  are opposed to each other while interposing the branch extending on the right side and provided in the direction perpendicular to the transfer direction of signal charges along the branch on the right side. An field-control voltage, which is different from that applied to the respective second field-control electrodes  42   p  and  42   q , is applied to the respective second field-control electrodes  44   p  and  44   q  to change a depletion potential in the branch extending on the right side, so as to control the movement of signal charge transferred in the branch extending on the right side. 
     The third transfer-control mechanism ( 31 ,  43   p ,  43   q ) implementing the range sensor according to the second embodiment includes a pair of third field-control electrodes  43   p  and  43   q  deposited on the pixel layer  22  via an insulating film while interposing the branch extending on the bottom side of the T-shaped topology. The insulating film is a “gate insulating film” of which the thickness immediately below the third field-control electrodes  43   p  and  43   q  is thinner than the other sites. The third field-control electrodes  43   p  and  43   q  are opposed to each other while interposing the branch extending on the bottom side and provided in the direction perpendicular to the transfer direction of signal charges along the branch on the bottom side. An field-control voltage, which is different from that applied to the first field-control electrodes  42   p  and  42   q  and the second field-control electrodes  44   p  and  44   q , is applied to the respective third field-control electrodes  43   p  and  43   q  to change a depletion potential in the branch extending on the bottom side, so as to control the movement of signal charge transferred in the branch extending on the bottom side. 
     The range sensor according to the second embodiment s also provided with additional branches extending adjacent to the light-receiving area in the direction perpendicular to the longitudinal direction of the trunk path of the surface-buried region  25 , as shown in the top view of  FIG. 15 . The respective tips of the branches are connected with a first extraction region  24   a  and a second extraction region  24   e  of n-type conductivity having a higher impurity concentration than the surface-buried region  25 . 
     As shown in  FIG. 15 , the range sensor according to the second embodiment includes a first extraction control mechanism ( 31 ,  41   a ) provided adjacent to the first extraction region  24   a  and configured to control extraction of charges toward the first extraction region  24   a  via one of the additional branches extending in the left direction of the surface-buried region  25 , and a second extraction control mechanism ( 31 ,  41   b ) provided adjacent to the second extraction region  24   e  and configured to control extraction of charges toward the second extraction region  24   e  via the other branch extending in the right direction of the surface-buried region  25 , as in the case of the range sensor according to the first embodiment. 
     The range sensor according to the second embodiment has substantially the same structure with regard to the other elements as the range sensor according to the first embodiment, in which the first extraction control mechanism ( 31 ,  41   a ) and the second extraction control mechanism ( 31 ,  41   b ) include an insulating film provided on the respective additional branches and further include a first extraction gate electrode  41   a  and a second extraction gate electrode  41   b , respectively, provided on the insulating film, as in the case of the cross section shown in  FIG. 2A ; a p-type pinning layer  27  is provided in contact with the surface of the surface-buried region  25 , as in the case of the cross sections of  FIG. 2  to  FIG. 4 ; and the pixel layer  22  is laminated on a p-type semiconductor substrate  21 . Thus, overlapping explanations are not repeated below. 
     As shown in  FIG. 15 , the range sensor according to the second embodiment is provided with the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  symmetrically arranged to interpose the trunk path in front of the branch portions of the T shape between the photodiode and the charge-modulation portion. Accordingly,the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  can switch the operation between the extraction of photoelectrons toward the first extraction region  24   a  and the second extraction region  24   e  and the transport of the photoelectrons toward the charge-modulation portion including the first transfer-control mechanism ( 31 ,  42   p ,  42   q ), the second transfer-control mechanism ( 31 ,  44   p ,  44   q ) and the third transfer-control mechanism ( 31 ,  43   p ,  43   q ). 
     The range sensor according to the second embodiment also has the function suitable for the time-of-flight measurement by use of the Eq. (4) or the Eq. (8) as described in the first embodiment. The range sensor according to the second embodiment thus can serve as a pixel (lock-in pixel) of a solid-state imaging device to achieve a time-of-flight range image sensor capable of rapid transfer of signal charges with high sensitivity at a low dark current even when a plurality of pixels each having a large area of which a pixel size is five micrometers square or greater are provided on a single semiconductor chip. 
     As in the case of the range sensor according to the first embodiment, the range sensor according to the second embodiment is provided with the static induction channel having a sufficient length from the trunk path adjacent to the light-receiving area to the charge-modulation portion, which is shielded by the shielding plate  51 , so as to achieve remarkable effects upon reducing the influence of background light while using a pulse at short intervals. 
     The range sensor and the solid-state imaging device according to the second embodiment exhibit the lateral electric field control effect by applying the field-control voltage to each of the first field-control electrodes  42   p  and  42   q , the second field-control electrodes  44   p  and  44   q  and the third field-control electrodes  43   p  and  43   q , so as to transfer charges to the first charge-accumulation region  24   b , the second charge-accumulation region  24   d  and the third charge-accumulation region  24   c  shown in  FIG. 15  more rapidly than the case of using “the transfer gate method” as described in the first embodiment. 
     (Third Embodiment) 
     A range sensor according to the third embodiment of the present invention is a lock-in pixel including a first surface-buried region  25   u  and a second surface-buried region  25   v  opposed in an interdigital manner inside a light-receiving area defined by a shielding plate  51  having a single aperture indicated by the dashed-dotted line as shown in the schematic plan view from above of the principal part in  FIG. 16 . Reference numeral  32   b  shown in  FIG. 16  denotes an edge of a field insulating film. Namely, an area surrounded by the edge  32   b  defines an active area of the lock-in pixel. The range sensor according to the third embodiment thus includes two surface-buried regions in one pixel. As in the case shown in the cross sections of  FIG. 2A ,  FIG. 3A  and  FIG. 4A , a p-type well region  23  is buried below the field insulating film (not shown). 
     When focused on the first surface-buried region  25   u  provided in the light-receiving area on the upper side in  FIG. 16 , the range sensor according to the third embodiment has a structure on one side of the lock-in pixel similar to that shown in the cross sections of  FIG. 2  to  FIG. 4 , including: a pixel layer  22  made of a semiconductor of a first conductivity type (p-type); the first surface-buried region  25   u  of a second conductivity type (n-type) selectively buried in the upper portion of the pixel layer  22  to implement a first photodiode in the light-receiving area by forming a junction with the pixel layer  22 , and having a first branch structure extending from the light-receiving area toward plural portions shielded by the shielding plate  51  along the upper portion of the pixel layer  22  so as to delineate a plurality of branches, implementing a T-shaped configuration on the end side; a first upper charge-accumulation region  24   ub , a second upper charge-accumulation region  24   ud  and a third upper charge-accumulation region  24   uc  of n-type conductivity, which are connected to the respective tips of the T-shaped first branch structure and having a higher impurity concentration than the first surface-buried region  25   u  ; a first upper transfer-control mechanism ( 31 ,  42   u ), a second upper transfer-control mechanism ( 31 ,  44   u ) and a third upper transfer-control mechanism ( 31 ,  43   u ) provided at the branches of the T-shaped first branch structure adjacent to the first upper charge-accumulation region  24   ub , the second upper charge-accumulation region  24   ud  and the third upper charge-accumulation region  24   uc,  respectively; to control a transfer of signal charges to the first upper charge-accumulation region  24   ub , the second upper charge-accumulation region  24   ud  and the third upper charge-accumulation region  24   uc;  and a first guide region  26   u  of n-type conductivity, which is buried in a part of the upper portion of the first surface-buried region  25   u  with one end provided below a part of the first surface-buried region  25   u  and other protruding-edges extending to at least a part of the respective transfer-control mechanisms, and having a higher impurity concentration than the first surface-buried region  25   u  and a lower impurity concentration than the first upper charge-accumulation region  24   ub , the second upper charge-accumulation region  24   ud  and the third upper charge-accumulation region  24   uc.    
     Although not shown in the drawings, the first upper charge-accumulation region  24   ub , the second upper charge-accumulation region  24   ud  and the third upper charge-accumulation region  24   uc  have the same cross sections as shown in  FIG. 2A ,  FIG. 3A  and  FIG. 4A  and are surrounded by the well region  23  to serve as floating diffusion layers provided above the pixel layer  22 . Although the range sensor according to the third embodiment has a structure for illustration purposes in which the number of the floating diffusion layers in which signal charges from the light-receiving area are accumulated is three, the number of the floating diffusion layers may be two or more than three. 
     The range sensor according to the third embodiment includes a first p-type pinning layer, which is in contact with the surface of the first surface-buried region  25   u  (not shown), as in the case shown in the cross-sectional views of  FIG. 2  to  FIG. 4 . The pixel layer  22  is laminated on a p-type semiconductor substrate. 
     In the range sensor according to the third embodiment, the first surface-buried region  25   u  of n-type conductivity, which implements the first photodiode, has a fork-like shape in a planar pattern to collect photoelectrons as signal charges generated in the light-receiving area shown in  FIG. 16  to the base of the fork-like shape so as to rapidly transfer the photoelectrons to a first charge-modulation portion including the first upper transfer-control mechanism ( 31 ,  42   u ), the second upper transfer-control mechanism ( 31 ,  44   u ) and the third upper transfer-control mechanism ( 31 ,  43   u ). Each of prongs of the fork-like shape has a stepped shape gradually increased in width from bottom to top in the drawing of  FIG. 16 . The range sensor according to the third embodiment generates a high drift field in the entire depleted branches of the first surface-buried region  25   u  having a fork-like shape gradually increased in width in the planar pattern in the light-receiving area, so as to rapidly transfer the photoelectrons as signal charges in the longitudinal direction of the prongs of the fork-like shape even when the light-receiving area has a large area of which a pixel size is five micrometers square or greater. 
     The first guide region  26   u  in the range sensor according to the third embodiment is a semiconductor region for guiding the photoelectrons collected in the central portion at the base of the fork-like shape to the respective narrow transfer channels in the charge-modulation portion. The first guide region  26   u  has a stepped shape of which the width in the direction perpendicular to the longitudinal direction (the vertical direction in  FIG. 16 ) of the first guide region  26   u  is gradually increased from bottom to top in the drawing of  FIG. 16 . The range sensor according to the third embodiment generates a high drift field in the entire depleted first guide region  26   u  gradually increased in width in the planar pattern. Accordingly, the photoelectrons as signal charges can rapidly be transferred along the first guide region  26   u  in the longitudinal direction. As shown in the plan view of  FIG. 16 , the narrow tip of the first guide region  26   u  is in contact with the first surface-buried region  25   u  to which carriers generated are transferred at a position corresponding to the bottom of the potential profile. 
     As in the case of the structure shown in  FIG. 3A  and  FIG. 4A , the first upper transfer-control mechanism ( 31 ,  42   u ), the second upper transfer-control mechanism ( 31 ,  44   u ) and the third upper transfer-control mechanism ( 31 ,  43   u ) implementing the range sensor according to the third embodiment include an insulating film  31  provided on the respective branches of the T-shaped first branch structure, and further include a first upper transfer gate electrode  42   u , a second upper transfer gate electrode  44   u  and a third upper transfer gate electrode  43   u , respectively, provided on the insulating film  31 . 
     As in the case of the structure shown in  FIG. 3A  and  FIG. 4A , the thickness of the insulating film  31  immediately below the respective first upper transfer gate electrode  42   u , second upper transfer gate electrode  44   u  and third upper transfer gate electrode  43   u  is thinner than the other sites, and the thinner parts of the insulating film  31  each serve as a “gate insulating film”. The voltage applied to each of the first upper transfer gate electrode  42   u , the second upper transfer gate electrode  44   u  and the third upper transfer gate electrode  43   u  controls potentials in the transfer channels defined by the respective branches of the T-shaped first branch structure, so as to control the signal charge transferred to the first upper charge-accumulation region  24   ub , the second upper charge-accumulation region  24   ud  and the third upper charge-accumulation region  24   uc.    
     The actual structure of the insulating film  31  may be a double-layered structure having a stepped shape obtained such that an interlayer insulating film is selectively deposited on a thin insulating film serving as a gate insulating film to surround the first upper transfer gate electrode  42   u , the second upper transfer gate electrode  44   u  and the third upper transfer gate electrode  43   u . Alternatively, the insulating film  31  having a stepped shape may be obtained such that an interlayer insulating film or a field insulating film having a thickness different from that of a gate insulating film is selectively deposited on a region other than the gate insulating film to surround the first upper transfer gate electrode  42   u , the second upper transfer gate electrode  44   u  and the third upper transfer gate electrode  43   u . The material used for the interlayer insulating film or the field insulating film may be the same as that used for the gate insulating film or may be a different dielectric. For example, the interlayer insulating film may be a dielectric of which relative dielectric constant is smaller than the gate insulating film. 
     Although the first guide region  26   u  cannot be viewed from above because of the upper elements such as the insulating film  31  and the first pinning layer, the first guide region  26   u  has a planar pattern in which the end portion of the first guide region  26   u  toward the first photodiode is exposed to the aperture of the shielding plate  51 , and other portions are shielded by the shielding plate  51 , in the plan view of the shielding plate  51  as viewed from above. 
     The range sensor according to the third embodiment is provided with additional branches extending on both sides of the trunk path in the direction perpendicular to the longitudinal direction of the trunk path which is a trunk portion of the first branch structure of the first surface-buried region  25   u . The respective tips of the additional branches extending in the lateral direction are connected with a first upper extraction region  24   ua  and a second upper extraction region  24   ue  of n-type conductivity having a higher impurity concentration than the first surface-buried region  25   u . As shown in  FIG. 16 , the range sensor according to the third embodiment further includes a first upper extraction control mechanism ( 31 ,  41   ua ) provided adjacent to the first upper extraction region  24   ua  and configured to control extraction of charges toward the first upper extraction region  24   ua  via one of the additional branches extending in the right direction of the first surface-buried region  25   u , and a second upper extraction control mechanism ( 31 ,  41   ub ) provided adjacent to the second upper extraction region  24   ue  and configured to control extraction of charges toward the second upper extraction region  24   ue  via the other branch extending in the left direction of the first surface-buried region  25   u.    
     As in the case of the structure shown in  FIG. 2A , the first upper extraction control mechanism ( 31 ,  41   ua ) and the second upper extraction control mechanism ( 31 ,  41   ub ) include an insulating film  31  provided on the respective additional branches of the first surface-buried region  25   u , and further include a first upper extraction gate electrode  41   ua  and a second upper extraction gate electrode  41   ub , respectively, provided on the insulating film  31 . As in the case of the structure shown in  FIG. 2A , the thickness of the insulating film  31  immediately below the respective first upper extraction gate electrode  41   ua  and the second upper extraction gate electrode  41   ub  is thinner than the other sites, and the thinner parts of the insulating film  31  each serve as a “gate insulating film”. 
     When focused on the second surface-buried region  25   v  provided in the light-receiving area on the lower side in  FIG. 16 , the range sensor according to the third embodiment has a structure on the other side of the lock-in pixel including: the second surface-buried region  25   v  of a second conductivity type (n-type) selectively buried in the upper portion of the pixel layer  22  to implement a second photodiode in the light-receiving area by forming a junction with the pixel layer  22 , and having a second branch structure extending from the light-receiving area toward plural portions shielded by the shielding plate  51  along the upper portion of the pixel layer  22  so as to delineate a plurality of branches, implementing a T-shaped configuration on the end side; a first lower charge-accumulation region  24   vb , a second lower charge-accumulation region  24   vd  and a third lower charge-accumulation region  24   vc  of n-type conductivity, which is connected to the tips of the T-shaped second branch structure and having a higher impurity concentration than the second surface-buried region  25   v ; a first lower transfer-control mechanism ( 31 ,  42   v ), a second lower transfer-control mechanism ( 31 ,  44   v ) and a third lower transfer-control mechanism ( 31 ,  43   v ) provided at the branches of the T-shaped second branch structure adjacent to the first lower charge-accumulation region  24   vb , the second lower charge-accumulation region  24   vd  and the third lower charge-accumulation region  24   vc , respectively, to control a transfer of signal charges to the first lower charge-accumulation region  24   vb , the second lower charge-accumulation region  24   vd  and the third lower charge-accumulation region  24   vc ; and a second guide region  26   v  of n-type conductivity, which is buried in a part of the upper portion of the second surface-buried region  25   v  with one end provided below a part of the second surface-buried region  25   v  and other protruding-edges extending to at least a part of the respective transfer-control mechanisms, and having a higher impurity concentration than the second surface-buried region  25   v  and a lower impurity concentration than the first lower charge-accumulation region  24   vb , the second lower charge-accumulation region  24   vd  and the third lower charge-accumulation region  24   vc.    
     Although not shown in the drawings, the first lower charge-accumulation region  24   vb , the second lower charge-accumulation region  24   vd  and the third lower charge-accumulation region  24   vc  have the same cross sections as shown in  FIG. 2A ,  FIG. 3A  and  FIG. 4A  and are surrounded by the well region  23  to serve as floating diffusion layers provided above the pixel layer  22 . Although the range sensor according to the third embodiment has a structure for illustration purposes in which the number of the floating diffusion layers in which signal charges from the light-receiving area are accumulated is three, the number of the floating diffusion layers may be two or more than three. 
     The range sensor according to the third embodiment includes a second p-type pinning layer, which is in contact with the surface of the second surface-buried region  25   v  (not shown), as in the case shown in the cross-sectional views of  FIG. 2  to  FIG. 4 . In the range sensor according to the third embodiment, the second surface-buried region  25   v  of n-type conductivity, which implements the second photodiode has a fork-like shape in a plan view to collect photoelectrons generated in the light-receiving area shown in  FIG. 16  to the base of the fork-like shape so as to rapidly transfer the photoelectrons to a second charge-modulation portion including the first lower transfer-control mechanism ( 31 ,  42   v ), the second lower transfer-control mechanism ( 31 ,  44   v ) and the third lower transfer-control mechanism ( 31 ,  43   v ). Each of prongs of the fork-like shape has a stepped shape gradually increased in width from upper-side to lower-side in the drawing of  FIG. 16 . The range sensor according to the third embodiment generates a high drift field in the entire depleted branches of the second surface-buried region  25   v  having a fork-like shape gradually increased in width in the planar pattern in the light-receiving area, so as to rapidly transfer the photoelectrons as signal charges in the longitudinal direction of the prongs of the fork-like shape even when the light-receiving area has a large area of which a pixel size is five micrometers square or greater. 
     The second guide region  26   v  in the range sensor according to the third embodiment is a semiconductor region for guiding the photoelectrons collected in the central portion at the base of the fork-like shape to the respective narrow transfer channels in the charge-modulation portion. The second guide region  26   v  has a stepped shape of which the width in the direction perpendicular to the longitudinal direction (the vertical direction in  FIG. 16 ) of the second guide region  26   v  is gradually increased from upper-side to lower-side in the drawing of  FIG. 16 . The range sensor according to the third embodiment generates a high drift field in the entire depleted second guide region  26   v  gradually increased in width in the planar pattern. Accordingly, the photoelectrons as signal charges can rapidly be transferred along the second guide region  26   v  in the longitudinal direction. As shown in the plan view of  FIG. 16 , the narrow tip of the second guide region  26   v  is in contact with the second surface-buried region  25   v  to which carriers generated are transferred at a position corresponding to the bottom of the potential profile. 
     For example, the first guide region  26   u  and the guide region  26   v  are formed as a semiconductor region having a higher impurity concentration than the first surface-buried region  25   u  and the second surface-buried region  25   v  such that parts of areas in which ions are implanted to form the respective first surface-buried region  25   u  and second surface-buried region  25   v  are doubly ion-implanted according to the planar pattern shown in  FIG. 16 . 
     As in the case of the structure shown in  FIG. 3A  and  FIG. 4A , the first lower transfer-control mechanism ( 31 ,  42   v ), the second lower transfer-control mechanism ( 31 ,  44   v ) and the third lower transfer-control mechanism ( 31 ,  43   v ) implementing the range sensor according to the third embodiment include the insulating film  31  provided on the respective branches of the T-shaped second branch structure, and further include a first lower transfer gate electrode  42   v , a second lower transfer gate electrode  44   v  and a third lower transfer gate electrode  43   v , respectively; provided on the insulating film  31 . 
     As in the case of the structure shown in  FIG. 3A  and  FIG. 4A , the thickness of the insulating film  31  immediately below the respective first lower transfer gate electrode  42   v , second lower transfer gate electrode  44   v  and third lower transfer gate electrode  43   v  is thinner than the other sites, and the thinner parts of the insulating film  31  each serve as a “gate insulating film”. The voltage applied to each of the first lower transfer gate electrode  42   v , the second lower transfer gate electrode  44   v  and the third lower transfer gate electrode  43   v  controls potentials in the transfer channels defined by the respective branches of the T-shaped second branch structure, so as to control the signal charge transferred to the first lower charge-accumulation region  24   vb , the second lower charge-accumulation region  24   vd  and the third lower charge-accumulation region  24   vc.    
     The actual structure of the insulating film  31  may be a double-layered structure having a stepped shape obtained such that an interlayer insulating film is selectively deposited on a thin insulating film serving as a gate insulating film to surround the first lower transfer gate electrode  42   v , the second lower transfer gate electrode  44   v  and the third lower transfer gate electrode  43   v . Alternatively, the insulating film  31  having a stepped shape may be obtained such that an interlayer insulating film or a field insulating film having a thickness different from that of a gate insulating film is selectively deposited on a region other than the gate insulating film to surround the first lower transfer gate electrode  42   v , the second lower transfer gate electrode  44   v  and the third lower transfer gate electrode  43   v.    
     Although the second guide region  26   v  cannot be viewed from above because of the upper elements such as the insulating film  31  and the second pinning layer, the second guide region  26   v  has a planar pattern in which the end portion of the second guide region  26   v  toward the second photodiode is exposed to the aperture of the shielding plate  51 , and other portions are shielded by the shielding plate  51 , in the plan view of the shielding plate  51  as viewed from above. 
     The range sensor according to the third embodiment is provided with additional branches extending on both sides of the trunk path in the direction perpendicular to the longitudinal direction of the trunk path which is a trunk portion of the second branch structure of the second surface-buried region  25   v . The respective tips of the additional branches extending in the lateral direction are connected with a first lower extraction region  24   va  and a second lower extraction region  24   ve  of n-type conductivity having a higher impurity concentration than the second surface-buried region  25   v . As shown in  FIG. 16 , the range sensor according to the third embodiment further includes a first lower extraction control mechanism ( 31 ,  41   va ) provided adjacent to the first lower extraction region  24   va  and configured to control extraction of charges toward the first lower extraction region  24   va  via one of the additional branches extending in the left direction of the second surface-buried region  25   v , and a second lower extraction control mechanism ( 31 ,  41   vb ) provided adjacent to the second lower extraction region  24   ve  and configured to control extraction of charges toward the second lower extraction region  24   ve  via the other branch extending in the right direction of the second surface-buried region  25   v.    
     As in the case of the structure shown in the cross-sectional view of  FIG. 2A , the first lower extraction control mechanism ( 31 ,  41   va ) and the second lower extraction control mechanism ( 31 ,  41   vb ) include the insulating film  31  provided on the respective additional branches of the second surface-buried region  25   v , and further include a first lower extraction gate electrode  41   va  and a second lower extraction gate electrode  41   vb , respectively, provided on the insulating film  31 . As in the case of the structure shown in the cross-sectional view of  FIG. 2A , the thickness of the insulating film  31  immediately below the respective first lower extraction gate electrode  41  and second lower extraction gate electrode  41   vb  is thinner than the other sites, and the thinner parts of the insulating film  31  each serve as a “gate insulating film”. 
     When a solid-state imaging device (image sensor) having a large pixel area and using a single photodiode, as in the case of the range sensors according to the first and second embodiments, cannot ensure a sufficiently rapid response, a plurality of photodiodes may be provided in one pixel, as shown in  FIG. 16 , so as to accumulate output from the plural photodiodes to amplify a signal. The range sensor according to the third embodiment corresponds to a case in which two range sensors each corresponding to the first and second embodiments are provided in one pixel. 
     As shown in  FIG. 16 , the range sensor according to the third embodiment is provided with the first upper extraction gate electrode  41   ua  and the second upper extraction gate electrode  41   ub  symmetrically arranged to interpose the trunk path in front of (below) the branch portions of the T shape between the first photodiode provided on the lower side of the range sensor and the first charge-modulation portion extending upward from the first photodiode on the right side. Accordingly, the first upper extraction gate electrode  41   ua  and the second upper extraction gate electrode  41   ub  can switch the operation between the extraction of photoelectrons toward the first upper extraction region  24   ua  and the second upper extraction region  24   ue  and the transport of the photoelectrons toward the first charge-modulation portion including the first upper transfer-control mechanism ( 31 ,  42   u ), the second upper transfer-control mechanism ( 31 ,  44   u ) and the third upper transfer-control mechanism ( 31 ,  43   u ). 
     Similarly, the range sensor according to the third embodiment is provided with the first lower extraction gate electrode  41   va  and the second lower extraction gate electrode  41   vb  symmetrically arranged to interpose the trunk path in front of (above) the branch portions of the T shape between the second photodiode provided on the upper side of the range sensor and the second charge-modulation portion extending downward from the second photodiode on the left side. Accordingly, the range sensor according to the third embodiment uses the first lower extraction gate electrode  41   va  and the second lower extraction gate electrode  41   vb  so as to switch the operation between the extraction of photoelectrons toward the first lower extraction region  24   va  and the second lower extraction region  24   ve  and the transport of the photoelectrons toward the second charge-modulation portion including the first lower transfer-control mechanism ( 31 ,  42   v ), the second lower transfer-control mechanism ( 31 ,  44   v ) and the third lower transfer-control mechanism ( 31 ,  43   v ). 
       FIG. 17  illustrates an equivalent circuit of the range sensor according to the third embodiment including two photodiodes in one pixel: a first photodiode Du ij  and a second photodiode Dv ij . The “first charge-modulation portion” is implemented by a first upper transfer transistor Qu 1T  as the first upper transfer-control mechanism ( 31 ,  42   u ), a second upper transfer transistor Qu 2T  as the second upper transfer-control mechanism ( 31 ,  44   u ), and a third upper transfer transistor Qu 3T  as the third upper transfer-control mechanism ( 31 ,  43   u ), as indicated in the middle in  FIG. 17 . The “second charge-modulation portion” provided on the right side of the first charge-modulation portion is implemented by a first lower transfer transistor Qv 1T  as the first lower transfer-control mechanism ( 31 ,  42   v ), a second lower transfer transistor Qv 2T  as the second lower transfer-control mechanism ( 31 ,  44   v ), and a third lower transfer transistor Qv 3T  as the third lower transfer-control mechanism ( 31 ,  43   v ). 
     The static induction channel from the first photodiode Du ij  indicated on the upper left side to the first charge-modulation portion is shown as a circuit indicated by the broken line in  FIG. 17 . The static induction channel is indicated on the upper left side by two first junction field-effect transistors Qu P1  and Qu P2  of which own gates are grounded. A source terminal of a first charge-extraction MOS transistor Qu D  for charge extraction is connected to a center tap of the two first junction field-effect transistors Qu P1  and Qu P1  connected in series, and a drain terminal of the first charge-extraction MOS transistor Qu D  is connected to a power source V DD  having a high potential. 
     The broken lines in  FIG. 17  on the upper left side indicate paths in which electrons flow rapidly due to a depletion electric field because of the connection in the semiconductor region in  FIG. 17 , photoelectrons generated in the first photodiode Du ij  immediately reach the first charge-modulation portion when the voltage G Du , applied to each of the first upper extraction gate electrode  41   ua  and the second upper extraction gate electrode  41   ub  implementing the first charge-extraction MOS transistor Qu D  is set at a low potential (L). Since the first charge-modulation portion includes the first upper transfer transistor QU 1T , the second upper transfer transistor Qu 2T  and the third upper transfer transistor Qu 3T , the equivalent circuit is illustrated with the case in which one end of each of the first upper transfer transistor Qu 1T , the second upper transfer transistor Qu 2T  and the third upper transfer transistor Qu 3T  is connected to the first junction field-effect transistor Qu P2  in the T-shaped manner. 
     In the circuit diagram, the other ends of the first upper transfer transistor Qu 1T , the second upper transfer transistor Qu 2T  and the third upper transfer transistor Qu 3T  are connected to the first upper charge-accumulation region  24   ub  serving as node D 1 , the second upper charge-accumulation region  24   ud  serving as node D 2  and the third upper charge-accumulation region  24   uc  serving as node D 3 , respectively. When the voltage at the intermediate potential (M) is applied to one of the first upper transfer gate electrode  42   u , the second upper transfer gate electrode  44   u  and the third upper transfer gate electrode  43   u , and the voltage at the low potential (L) is applied to the other two electrodes, the photoelectrons are transferred to one of the first upper charge-accumulation region  24   ub , the second upper charge-accumulation region  24   ud  and the third upper charge-accumulation region  24   uc.    
     The static induction channel from the second photodiode Dv ij  indicated on the lower left side to the lower side of the second charge-modulation portion indicated in the middle is indicated by two second junction field-effect transistors Qv P1  and Qv P2  of which own gates are grounded.  FIG. 17  indicates on the lower left side a circuit diagram in which a source terminal of a second charge-extraction MOS transistor Qv D  for charge extraction is connected to a center tap of the two second junction field-effect transistors Qv P1  and Qv P2  is connected in series, and a drain terminal of the second charge-extraction MOS transistor Qv D  is connected to a power source V DD  having a high potential. 
     The broken lines in  FIG. 17  on the lower left side indicate paths in which electrons flow rapidly due to a depletion electric field because of the connection in the semiconductor region in  FIG. 17 , photoelectrons generated in the second photodiode Dv ij  immediately reach the second charge-modulation portion when the voltage G Dv  applied to each of the first lower extraction gate electrode  41   va  and the second lower extraction gate electrode  41   vb  implementing the second charge-extraction MOS transistor Qv D  is set at a low potential (L). Since the second charge-modulation portion includes the first lower transfer transistor Qv 1T , the second lower transfer transistor Qv 2T  and the third lower transfer transistor Qv 3T , the equivalent circuit is illustrated with the case in which one end of each of the first lower transfer transistor Qv 1T , the second lower transfer transistor Qv 2T  and the third lower transfer transistor Qv 3T  is connected to the second junction field-effect transistor Qv P2  in the T-shaped manner. 
     The other ends of the first lower transfer transistor Qv 1T , the second lower transfer transistor Qv 2T  and the third lower transfer transistor Qv 3T  are connected to the first lower charge-accumulation region  24   vb , the second lower charge-accumulation region  24   vd  and the third lower charge-accumulation region  24   vc , respectively. 
     The first lower charge-accumulation region  24   vb  is short-circuited with the first upper charge-accumulation region  24   ub  via a surface line such as a metal wire, the second lower charge-accumulation region  24   vd  is short-circuited with the second upper charge-accumulation region  24   ud  via a surface line, and the third lower charge-accumulation region  24   vc  is short-circuited with the third upper charge-accumulation region  24   uc  via a surface line, although not shown in  FIG. 16 . When the voltage at the intermediate potential (M) is applied to one of the first lower transfer gate electrode  42   v , the second lower transfer gate electrode  44   v  and the third lower transfer gate electrode  43   v , and the voltage at the low potential (L) is applied to the other two electrodes, the photoelectrons are transferred to one of the first lower charge-accumulation region  24   vb  serving as common node D 1 , the second lower charge-accumulation region  24   vd  serving as common node D 2  and the third lower charge-accumulation region  24   vc  serving as common node D 3 . 
     The three common nodes D 1 , D 3  and D 2  shown in  FIG. 17  are connected to capacitors C 1 , C 3  and C 2  for charge-accumulation. The capacitors C 1 , C 3  and C 2  are preferably depletion-mode MOS capacitors in which a threshold voltage is set at a negative voltage so as to reduce voltage dependence. The first common node D 1  is connected to a gate terminal of a first amplification transistor Q 1A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the first common node D 1 . The first common node D 1  is further connected to a first reset transistor Q 1R , for initializing signal charges after signal readout. A source terminal of the first amplification transistor Q 1A  is connected to a first selection transistor Q 1S , serving as a switch for readout image selection. An output of first selection transistor Q 1S  is connected to a signal readout line extending in the vertical direction. 
     The second common node D 2  is connected to a gate terminal of a second amplification transistor Q 2A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the second common node D 2 . The second common node D 2  is further connected to a second reset transistor Q 2R  for initializing signal charges after signal readout. A source terminal of the second amplification transistor Q 2A  is connected to a second selection transistor Q 2S , serving as a switch for readout image selection. An output of the second selection transistor Q 2S  is connected to a signal readout line extending in the vertical direction. The third common node D 3  is connected to a gate terminal of a third amplification transistor Q 3A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the third common node D 3 . The third common node D 3  is further connected to a third reset transistor Q 3R  for initializing signal charges after signal readout. A source terminal of the third amplification transistor Q 3A  is connected to a third selection transistor Q 3S , serving as a switch for readout image selection. An output of the third selection transistor Q 3S  is connected to a signal readout line extending in the vertical direction. 
     The signal readout method in the range sensor according to the third embodiment may be either a method of reading out a signal from each of three signal readout lines arranged in parallel as shown in  FIG. 17 , or a method of turning on a switch sequentially by selection signals SL 1 , SL 2  and SL 3  to read out each signal as a time-series signal from a single signal readout line as shown in  FIG. 11 . 
     In the range sensor and the solid-state imaging device according to the third embodiment, the first lower charge-accumulation region  24   vb  is short-circuited with the first upper charge-accumulation region  24   ub , the second lower charge-accumulation region  24   vd  is short-circuited with the second upper charge-accumulation region  24   ud,  and the third lower charge-accumulation region  24   vc  is short-circuited with the third upper charge-accumulation region  24   uc , so as to receive light in both the first photodiode Du ij  and the second photodiode Dv ij , and accumulate signals as charges after subjected to charge modulation in both the first and second charge-modulation portions to amplify the signals. 
     As in the case of the range sensor according to the first and second embodiments, the range sensor according to the third embodiment is provided with the static induction channel having a sufficient length from the trunk path adjacent to the light-receiving area to the respective first and second charge-modulation portions, which are shielded by the shielding plate  51 , so as to achieve remarkable effects upon reducing the influence of background light while using a pulse at short intervals. 
     The range sensor according to the third embodiment has the function suitable for the time-of-flight measurement by use of the Eq. (4) or the Eq. (8) as described in the first embodiment. The range sensor according to the third embodiment thus can serve as a pixel (lock-in pixel) of a solid-state imaging device to achieve a time-of-flight range image sensor capable of rapid transfer of signal charges with high sensitivity at a low dark current even when a plurality of pixels each having a large area of which a pixel size is five micrometers square or greater are provided on a single semiconductor chip. 
     (Fourth Embodiment) 
     A range sensor according to the fourth embodiment of the present invention is a lock-in pixel including a shielding plate  51  having two apertures indicated by the dashed-dotted lines to define a plurality of light-receiving areas in one pixel as shown in the schematic plan view from above of the principal part in  FIG. 18 . Reference numerals  32   a  and  32   e  shown in  FIG. 18  each denote an edge of a field insulating film. Namely, the edges  32   a  and  32   e  define active areas of the lock-in pixel. The range sensor according to the fourth embodiment thus includes two active areas in one pixel. A p-type well region  23  is buried below the respective field insulating films (not shown), as in the case shown in the cross sections of  FIG. 2A ,  FIG. 3A  and  FIG. 4A . 
     When focused on the first aperture indicated by the dashed-dotted line provided on the lower side in  FIG. 18 , the range sensor according to the fourth embodiment has a structure on one side of the lock-in pixel similar to that shown in the cross sections of  FIG. 2  to  FIG. 4 , including: a pixel layer  22  made of a semiconductor of a first conductivity type (p-type); a first surface-buried region  25   a  of a second conductivity type (n-type) selectively buried in the upper portion of the pixel layer  22  to implement a first photodiode in a first light-receiving area defined by a first aperture by forming a junction with the pixel layer  22 , and having a first branch structure extending from the light-receiving area toward plural portions shielded by the shielding plate  51  along the upper portion of the pixel layer  22  so as to delineate a plurality of branches, implementing a T-shaped configuration on the end side; a first left charge-accumulation region  24   ab , a second left charge-accumulation region  24   ad  and a third left charge-accumulation region  24   ac  of n-type conductivity, which is connected to the tips of the T-shaped first branch structure and having a higher impurity concentration than the first surface-buried region  25   a  ; a first left transfer-control mechanism ( 31 ,  42   a ), a second left transfer-control mechanism ( 31 ,  44   a ) and a third left transfer-control mechanism ( 31 ,  43   a ) provided at the branches of the T-shaped first branch structure adjacent to the first left charge-accumulation region  24   ab , the second left charge-accumulation region  24   ad  and the third left charge-accumulation region  24   a  c, respectively, to control a transfer of signal charges to the first left charge-accumulation region  24   ab , the second left charge-accumulation region  24   ad  and the third left charge-accumulation region  24   ac;  and a first guide region  26   d  of n-type conductivity; which is buried in a part of the upper portion of the first surface-buried region  25   a  with one end provided below a part of the first aperture and other protruding-edges extending to at least a part of the respective transfer-control mechanisms, and having a higher impurity concentration than the first surface-buried region  25   a  and a lower impurity concentration than the first left charge-accumulation region  24   ab , the second left charge-accumulation region  24   ad  and the third left charge-accumulation region  24   ac.    
     Although not shown in the drawings, the first left charge-accumulation region  24   ab , the second left charge-accumulation region  24   ad  and the third left charge-accumulation region  24   ac  have the same cross sections as shown in  FIG. 2A ,  FIG. 3A  and  FIG. 4A  and are surrounded by the well region  23  to serve as floating diffusion layers provided above the pixel layer  22 . Although the range sensor according to the fourth embodiment has a structure for illustration purposes in which the number of the floating diffusion layers in which signal charges from the first light-receiving area are accumulated is three, the number of the floating diffusion layers may be two or more than three. 
     The range sensor according to the fourth embodiment includes a first p-type pinning layer, which is in contact with the surface of the first surface-buried region  25   a  (not shown), as in the case shown in the cross-sectional views of  FIG. 2  to  FIG. 4 . The pixel layer  22  is laminated on a p-type semiconductor substrate. 
     In the range sensor according to the fourth embodiment, the first surface-buried region  25   a  of n-type conductivity, which implements the first photodiode has a gun shape in a planar pattern to collect photoelectrons as signal charges generated in the first light-receiving area shown in  FIG. 18  to a grip portion of the gun shape so as to rapidly transfer the photoelectrons to a first charge-modulation portion including the first left transfer-control mechanism ( 31 ,  42   a ), the second left transfer-control mechanism ( 31 ,  44   a ) and the third left transfer-control mechanism ( 31 ,  43   a ). 
     The first guide region  26   d  in the range sensor according to the fourth embodiment is a semiconductor region for guiding the photoelectrons collected to the grip portion of the gun shape to the respective narrow transfer channels in the first charge-modulation portion, and has a higher impurity concentration than the first surface-buried region  25   a . As in the case of the structure shown in  FIG. 3A  and  FIG. 4A , the first left transfer-control mechanism ( 31 ,  42   a ), the second left transfer-control mechanism ( 31 ,  44   a ) and the third left transfer-control mechanism ( 31 ,  43   a ) implementing the range sensor according to the fourth embodiment include an insulating film  31  provided on the respective branches of the T-shaped first branch structure, and further include a first left transfer gate electrode  42   a , a second left transfer gate electrode  44   a  and a third left transfer gate electrode  43   a , respectively, provided on the insulating film  31 . 
     As in the case of the structure shown in  FIG. 3A  and  FIG. 4A , the thickness of the insulating film  31  immediately below the respective first left transfer gate electrode  42   a , second left transfer gate electrode  44   a  and third left transfer gate electrode  43   a  is thinner than the other sites, and the thinner parts of the insulating film  31  each serve as a “gate insulating film”. The voltage applied to each of the first left transfer gate electrode  42   a , the second left transfer gate electrode  44   a  and the third left transfer gate electrode  43   a  controls potentials in the transfer channels defined by the respective branches of the T-shaped first branch structure, so as to control the signal charge transferred to the first left charge-accumulation region  24   ab , the second left charge-accumulation region  24   ad  and the third left charge-accumulation region  24   ac.    
     The actual structure of the insulating film  31  may be a double-layered structure having a stepped shape obtained such that an interlayer insulating film is selectively deposited on a thin insulating film serving as a gate insulating film to surround the first left transfer gate electrode  42   a , the second left transfer gate electrode  44   a  and the third left transfer gate electrode  43   a . Alternatively, the insulating film  31  having a stepped shape may be obtained such that an interlayer insulating film or a field insulating film having a thickness different from that of a gate insulating film is selectively deposited on a region other than the gate insulating film to surround the first left transfer gate electrode  42   a , the second left transfer gate electrode  44   a  and the third left transfer gate electrode  43   a . The material used for the interlayer insulating film or the field insulating film may be the same as that used for the gate insulating film or may be a different dielectric. For example, the interlayer insulating film may be a dielectric of which relative dielectric constant is smaller than the gate insulating film. 
     Although the first guide region  26   d  cannot be viewed from above because of the upper elements such as the insulating film  31  and the first pinning layer, the first guide region  26   d  has a planar pattern in which the end portion of the first guide region  26   d  toward the first photodiode is exposed to the aperture of the shielding plate  51 , and other portions are shielded by the shielding plate  51 , in the plan view of the shielding plate  51  as viewed from above. 
     The range sensor according to the fourth embodiment is provided with additional branches extending on both sides of the trunk path in the direction perpendicular to the longitudinal direction of the trunk path which is a trunk portion of the first branch structure of the first surface-buried region  25   a . The respective tips of the additional branches extending in the lateral direction are connected with a first left extraction region  24   aa  and a second left extraction region  24   ae  of n-type conductivity having a higher impurity concentration than the first surface-buried region  25   a.    
     As shown in  FIG. 18 , the range sensor according to the fourth embodiment further includes a first left extraction control mechanism ( 31 ,  41   aa ) provided adjacent to the first left extraction region  24   aa  and configured to control extraction of charges toward the first left extraction region  24   aa  via one of the additional branches extending in the right direction of the first surface-buried region  25   a , and a second left extraction control mechanism ( 31 ,  41   ab ) provided adjacent to the second left extraction region  24   ae  and configured to control extraction of charges toward the second left extraction region  24   ae  via the other branch extending in the left direction of the first surface-buried region  25   a.    
     As in the case of the structure shown in  FIG. 2A , the first left extraction control mechanism ( 31 ,  41   aa ) and the second left extraction control mechanism ( 31 ,  41   ab ) include an insulating film  31  provided on the respective additional branches of the first surface-buried region  25   a , and further include a first left extraction gate electrode  41   aa  and a second left extraction gate electrode  41   ab , respectively, provided on the insulating film  31 . As in the case of the structure shown in  FIG. 2A , the thickness of the insulating film  31  immediately below the respective first left extraction gate electrode  41   aa  and second left extraction gate electrode  41   ab  is thinner than the other sites, and the thinner parts of the insulating film  31  each serve as a “gate insulating film”. 
     When focused on the second aperture indicated by the dashed-dotted line provided on the upper side in  FIG. 18 , the range sensor according to the fourth embodiment has a structure on the other side of the lock-in pixel including: a second surface-buried region  25   b  of a second conductivity type (n-type) selectively buried in the upper portion of the pixel layer  22  to implement a second photodiode in a second light-receiving area by forming a junction with the pixel layer  22 , and having a second branch structure extending from the light-receiving area toward plural portions shielded by the shielding plate  51  along the upper portion of the pixel layer  22  so as to delineate a plurality of branches, implementing a T-shaped configuration on the end side; a first right charge-accumulation region  24   bb , a second right charge-accumulation region  24   bd  and a third right charge-accumulation region  24   bc  of n-type conductivity, which is connected to the tips of the T-shaped second branch structure and having a higher impurity concentration than the second surface-buried region  25   b;  a first right transfer-control mechanism ( 31 ,  42   b ), a second right transfer-control mechanisms ( 31 ,  42   b ) and a third right transfer-control mechanism ( 31 ,  43   b ) provided at the branches of the T-shaped second branch structure adjacent to the first right charge-accumulation region  24   bb , the second right charge-accumulation region  24   bd  and the third right charge-accumulation region  24   bc , respectively, to control a transfer of signal charges to the first right charge-accumulation region  24   bb , the second right charge-accumulation region  24   bd  and the third right charge-accumulation region  24   bc;  and a second guide region  26   e  of n-type conductivity; which is buried in a part of the upper portion of the second surface-buried region  25   b  with one end provided below a part of the second aperture and other protruding-edges extending to at least a part of the respective transfer-control mechanisms, and having a higher impurity concentration than the second surface-buried region  25   b  and a lower impurity concentration than the first right charge-accumulation region  24   bb , the second right charge-accumulation region  24   bd  and the third right charge-accumulation region  24   bc.    
     Although not shown in the drawings, the first right charge-accumulation region  24   bb , the second right charge-accumulation region  24   bd  and the third right charge-accumulation region  24   bc  have the same cross sections as shown in  FIG. 2A ,  FIG. 3A  and  FIG. 4A  and are surrounded by the well region  23  to serve as floating diffusion layers provided above the pixel layer  22 . Although the range sensor according to the fourth embodiment has a structure for illustration purposes in which the number of the floating diffusion layers in which signal charges from the second light-receiving area are accumulated is three, the number of the floating diffusion layers may be two or more than three. 
     The range sensor according to the third embodiment includes a second p-type pinning layer, which is in contact with the surface of the second surface-buried region  25   b  (not shown), as in the case shown in the cross-sectional views of  FIG. 2  to  FIG. 4 . In the range sensor according to the fourth embodiment, the second surface-buried region  25   b  of n-type conductivity; which implements the second photodiode has a gun shape in a planar pattern to collect photoelectrons generated in the light-receiving area shown in  FIG. 18  to the grip portion of the gun shape so as to rapidly transfer the photoelectrons to a second charge-modulation portion including the first right transfer-control mechanism ( 31 ,  42   b ), the second right transfer-control mechanisms ( 31 ,  42   b ) and the third right transfer-control mechanism ( 31 ,  43   b ). 
     The second guide region  26   e  in the range sensor according to the fourth embodiment is a semiconductor region for guiding the photoelectrons collected to the grip portion of the gun shape to the respective narrow transfer channels in the second charge-modulation portion, and has a higher impurity concentration than the second surface-buried region  25   b . As in the case of the structure shown in  FIG. 3A  and  FIG. 4A , the first right transfer-control mechanism ( 31 ,  42   b ), the second right transfer-control mechanisms ( 31 ,  42   b ) and the third right transfer-control mechanism ( 31 ,  43   b ) implementing the range sensor according to the fourth embodiment include the insulating film  31  provided on the respective branches of the T-shaped second branch structure, and further include a first right transfer gate electrode  42   b , a second right transfer gate electrode  44   b  and a third right transfer gate electrode  43   b,  respectively; provided on the insulating film  31 . 
     As in the case of the structure shown in  FIG. 3A  and  FIG. 4A , the thickness of the insulating film  31  immediately below the respective first right transfer gate electrode  42   b , second right transfer gate electrode  44   b  and third right transfer gate electrode  43   b  is thinner than the other sites, and the thinner parts of the insulating film  31  each serve as a “gate insulating film”. The voltage applied to each of the first right transfer gate electrode  42   b , the second right transfer gate electrode  44   b  and the third right transfer gate electrode  43   b  controls potentials in the transfer channels defined by the respective branches of the T-shaped second branch structure, so as to control the signal charge transferred to the first right charge-accumulation region  24   bb , the second right charge-accumulation region  24   bd  and the third right charge-accumulation region  24   bc.    
     The actual structure of the insulating film  31  may be a double-layered structure having a stepped shape obtained such that an interlayer insulating film is selectively deposited on a thin insulating film serving as a gate insulating film to surround the first right transfer gate electrode  42   b , the second right transfer gate electrode  44   b  and the third right transfer gate electrode  43   b . Alternatively, the insulating film  31  having a stepped shape may be obtained such that an interlayer insulating film or a field insulating film having a thickness different from that of a gate insulating film is selectively deposited on a region other than the gate insulating film to surround the first right transfer gate electrode  42   b , the second right transfer gate electrode  44   b  and the third right transfer gate electrode  43   b.    
     Although the second guide region  26   e  cannot be viewed from above because of the upper elements such as the insulating film  31  and the second pinning layer, the second guide region  26   e  has a planar pattern in which the end portion of the second guide region  26   e  toward the second photodiode is exposed to the aperture of the shielding plate  51 , and other portions are shielded by the shielding plate  51 , in the plan view of the shielding plate  51  as viewed from above. 
     The range sensor according to the fourth embodiment is provided with additional branches extending on both sides of the trunk path in the direction perpendicular to the longitudinal direction of the trunk path which is a trunk portion of the second branch structure of the second surface-buried region  25   b . The respective tips of the additional branches extending in the lateral direction are connected with a first right extraction region  24   ba  and a second right extraction region  24   be  of n-type conductivity having a higher impurity concentration than the second surface-buried region  25   b . As shown in  FIG. 18 , the range sensor according to the fourth embodiment further includes a first right extraction control mechanism ( 31 ,  41   ba ) provided adjacent to the first right extraction region  24   ba  and configured to control extraction of charges toward the first right extraction region  24   ba  via one of the additional branches extending in the left direction of the second surface-buried region  25   b , and a second right extraction control mechanism ( 31 ,  41   bb ) provided adjacent to the second right extraction region  24   be  and configured to control extraction of charges toward the second right extraction region  24   be  via the other branch extending in the right direction of the second surface-buried region  25   b.    
     As in the case of the structure shown in the cross-sectional view of  FIG. 2A , the first right extraction control mechanism ( 31 ,  41   ba ) and the second right extraction control mechanism ( 31 ,  41   bb ) include the insulating film  31  provided on the respective additional branches of the second surface-buried region  25   b , and further include a first right extraction gate electrode  41   ba  and a second right extraction gate electrode  41   bb,  respectively, provided on the insulating film  31 . As in the case of the structure shown in the cross-sectional view of  FIG. 2A , the thickness of the insulating film  31  immediately below the respective first right extraction gate electrode  41   ba  and second right extraction gate electrode  41   bb  is thinner than the other sites, and the thinner parts of the insulating film  31  each serve as a “gate insulating film”. 
     When a solid-state imaging device (image sensor) having a large pixel area and using a single photodiode, as in the case of the range sensors according to the first and second embodiments, cannot ensure a sufficiently rapid response, a plurality of photodiodes may be provided in one pixel, as shown in  FIG. 18 , so as to accumulate output from the plural photodiodes to amplify a signal. The range sensor according to the fourth embodiment corresponds to a case in which two range sensors each corresponding to the first and second embodiments are provided in one pixel. 
     As shown in  FIG. 18 , the range sensor according to the fourth embodiment is provided with the first left extraction gate electrode  41   aa  and the second left extraction gate electrode  41   ab  symmetrically arranged to interpose the trunk path in front of (below) the branch portions of the T shape between the first photodiode provided on the lower side of the range sensor and the first charge-modulation portion extending upward from the first photodiode on the left side. Accordingly; the first left extraction gate electrode  41   aa  and the second left extraction gate electrode  41   ab  can switch the operation between the extraction of photoelectrons toward the first left extraction region  24   aa  and the second left extraction region  24   ae  and the transport of the photoelectrons toward the first charge-modulation portion including the first left transfer-control mechanism ( 31 ,  42   a ), the second left transfer-control mechanism ( 31 ,  44   a ) and the third left transfer-control mechanism ( 31 ,  43   a ). 
     Similarly, the range sensor according to the fourth embodiment is provided with the first right extraction gate electrode  41   ba  and the second right extraction gate electrode  41   bb  symmetrically arranged to interpose the trunk path in front of (above) the branch portions of the T shape between the second photodiode provided on the upper side of the range sensor and the second charge-modulation portion extending downward from the second photodiode on the right side. Accordingly, the first right extraction gate electrode  41   ba  and the second right extraction gate electrode  41   bb  can switch the operation between the extraction of photoelectrons toward the first right extraction region  24   va  and the second right extraction region  24   ve  and the transport of the photoelectrons toward the second charge-modulation portion including the first right transfer-control mechanism ( 31 ,  42   b ), the second right transfer-control mechanisms ( 31 ,  42   b ) and the third right transfer-control mechanism ( 31 ,  43   b ). 
       FIG. 19  illustrates an equivalent circuit of the range sensor according to the fourth embodiment including two photodiodes in one pixel: a first photodiode Da ij  and a second photodiode Db ij . The “first charge-modulation portion” in the range sensor according to the fourth embodiment is implemented by a first left transfer transistor Qa 1T  as the first left transfer-control mechanism ( 31 ,  42   a ), a second left transfer transistor Qa 2T  as the second left transfer-control mechanism ( 31 ,  44   a ), and a third left transfer transistor Qa 3T  as the third left transfer-control mechanism ( 31 ,  43   a ), as indicated in the middle in  FIG. 19 . The “second charge-modulation portion” provided on the right side of the first charge-modulation portion is implemented by a first right transfer transistor Qb 1T  as the first right transfer-control mechanism ( 31 ,  42   b ), a second right transfer transistor Qb 2T , as the second right transfer-control mechanism ( 31 ,  44   b ), and a third right transfer transistor Qb 3T  as the third lower transfer-control mechanism ( 31 ,  43   b ). 
     The static induction channel from the first photodiode Da ij  indicated on the upper left side to the first charge-modulation portion is shown as a circuit indicated by the broken line in  FIG. 19 . The static induction channel is indicated on the upper left side by two first junction field-effect transistors Qa P1  and Qa P2  of which own gates are grounded. A source terminal of a first charge-extraction MOS transistor Qa D  for charge extraction is connected to a center tap of the two first junction field-effect transistors Qa P1  and Qa P2  connected in series, and a drain terminal of the first charge-extraction MOS transistor Qa D  is connected to a power source V DD  having a high potential. 
     The broken lines in  FIG. 19  on the upper left side indicate paths in which electrons flow rapidly due to a depletion electric field because of the connection in the semiconductor region in  FIG. 19 , photoelectrons generated in the first photodiode Da ij  immediately reach the first charge-modulation portion when the voltage Ga D , applied to each of the first left extraction gate electrode  41   aa  and the second left extraction gate electrode  41   ab  implementing the first charge-extraction MOS transistor Qa D  is set at a low potential (L). Since the first charge-modulation portion includes the first left transfer transistor Qa 1T , the second left transfer transistor Qa 2T  and the third left transfer transistor Qa 3T , the equivalent circuit is illustrated with the case in which one end of each of the first left transfer transistor Qa 1T , the second left transfer transistor Qa 2T  and the third left transfer transistor Qa 3T  is connected to the first junction field-effect transistor Qa P2  in the T-shaped manner. 
     In the circuit diagram, the other ends of the first left transfer transistor Qa 1T , the second left transfer transistor Qa 2T  and the third left transfer transistor Qa 3T  are connected to the first left charge-accumulation region  24   ab  serving as node D 1 , the second left charge-accumulation region  24   ad  serving as node D 2  and the third left charge-accumulation region  24   ac  serving as node D 3 , respectively. When the voltage at the intermediate potential (M) is applied to one of the first left transfer gate electrode  42   a , the second left transfer gate electrode  44   a  and the third left transfer gate electrode  43   a , and the voltage at the low potential (L) is applied to the other two electrodes, the photoelectrons are transferred to one of the first left charge-accumulation region  24   ab,  the second left charge-accumulation region  24   ad  and the third left charge-accumulation region  24   ac.    
     The static induction channel from the second photodiode Db ij  indicated on the lower left side to the lower side of the second charge-modulation portion indicated in the middle is indicated by two second junction field-effect transistors Qb P1  and Qb P2 , of which own gates are grounded in  FIG. 19 .  FIG. 19  indicates on the lower left side a circuit diagram in which a source terminal of a second ch/arge-extraction MOS transistor Qb D  for charge extraction is connected to a center tap of the two second junction field-effect transistors Qb P1  and Qb P2  connected in series, and a drain terminal of the second charge-extraction MOS transistor QU D  is connected to a power source V DD  having a high potential. 
     The broken lines in  FIG. 19  on the lower left side indicate paths in which electrons flow rapidly due to a depletion electric field because of the connection in the semiconductor region in  FIG. 19 , photoelectrons generated in the second photodiode Db ij  immediately reach the second charge-modulation portion when the voltage Gb D  applied to each of the first right extraction gate electrode  41   ba  and the second right extraction gate electrode  41   bb  implementing the second charge-extraction MOS transistor Qb D  is set at a low potential (L), Since the second charge-modulation portion includes the first right transfer transistor QU 1T , the second right transfer transistor Qb 2T  and the third right transfer transistor Qb 3T , the equivalent circuit is illustrated with the case in which one end of each of the first right transfer transistor Qb 1T , the second right transfer transistor Qb 2T  and the third right transfer transistor Qb 3T , is connected to the second junction field-effect transistor Qb P2  in the T-shaped manner. 
     The other ends of the first right transfer transistor Qb 1T , the second right transfer transistor Qb 1T  and the third right transfer transistor Qb 3T  are connected to the first right charge-accumulation region  24   bb , the second right charge-accumulation region  24   bd  and the third right charge-accumulation region  24   bc, respectively.    
     The first right charge-accumulation region  24   bb  is short-circuited with the first left charge-accumulation region  24   ab  via a surface line such as a metal wire, the second right charge-accumulation region  24   bd  is short-circuited with the second left charge-accumulation region  24   ad  via a surface line, and the third right charge-accumulation region  24   bc  is short-circuited with the third left charge-accumulation region  24   ac  via a surface line, although not shown in  FIG. 18 . When the voltage at the intermediate potential (M) is applied to one of the first right transfer gate electrode  42   b , the second right transfer gate electrode  44   b  and the third right transfer gate electrode  43   b , and the voltage at the low potential (L) is applied to the other two electrodes, the photoelectrons are transferred to one of the first right charge-accumulation region  24   bb  serving as common node D 1 , the second right charge-accumulation region  24   bd  serving as common node D 2  and the third right charge-accumulation region  24   bc  serving as common node D 3 . 
     The three common nodes D 1 , D 3  and D 2  shown in  FIG. 19  are connected to capacitors C 1 , C 3  and C 2  for charge-accumulation. The capacitors C 1 , C 3  and C 2  are preferably depletion-mode MOS capacitors in which a threshold voltage is set at a negative voltage so as to reduce voltage dependence. The first common node D 1  is connected to a gate terminal of a first amplification transistor Q 1A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the first common node D 1 . The first common node D 1  is further connected to a first reset transistor Q 1R  for initializing signal charges after signal readout. A source terminal of the first amplification transistor Q 1A  is connected to a first selection transistor Q 1S , serving as a switch for readout image selection. An output of the first selection transistor Q 1S  is connected to a signal readout line extending in the vertical direction. 
     The second common node D 2  is connected to a gate terminal of a second amplification transistor Q 2A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the second common node D 2 . The second common node is further connected to a second reset transistor Q 2R  for initializing signal charges after signal readout. A source terminal of the second amplification transistor Q 2A  is connected to a second selection transistor Q 2S , serving as a switch for readout image selection. An output of the second selection transistor Q 2s  is connected to a signal readout line extending in the vertical direction. The third common node D 3  is connected to a gate terminal of a third amplification transistor Q 3A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the third common node D 3 . The third common node D 3  is further connected to a third reset transistor Q 3R  for initializing signal charges after signal readout. A source terminal of the third amplification transistor Q 3A  is connected to a third selection transistor Q 3S , serving as a switch for readout image selection. An output of the third selection transistor Q 3S  is connected to a signal readout line extending in the vertical direction. 
     The signal readout method in the range sensor according to the fourth embodiment may be either a method of reading out a signal from each of three signal readout lines arranged in parallel as shown in  FIG. 19 , or a method of turning on a switch sequentially by selection signals SL 1 , SL 2  and SL 3  to read out each signal as a time-series signal from a single signal readout line as shown in  FIG. 11 . 
     In the range sensor and the solid-state imaging device according to the fourth embodiment, the first right charge-accumulation region  24   bb  is short-circuited with the first left charge-accumulation region  24   ab , the second right charge-accumulation region  24   bd  is short-circuited with the second left charge-accumulation region  24   ad , and the third right charge-accumulation region  24   bc  is short-circuited with the third left charge-accumulation region  24   ac , so as to receive light in both the first photodiode Da ij ; and the second photodiode Db ij , and accumulate signals as charges after subjected to charge modulation in both the first and second charge-modulation portions to amplify the signals. 
     As in the case of the range sensor according to the first and second embodiments, the range sensor according to the fourth embodiment is provided with the static induction channel having a sufficient length from the trunk path adjacent to the first and second light-receiving areas to the respective first and second charge-modulation portions, which are shielded by the shielding plate  51 , so as to achieve remarkable effects upon reducing the influence of background light while using a pulse at short intervals. 
     The range sensor according to the fourth embodiment has the function suitable for the time-of-flight measurement by use of the Eq. (4) or the Eq. (8) as described in the first embodiment. The range sensor according to the fourth embodiment thus can serve as a pixel (lock-in pixel) of a solid-state imaging device to achieve a time-of-flight range image sensor capable of rapid transfer of signal charges with high sensitivity at a low dark current even when a plurality of pixels each having a large area of which a pixel size is five micrometers square or greater are provided on a single semiconductor chip. 
     (Fifth Embodiment) 
     A range sensor according to the fifth embodiment of the present invention is a lock-in pixel including a shielding plate  51  having four apertures: a first aperture indicated by the dashed-dotted line on the upper left side, a second aperture indicated by the dashed-dotted line on the lower left side, a third aperture indicated by the dashed-dotted line on the lower right side, and a fourth aperture indicated by the dashed-dotted line on the upper right side, to define tour light-receiving areas, as shown in the schematic plan view from above of the principal part in  FIG. 20 . 
     As indicated by the two-dot dashed lines in  FIG. 20 , a first microlens  53   p  is provided over a first light-receiving area defined by the first aperture, a second microlens  53   q  is provided over a second light-receiving area defined by the second aperture, a third microlens  53   r  is provided over a third light-receiving area defined by the third aperture, and a fourth microlens  53   s  is provided over a fourth light-receiving area defined by the fourth aperture. Four photodiodes provided in the four light-receiving areas receive light collected by the independent microlenses  53   p ,  53   q ,  53   r  and  53   s . The microlenses  53   p ,  53   q ,  53   r  and  53   s  are not necessarily provided and may be omitted when the intensity of incident light (incoming light) is high. 
     When focused on the first aperture provided on the upper left side and the second aperture provided on the lower left side in  FIG. 20 , the range sensor according to the fifth embodiment has a structure similar to that shown in the cross sections of  FIG. 2  to  FIG. 4 , including: a pixel layer  22  made of a semiconductor of a first conductivity type (p-type); and a surface-buried region  25   p  of a second conductivity type (n-type) selectively buried in the upper portion of the pixel layer  22  to implement a first photodiode in the first light-receiving area defined by the first aperture by forming a junction with the pixel layer  22  and a second photodiode in the second light-receiving area defined by the second aperture by forming a junction with the pixel layer  22 , connecting the upper and second light-receiving areas in the vertical direction, and including a charge-modulation arrangement-region extending from the portion where the first and second light-receiving areas are connected toward the portion shielded by the shielding plate  51  along the upper portion of the pixel layer  22  in the right direction. 
     The surface-buried region  25   p  in the range sensor according to the fifth embodiment includes first to fourth light-receiving terminal-portions extending into a four-leaf clover shape on both sides of the charge-modulation arrangement-region in which the first light-receiving terminal-portion extending on the upper left side protrudes toward the first light-receiving area, the second light-receiving terminal-portion extending on the lower left side protrudes toward the second light-receiving area, the third light-receiving terminal-portion extending on the lower right side protrudes toward the third light-receiving area, and the fourth light-receiving terminal-portion extending on the upper right side protrudes toward the fourth light-receiving area. 
     As shown in the enlarged plan view of  FIG. 21 , the charge-modulation arrangement-region of the surface-buried region  25   p  further extends toward the right on the pixel layer  22  shielded by the shielding plate  51  to implement a third photodiode in the third light-receiving area defined below the third aperture indicated on the lower right side by forming a junction with the pixel layer  22  and a fourth photodiode in the fourth light-receiving area defined below the fourth aperture indicated on the upper right side by forming a junction with the pixel layer  22 . Thus, the entire surface-buried region  25   p  has a symmetrical four-leaf clover shape as shown in  FIG. 20  and is provided with the charge-modulation arrangement-region in the central portion to implement a continuous semiconductor region. 
     As shown in the enlarged plan view of  FIG. 21 , the charge-modulation arrangement-region in the central portion of the surface-buried region  25   p  has a polygonal shape including fishbone-shaped branches extending in the vertical direction, instead of a rectangular shape extending straight in the horizontal direction. In other words, the surface-buried region  25   p  including the charge-modulation arrangement-region in the central portion is provided with first and second branches extending downward in parallel on the lower side of the charge-modulation arrangement-region, and a third branch extending upward on the upper side of the charge-modulation arrangement-region on the opposite side of the first and second branches. 
     As shown in  FIG. 20 , the first light-receiving terminal-portion on the upper left side has an area sufficient to cover substantially the entire first aperture, the second light-receiving terminal-portion on the lower left side has an area sufficient to cover substantially the entire second aperture, the third light-receiving terminal-portion on the lower right side has an area sufficient to cover substantially the entire third aperture, and the fourth light-receiving terminal-portion on the upper right side has an area sufficient to cover substantially the entire fourth aperture. 
     In the central portion of the charge-modulation arrangement-region, a first charge-accumulation region  24   h  and a second charge-accumulation region  24   i  of n-type conductivity having a higher impurity concentration than the first surface-buried region  25   p  are connected to the tips of the branches of the first and second branches, respectively, and a third charge-accumulation region  24   l  of n-type conductivity having a higher impurity concentration than the first surface-buried region  25   p  is connected to the tip of the branch of the third branch. 
     Reference numeral  32   p  shown in  FIG. 20  denotes an edge of a thick field insulating film. A well region of p-type conductivity is buried below the field insulating film (not shown), as in the case shown in the cross sections of  FIG. 2A ,  FIG. 3A  and  FIG. 4A . The first charge-accumulation region  24   h , the second charge-accumulation region  24   i  and the third charge-accumulation region  24   l  are surrounded by the well region to serve as floating diffusion layers provided above the pixel layer  22 . Although the range sensor according to the fifth embodiment has a structure for illustration purposes in which the number of the floating diffusion layers in which signal charges from the four light-receiving areas are accumulated is three, the number of the floating diffusion layers may be two or more than three. 
     The range sensor according to the fifth embodiment includes a first transfer-control mechanism ( 31 ,  42   h ), a second transfer-control mechanism ( 31 ,  44   h ) and a third transfer-control mechanism ( 31 ,  43   h ) provided adjacent to the first charge-accumulation region  24   h , the second charge-accumulation region  24   i  and the third charge-accumulation region  24   l , respectively; to control a transfer of signal charges to the first charge-accumulation region  24   h , the second charge-accumulation region  24   i  and the third charge-accumulation region  24   l.    
     The range sensor according to the fifth embodiment further includes a guide region  26   f  of n-type conductivity, which is buried in a part of the upper portion of the surface-buried region  25   p , having an H-shape including first and second protruding-edges connecting the ends of the first and second photodiodes in the vertical direction on the left side in  FIG. 21  and third and fourth protruding-edges connecting the ends of the third and fourth photodiodes in the vertical direction on the right side in  FIG. 21 , and having a higher impurity concentration than the first surface-buried region  25   p  and a lower impurity concentration than the first charge-accumulation region  24   h , the second charge-accumulation region  24   i  and the third charge-accumulation region  24   l.    
     Since the range sensor according to the fifth embodiment is a lock-in pixel including the four light-receiving areas, the H-shaped guide region  26   f  includes the four protruding-edges corresponding to the four apertures. The central portion surrounded by the four protruding-edges of the H-shaped guide region  26   f  is not formed into a rectangular shape extending straight in the horizontal direction but is provided with branches extending in the vertical direction as shown in the enlarged plan view of  FIG. 21 . The entire shape of the guide region  26   f  is thus not an exact H-shape. 
     The range sensor according to the fifth embodiment includes a first p-type pinning layer, which is in contact with the surface of the surface-buried region  25   p  (not shown), as in the case shown in the cross-sectional views of  FIG. 2  to  FIG. 4 . The pixel layer  22  is laminated on a p-type semiconductor substrate. 
     The range sensor according to the fifth embodiment includes a charge-modulation portion implemented by the first transfer-control mechanism ( 31 ,  42   h ), the second transfer-control mechanism ( 31 ,  44   h ) and the third transfer-control mechanism ( 31 ,  43   h ). The guide region  26   f  having a substantially H-shaped planar structure is a semiconductor region for collecting photoelectrons as signal charges from the four regions and guiding the collected photoelectrons to the narrow transfer channels in the charge-modulation portion provided in the central portion of the H-shape, and has a higher impurity concentration than the first surface-buried region  25   p.    
     As in the case of the structure shown in  FIG. 3A  and  FIG. 4A , the first transfer-control mechanism ( 31 ,  42   h ), the second transfer-control mechanism ( 31 ,  44   h ) and the third transfer-control mechanism ( 31 ,  43   h ) implementing the range sensor according to the fifth embodiment respectively include an insulating film  31  provided on the respective first, second and third branches, and further include a first transfer gate electrode  42   h , a second transfer gate electrode  44   h  and a third transfer gate electrode  43   h  provided on the insulating film  31 . 
     As in the case of the structure shown in  FIG. 3A  and  FIG. 4A , the thickness of the insulating film  31  immediately below the respective first transfer gate electrode  42   h,  second transfer gate electrode  44   h  and third transfer gate electrode  43   h  is thinner than the other sites, and the thinner parts of the insulating film  31  each serve as a “gate insulating film”. 
     Although the guide region  26   f  cannot be viewed from above because of the upper elements such as the insulating film  31  and the first pinning layer, the guide region  26   f  has a planar pattern in which the four ends of the substantially H-shaped guide region  26   f  toward the four photodiodes are exposed to the four apertures of the shielding plate  51 , and the other area is shielded by the shielding plate  51 , in the plan view of the shielding plate  51  as viewed from above. 
     In the range sensor according to the fifth embodiment, the voltage applied to each of the first transfer gate electrode  42   h , the second transfer gate electrode  44   h  and the third transfer gate electrode  43   h  controls potentials in the transfer channels defined by the respective branches, so as to control the signal charge transferred to the first charge-accumulation region  24   h , the second charge-accumulation region  24   i  and the third charge-accumulation region  24   l.    
     As described above, the surface-buried region  25   p  in the range sensor according to the fifth embodiment has a fishbone shape in the central portion. The charge-modulation arrangement-region corresponding to a backbone part of the fishbone shape is provided at both ends with additional branches extending in the vertical direction perpendicular to the extending direction of the charge-modulation arrangement-region, a first extraction region  24   g  and a second extraction region  24   f  of n-type conductivity having a higher impurity concentration than the surface-buried region  25   p  are connected to the tips of the additional branches extending in the vertical direction on the left side of the charge-modulation arrangement-region, a third extraction region  24   j  and a fourth extraction region  24   k  of n-type conductivity having a higher impurity concentration than the surface-buried region  25   p  are connected to the tips of the additional branches extending in the vertical direction on the right side of the charge-modulation arrangement-region. 
     As shown in  FIG. 20 , the range sensor according to the fifth embodiment further includes a first extraction control mechanism ( 31 ,  41   m ) provided adjacent to the first extraction region  24   g  and configured to control extraction of charges toward the first extraction region  24   g  via the branch extending in the lower direction of the surface-buried region  25   p , a second extraction control mechanism ( 31 ,  41   l ) provided adjacent to the second extraction region  24   f  and configured to control extraction of charges toward the second extraction region  24   f  via the branch extending in the upper direction of the surface-buried region  25   p , a third extraction control mechanism ( 31 ,  41   n ) provided adjacent to the third extraction region  24   j  and configured to control extraction of charges toward the third extraction region  24   j  via the branch extending in the lower direction of the surface-buried region  25   p , and a fourth extraction control mechanism ( 31 ,  41   o ) provided adjacent to the fourth extraction region  24   k  and configured to control extraction of charges toward the fourth extraction region  24   k  via the branch extending in the upper direction of the surface-buried region  25   p.    
     As in the case of the structure shown in  FIG. 2A , the first extraction control mechanism ( 31 ,  41   m ), the second extraction control mechanism ( 31 ,  41   l ), the third extraction control mechanism ( 31 ,  42   n ) and the fourth extraction control mechanism ( 31 ,  41   o ) include the insulating film  31  provided on the respective additional branches of the surface-buried region  25   p , and further include a first extraction gate electrode  41   m , a second extraction gate electrode  41   l , third extraction gate electrode  41   n  and a fourth extraction gate electrode  41   o , respectively, provided on the insulating film  31 . As in the case of the structure shown in  FIG. 2A , the thickness of the insulating film  31  immediately below the respective first extraction gate electrode  41   m , second extraction gate electrode  41   l , third extraction gate electrode  41   n  and fourth extraction gate electrode  41   o  is thinner than the other sites, and the thinner parts of the insulating film  31  each serve as a “gate insulating film”. The first extraction gate electrode  41   m , the second extraction gate electrode  41   l , the third extraction gate electrode  41   n  and the fourth extraction gate electrode  41   o  are symmetrically provided at four corners of the planar structure of the substantially H-shaped guide region  26   f.    
     When a solid-state imaging device (image sensor) having a large pixel area and using a single photodiode, as in the case of the range sensors according to the first and second embodiments, cannot ensure a sufficiently rapid response, a plurality of photodiodes may be provided in one pixel, as shown in  FIG. 20 , so as to accumulate output from the plural photodiodes to amplify a signal. The range sensor according to the fifth embodiment shown in  FIG. 20  corresponds to a case in which four range sensors each corresponding to the first and second embodiments are provided in one pixel. 
     As shown in  FIG. 20 , the range sensor according to the fifth embodiment is provided with the first extraction gate electrode  41   m  and the second extraction gate electrode  41   l  symmetrically arranged to interpose the charge-modulation arrangement-region in the vertical direction between a portion vertically connecting the first and second photodiodes provided on the upper left side and the lower left side and the charge-modulation portion provided in the central portion of the charge-modulation arrangement-region extending toward the right from the connecting portion. Accordingly, the first extraction gate electrode  41   m  and the second extraction gate electrode  41   l  can switch the operation between the extraction of photoelectrons toward the first extraction region  24   g  and the second extraction region  24   f  and the transport of the photoelectrons toward the charge-modulation portion including the first transfer-control mechanism ( 31 ,  42   h ), the second transfer-control mechanism ( 31 ,  44   h ) and the third transfer-control mechanism ( 31 ,  43   h ). 
     Similarly, the range sensor according to the fifth embodiment is provided with the third extraction gate electrode  41   n  and the fourth extraction gate electrode  41   o  symmetrically arranged to interpose the charge-modulation arrangement-region in the vertical direction between a portion vertically connecting the third and fourth photodiodes provided on the lower right side and the upper right side and the charge-modulation portion provided in the central portion of the charge-modulation arrangement-region extending toward the left from the connecting portion. Accordingly; the third extraction gate electrode  41   n  and the fourth extraction gate electrode  41   o  can switch the operation between the extraction of photoelectrons toward the third extraction region  24   j  and the fourth extraction region  24   k  and the transport of the photoelectrons toward the charge-modulation portion including the first transfer-control mechanism ( 31 ,  42   h ), the second transfer-control mechanism ( 31 ,  44   h ) and the third transfer-control mechanism ( 31 ,  43   h ). 
       FIG. 22  illustrates an equivalent circuit of the range sensor according to the fifth embodiment including four photodiodes in one pixel: a first photodiode Dp ij , a second photodiode Dq ij , a third photodiode and a fourth photodiode Ds ij . The charge-modulation portion common to the four photodiodes in the range sensor according to the fifth embodiment is implemented by three transistors arranged in parallel: a first transfer transistor Q 1T  as the first transfer-control mechanism ( 31 ,  42   h ), a second transfer transistor Q 2T  as the second transfer-control mechanism ( 31 ,  44   h ), and a third transfer transistor Q 3T  as the third left transfer-control mechanism ( 31 ,  43   h ), as indicated in the middle in  FIG. 22 . 
     The static induction channels from the respective first photodiode Dp ij  and second photodiode Dq ij  indicated on the upper left side to the “common charge-modulation portion” are shown as a circuit indicated by the broken lines in  FIG. 22 . The static induction channel is indicated on the upper left side by two first junction field-effect transistors Qu P1  and Qu P2  of which own gates are grounded. A source terminal of a first charge-extraction MOS transistor Qu D  for charge extraction is connected to a center tap of the two first junction field-effect transistors Qu P1  and Qu P2  connected in series, and a drain terminal of the first charge-extraction MOS transistor Qu D  is connected to a power source V DD  having a high potential. 
     The broken lines in  FIG. 22  on the upper left side indicate paths in which electrons flow rapidly due to a depletion electric field because of the connection in the semiconductor region in  FIG. 22 , photoelectrons generated in the first photodiode Dp ij  and the second photodiode Dq ij  immediately reach the common charge-modulation portion as signal charges when the voltage G D  applied to each of the first extraction gate electrode  41   m  and the second extraction gate electrode  41   l  implementing the first charge-extraction MOS transistor Qu D  is set at a low potential (L). The equivalent circuit is illustrated such that one end of each of the first transfer transistor Q 1T , the second transfer transistor Q 2T  and the third transfer transistor Q 3T  implementing the common charge-modulation portion is connected to the first junction field-effect transistor QU P2  in the T-shaped manner. 
     The static induction channel from the respective third photodiode Dr ij  and fourth photodiode Ds ij  indicated on the lower left side to the common charge-modulation portion is indicated by two second junction field-effect transistors Qv P1  and Qv P2  of which own gates are grounded. A source terminal of a second charge-extraction MOS transistor Qv D  for charge extraction is connected to a center tap of the two second junction field-effect transistors Qv P1  and Qv P2  connected in series, and a drain terminal of the second charge-extraction MOS transistor Qv D  is connected to a power source V DD  having a high potential. 
     The broken lines in  FIG. 22  on the lower left side indicate paths in which electrons flow rapidly due to a depletion electric field because of the connection in the semiconductor region in  FIG. 22 , photoelectrons generated in the third photodiode and the fourth photodiode Ds ij  immediately reach the common charge-modulation portion when the voltage G D  applied to each of the third extraction gate electrode  41   n  and the fourth extraction gate electrode  41   o  implementing the second charge-extraction MOS transistor Qv D  is set at a low potential (L). 
     The equivalent circuit is illustrated such that one end of each of the first transfer transistor Q 1T , the second transfer transistor Q 2T  and the third transfer transistor Q 3T  is connected to the second junction field-effect transistor Qv P2  in the T-shaped manner. 
     Namely, the output terminal of the first junction field-effect transistor Qu P2  and the output terminal of the second junction field-effect transistor Qv P2  are connected to the input terminal of each of the first transfer transistor Q 1T , the second transfer transistor Q 2T  and the third transfer transistor Q 3T  arranged in parallel in the T-shaped manner. 
     In the circuit diagram, the other ends of the first transfer transistor Q 1T , the second transfer transistor Q 2T  and the third transfer transistor Q 3T  are connected to the first charge-accumulation region  24   h  serving as node D 1 , the second charge-accumulation region  24   i  serving as node D 2  and the third charge-accumulation region  24   l  serving as node D 3 , respectively. When the voltage at the intermediate potential (M) is applied to one of the first transfer gate electrode  42   h , the second transfer gate electrode  44   h  and the third transfer gate electrode  43   h , and the voltage at the low potential (L) is applied to the other two electrodes, the photoelectrons are transferred to one of the first charge-accumulation region  24   h , the second charge-accumulation region  24   i  and the third charge-accumulation region  24   l.    
     The three common nodes D 1 , D 3  and D 2  shown in  FIG. 22  are connected to capacitors C 1 , C 3  and C 2  for charge-accumulation. The capacitors C 1 , C 3  and C 2  are preferably depletion-mode MOS capacitors in which a threshold voltage is set at a negative voltage so as to reduce voltage dependence. The first node D 1  is connected to a gate terminal of a first amplification transistor Q 1A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the first node D 1 . The first node D 1  is further connected to a first reset transistor Q 1R  for initializing signal charges after signal readout. A source terminal of the first amplification transistor QiA is connected to a first selection transistor Q 1S , serving as a switch for readout image selection. An output of the first selection transistor Q 1S  is connected to a signal readout line extending in the vertical direction. 
     The second node D 2  is connected to a gate terminal of a second amplification transistor Q 2A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the second node D 2 . The second node D 2  is further connected to a second reset transistor Q 2R  for initializing signal charges after signal readout. A source terminal of the second amplification transistor Q 2A  is connected to a second selection transistor Q 2S , serving as a switch for readout image selection. An output of the second selection transistor Q 2S  is connected to a signal readout line extending in the vertical direction. The third node D 3  is connected to a gate terminal of a third amplification transistor Q 3A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the third node D 3 . The third node D 3  is further connected to a third reset transistor Q 3R  for initializing signal charges after signal readout. A source terminal of the third amplification transistor Q 3A  is connected to a third selection transistor Q 3S , serving as a switch for readout image selection. An output of the third selection transistor Q 3S  is connected to a signal readout line extending in the vertical direction. 
     The signal readout method in the range sensor according to the fifth embodiment may be either a method of reading out a signal from each of three signal readout lines arranged in parallel as shown in  FIG. 22 , or a method of turning on a switch sequentially by selection signals SL 1 , SL 2  and SL 3  to read out each signal as a time-series signal from a single signal readout line as shown in  FIG. 11 . 
     In the range sensor and the solid-state imaging device according to the fifth embodiment, the first photodiode Dp ij , the second photodiode Dq ij , the third photodiode Dr ij  and the fourth photodiode Ds ij  each receive light and accumulate signals as charges after subjected to charge modulation in the common charge-modulation portion provided in the central portion of the pixel so as to amplify the signals. 
     When a solid-state imaging device (image sensor) having a large pixel area and using a single photodiode cannot ensure a sufficiently rapid response, a plurality of photodiodes, each based on the structure according to the first embodiment, may be provided around the pixel as illustrated in the fifth embodiment to merge the common part in the central portion of the pixel, so as to strengthen the function of the range sensor according to the first embodiment and exhibit a more rapid response and higher-speed performance (improve the efficiency of charge collection). 
     The range sensor according to the fifth embodiment has the function suitable for the time-of-flight measurement by use of the Eq. (4) the Eq. (8), as described in the first embodiment. The range sensor according to the fifth embodiment thus can serve as a pixel (lock-in pixel) of a solid-state imaging device to achieve a time-of-flight range image sensor capable of rapid transfer of signal charges with high sensitivity at a low dark current even when a plurality of pixels each having a large area of which a pixel size is five micrometers square or greater are provided on a single semiconductor chip. 
     As in the case of the range sensor according to the respective first to fourth embodiments, the range sensor according to the fifth embodiment is provided with the static induction channel having a sufficient length from each end of the charge-modulation arrangement-region to the central portion of the charge-modulation portion, which is shielded by the shielding plate  51 , so as to achieve remarkable effects upon reducing the influence of background light while using a pulse at short intervals. 
     Although the range sensor according to the fifth embodiment has a structure for illustration purposes in which the four photodiodes are provided around the pixel to each receive light to accumulate signals as charges after subjected to charge modulation in the common charge-modulation portion provided in the central portion of the pixel to amplify the signals, the number of photodiodes provided around the pixel may be two or the other number such as six or eight, depending on the size of the pixel area or the response speed or sensitivity to be required. 
     &lt;Modified Example 1 of Fifth Embodiment&gt; 
     A range sensor according to a modified example (modified example 1) of the fifth embodiment of the present invention is a lock-in pixel having the same structure as shown in  FIG. 20  and  FIG. 21  including a shielding plate  51  having four apertures: a first aperture on the upper left side, a second aperture on the lower left side, a third aperture on the lower right side, and a fourth aperture on the upper right side, each indicated by the dashed-dotted line, to define four light-receiving areas, as shown in the schematic plan view from above of the principal part in  FIG. 23 . The range sensor of this example differs in a planar pattern of a surface-buried region  25   q  of n-type conductivity, which is buried in the pixel layer  22  made of the n-type semiconductor, from the surface-buried region  25   p  shown in  FIG. 20  and  FIG. 21  having a four-leaf clover shape in the plan view. 
     The surface-buried region  25   q  of the range sensor according to modified example 1 of the fifth embodiment is selectively buried in the upper portion of the pixel layer  22  to implement a first photodiode in a first light-receiving area defined by a first aperture and a second photodiode in a second light-receiving area defined by a second aperture, in which first and second light-receiving terminal-portions are connected to provide a vertically-integrated region in a planar pattern. This topology differs from the surface-buried region  25   p  in the planar pattern shown in  FIG. 20  and  FIG. 21 . 
     The surface-buried region  25   q  is further selectively buried in the upper portion of the pixel layer  22  to implement a third photodiode in a third light-receiving area defined by a third aperture and a fourth photodiode in a fourth light-receiving area defined by a fourth aperture, in which third and fourth light-receiving terminal-portions are connected to provide a vertically-integrated region in a planar pattern. This topology also differs from the surface-buried region  25   p  shown in  FIG. 20  and  FIG. 21 . 
     As shown in the plan view of  FIG. 23 , the first light-receiving terminal-portion in the surface-buried region  25   q  on the upper left side has an area sufficient to cover substantially the entire first aperture, the second light-receiving terminal-portion in the surface-buried region  25   q  on the lower left side has an area sufficient to cover substantially the entire second aperture, the third light-receiving terminal-portion in the surface-buried region  25   q  on the lower right side has an area sufficient to cover substantially the entire third aperture, and the fourth light-receiving terminal-portion in the surface-buried region  25   q  on the upper right side has an area sufficient to cover substantially the entire fourth aperture. 
     Since the first and second light-receiving terminal-portions are integrated in a planar pattern, and the third and fourth light-receiving terminal-portions are integrated in a planar pattern, the first and second light-receiving areas implement the common left side of the surface-buried region  25   q , and the third and fourth light-receiving areas implement the common right side of the surface-buried region  25   q , so as to provide a simpler planar pattern in lithography. Further, since the first and second light-receiving terminal-portions are integrated in a planar pattern, and the third and fourth light-receiving terminal-portions are integrated in a planar pattern, the area covered with the shielding plate  51  is larger than that in the planar pattern of the surface-buried region  25   p  shown in  FIG. 20  and  FIG. 21 . 
     The topology of the surface-buried region  25   q  in which the integrated region of the first and second light-receiving terminal-portions on the left side and the integrated region of the third and fourth light-receiving terminal-portions on the right side are connected by a charge-modulation arrangement-region including fishbone-shaped branches extending in the vertical direction, is the same as that of the surface-buried region  25   p  shown in  FIG. 20  and  FIG. 21 . 
     The topology in the charge-modulation arrangement-region in the central portion of the surface-buried region  25   q , including a first charge-accumulation region  24   h  and a second charge-accumulation region  24   i  of n-type conductivity having a higher impurity concentration than the surface-buried region  25   q  which are connected to the tips of branches of first and second branches, respectively, a third charge-accumulation region  24   l  of n-type conductivity having a higher impurity concentration than the surface-buried region  25   q  which is connected to the tip of a branch of a third branch, and a guide region  26   f , is the same as that of the surface-buried region  25   p  shown in  FIG. 20  and  FIG. 21 , and overlapping explanations are thus not repeated below. 
     The range sensor according to modified example 1 of the fifth embodiment having a simple planar pattern in which the first and second light-receiving terminal-portions are connected to provide an integrated region, and the third and fourth light-receiving terminal-portions are connected to provide an integrated region, as shown in  FIG. 23 , can also receive light in the four photodiodes and accumulate signals as charges after subjected to charge modulation in the common charge-modulation portion provided in the central portion of the pixel so as to amplify the signals. 
     The range sensor according to modified example 1 of the fifth embodiment including a plurality of photodiodes provided in a simpler planar pattern and having a merged common part in the central portion, can strengthen the function of the range sensor according to the first embodiment and exhibit a more rapid response and higher-speed performance (improve the efficiency of charge collection). 
     &lt;Modified example 2 of fifth embodiment&gt; 
     A range sensor according to modified example 2 of the fifth embodiment of the present invention is a lock-in pixel having the same structure as shown in  FIG. 20  including a shielding plate  51  having four apertures: a first aperture as indicated by the dashed-dotted line on the upper left side, a second aperture as indicated by the dashed-dotted line on the lower left side, a third aperture as indicated by the dashed-dotted line on the lower right side, and a fourth aperture as indicated by the dashed-dotted line on the upper right side, to define four light-receiving areas, as shown in the schematic plan view from above of the principal part in  FIG. 24 . The lock-in pixel shown in  FIG. 24  also includes four photodiodes provided in the respective four light-receiving areas and receiving light collected in the respective independent microlenses  53   p ,  53   q ,  53   r  and  53   s , as in the case of the lock-in pixel shown in  FIG. 20 . 
     The range sensor according to modified example 2 of the fifth embodiment includes a guide region  26   g  including a first protruding-edge tapered into a Z-shape and extending in the upper left direction toward the first photodiode provided on the upper left side in  FIG. 24 , and a second protruding-edge tapered into a Z-shape and extending in the lower left direction toward the second photodiode provided on the lower left side in  FIG. 24 . The guide region  26   g  further includes a third protruding-edge tapered into a Z-shape and extending in the lower left direction toward the third photodiode provided on the lower left side in  FIG. 24 , and a fourth protruding-edge tapered into a Z-shape and extending in the upper right direction toward the fourth photodiode provided on the upper right side in  FIG. 24 . 
     The planar pattern of the lock-in pixel shown in  FIG. 24  differs from that shown in  FIG. 20  in that the guide region  26   g  connects the first and second photodiodes in the vertical direction with the Z-shaped first and second protruding-edges and connects the third and fourth photodiodes in the vertical direction with the Z-shaped third and fourth protruding-edges. 
     As shown in  FIG. 24 , although the guide region  26   g  according to modified example 2 of the fifth embodiment has a tour-legged (bronze vessel) shape implemented by the four protruding-edges including the Z-shaped first and second protruding-edges symmetrically opposed in the vertical direction on the left side and the Z-shaped third and fourth protruding-edges symmetrically opposed in the vertical direction on the right side, the guide region  26   g  entirely has an H-shape and is buried in a part of the surface-buried region  25   p  on the upper side. The guide region  26   g  is a semiconductor region of n-type conductivity, which is the same conductivity type as the surface-buried region  25   p , and has a higher impurity concentration than the surface-buried region  25   p.    
     However, the central portion surrounded by the four protruding-edges of the H-shaped guide region  26   g  is not formed into a rectangular shape extending straight in the horizontal direction but is provided with branches extending in the vertical direction as shown in the plan view of  FIG. 24 . The entire shape of the guide region  26   g  is thus not an actual H-shape. The range sensor according to modified example 2 of the fifth embodiment includes a charge-modulation portion implemented by a first transfer-control mechanism ( 31 ,  42   h ), a second transfer-control mechanism ( 31 ,  44   h ) and a third transfer-control mechanism ( 31 ,  43   h ). The guide region  26   g  having a substantially H-shaped planar structure is a semiconductor region for collecting photoelectrons as signal charges from the four regions and guiding the collected photoelectrons to the narrow transfer channels in the charge-modulation portion provided in the central portion of the H-shape, and has a higher impurity concentration than the surface-buried region  25   p.    
     The first to fourth protruding-edges of the guide region  26   g  each have a stepped shape gradually increased in width from the respective edges on the photodiode side to the central portion of the H-shape. The range sensor according to modified example 2 of the fifth embodiment generates a high drift field in the entire region of the respective first to fourth protruding-edges of the depleted guide region  26   g  gradually increased in width toward the central portion of the H-shape. Accordingly, photoelectrons as signal charges can rapidly be transferred in the longitudinal direction of the first to fourth protruding-edges of the guide region  26   g  toward the central portion of the H-shape. 
     As in the case of the structure shown in  FIG. 3A  and  FIG. 4A , and particularly the structure shown in  FIG. 20 , the first transfer-control mechanism ( 31 ,  42   h ), the second transfer-control mechanism ( 31 ,  44   h ) and the third transfer-control mechanism ( 31 ,  43   h ) implementing the range sensor according to modified example 2 of the fifth embodiment respectively include an insulating film  31  provided on the respective first, second and third branches, and further include a first transfer gate electrode  42   h , a second transfer gate electrode  44   h  and a third transfer gate electrode  43   h  provided on the insulating film  31 . The range sensor according to modified example 2 of the fifth embodiment has the same topology as that shown in  FIG. 20  in which the surface-buried region  25   p  has a fishbone shape in the central portion, a first extraction region  24   g  and a second extraction region  24   f  of n-type conductivity having a higher impurity concentration than the surface-buried region  25   p  are connected to the tips of additional branches extending in the vertical direction on the left side of the charge-modulation arrangement-region, and a third extraction region  24   j  and a fourth extraction region  24   k  of n-type conductivity having a higher impurity concentration than the surface-buried region  25   p  are connected to the tips of additional branches extending in the vertical direction on the right side of the charge-modulation arrangement-region. 
     As shown in  FIG. 24 , the range sensor according to modified example 2 of the fifth embodiment further includes a first extraction control mechanism ( 31 ,  41   q ) provided adjacent to the first extraction region  24   g  and configured to control extraction of charges toward the first extraction region  24   g  via the branch extending in the lower direction of the surface-buried region  25   p , a second extraction control mechanism ( 31 ,  41   p ) provided adjacent to the second extraction region  24   f  and configured to control extraction of charges toward the second extraction region  24   f  via the branch extending in the upper direction of the surface-buried region  25   p , a third extraction control mechanism ( 31 ,  41   r ) provided adjacent to the third extraction region  24   j  and configured to control extraction of charges toward the third extraction region  24   j  via the branch extending in the lower direction of the surface-buried region  25   p , and a fourth extraction control mechanism ( 31 ,  41   s ) provided adjacent to the fourth extraction region  24   k  and configured to control extraction of charges toward the fourth extraction region  24   k  via the branch extending in the upper direction of the surface-buried region  25   p.  The first extraction control mechanism ( 31 ,  41   q ), the second extraction control mechanism ( 31 ,  41   p ), the third extraction control mechanism ( 31 ,  41   r ) and the fourth extraction control mechanism ( 31 ,  41   s ) differ from the respective mechanisms shown in  FIG. 20  in that a first extraction gate electrode  41   q , a second extraction gate electrode  41   p , a third extraction gate electrode  41   r  and a fourth extraction gate electrode  41   s  implementing the first extraction control mechanism ( 31 ,  41   q ), the second extraction control mechanism ( 31 ,  41   p ), the third extraction control mechanism ( 31 ,  41   r ) and the fourth extraction control mechanism ( 31 ,  41   s ) each have an L-shape in a planar pattern. 
     As shown in  FIG. 24 , a plurality of photodiodes is provided in one pixel, so as to accumulate output from the plural photodiodes to amplify a signal per pixel. As shown in  FIG. 24 , the range sensor according to modified example 2 of the fifth embodiment is provided with the first extraction gate electrode  41   q  and the second extraction gate electrode  41   p  symmetrically arranged to interpose the charge-modulation arrangement-region in the vertical direction between a portion vertically connecting the first and second photodiodes provided on the upper left side and the lower left side and the charge-modulation portion provided in the central portion of the charge-modulation arrangement-region extending toward the right from the connecting portion. Accordingly, the first extraction gate electrode  41   q  and the second extraction gate electrode  41   p  can switch the operation between the extraction of photoelectrons toward the first extraction region  24   g  and the second extraction region  24   f  and the transport of the photoelectrons toward the charge-modulation portion including the first transfer-control mechanism ( 31 ,  42   h ), the second transfer-control mechanism ( 31 ,  44   h ) and the third transfer-control mechanism ( 31 ,  43   h ). 
     Similarly, the range sensor according to modified example 2 of the fifth embodiment is provided with the third extraction gate electrode  41   r  and the fourth extraction gate electrode  41   s  symmetrically arranged to interpose the charge-modulation arrangement-region in the vertical direction between a portion vertically connecting the third and fourth photodiodes provided on the lower right side and the upper right side and the charge-modulation portion provided in the central portion of the charge-modulation arrangement-region extending toward the left from the connecting portion. Accordingly, the third extraction gate electrode  41   r  and the fourth extraction gate electrode  41   s  can switch the operation between the extraction of photoelectrons toward the third extraction region  24   j  and the fourth extraction region  24   k  and the transport of the photoelectrons toward the charge-modulation portion including the first transfer-control mechanism ( 31 ,  42   h ), the second transfer-control mechanism ( 31 ,  44   h ) and the third transfer-control mechanism ( 31 ,  43   h ), as in the case shown in  FIG. 20 . 
     &lt;Modified example 3 of fifth embodiment&gt; 
     A range sensor according to modified example 3 of the fifth embodiment of the present invention is a lock-in pixel including a shielding plate  51  having four apertures: a first aperture as indicated by the dashed-dotted line on the upper left side, a second aperture as indicated by the dashed-dotted line on the lower left side, a third aperture as indicated by the dashed-dotted line on the lower right side, and a fourth aperture as indicated by the dashed-dotted line on the upper right side, to define four light-receiving areas in one pixel, as shown in the schematic plan view from above of the principal part in  FIG. 25 . The lock-in pixel shown in  FIG. 25  also includes four photodiodes provided in the respective four light-receiving areas to receive light collected in the respective independent microlenses  53   p ,  53   q ,  53   r  and  53   s , as in the case of the lock-in pixel shown in  FIG. 20  and  FIG. 24 . 
     When focused on the first aperture provided on the upper left side and the second aperture provided on the lower left side in  FIG. 25 , the range sensor according to modified example 3 of the fifth embodiment has a structure similar to that shown in the cross sections of  FIG. 2  to  FIG. 4 , including: a pixel layer  22  made of a semiconductor of a first conductivity type (p-type); and a surface-buried region  25   r  of a second conductivity type (n-type) selectively buried in the upper portion of the pixel layer  22  to implement a first photodiode in the first light-receiving area defined by the first aperture by forming a junction with the pixel layer  22  and a second photodiode in the second light-receiving area defined by the second aperture by forming a junction with the pixel layer  22 , connecting the upper and second light-receiving areas in the vertical direction, and including a charge-modulation arrangement-region extending from the portion where the first and second light-receiving areas are connected toward the portion shielded by the shielding plate  51  along the upper portion of the pixel layer  22  in the right direction. 
     When focused on a planar pattern of the surface-buried region  25   r , the surface-buried region  25   r  has a planar pattern on the left side in which a first light-receiving terminal-portion having a trilobate leaf shape extends toward a first light-receiving area in the upper left direction, and a second light-receiving terminal-portion having a trilobate leaf shape extends toward a second light-receiving area in the lower left direction. As used herein, the “trilobate leaf shape” is a shape of a leaf of trident maple divided into three by two slits. 
     The surface-buried region  25   r  has a planar pattern on the right side in which a third light-receiving terminal-portion having a trilobate leaf shape extends toward a third light-receiving area in the lower right direction, and a fourth light-receiving terminal-portion having a trilobate leaf shape extends toward a fourth light-receiving area in the upper right direction. The surface-buried region  25   r  on the right side in  FIG. 25  thus includes a third photodiode in the third light-receiving area defined by a third aperture by forming a junction with the pixel layer  22  and a fourth photodiode in the fourth light-receiving area defined by a fourth aperture by forming a junction with the pixel layer  22 . 
     The two trilobate leaf shapes of the first and second light-receiving areas on the eft side and the two trilobate leaf shapes of the third and fourth light-receiving areas on the right side are connected to each other via the charge-modulation arrangement-region interposed therebetween and extending in the lateral direction. Namely, the surface-buried region  25   r  in the range sensor according to modified example 3 of the fifth embodiment includes the trilobate leaf-shaped first to fourth light-receiving terminal-portions in the first to fourth light-receiving areas provided into a tetracyclic flower shape in a planar pattern. 
     As shown in  FIG. 25 , the trilobate leaf-shaped first light-receiving terminal-portion on the upper left side has an area sufficient to cover substantially the entire first aperture, the trilobate leaf-shaped second light-receiving terminal-portion on the lower left side has an area sufficient to cover substantially the entire second aperture, the trilobate leaf-shaped third light-receiving terminal-portion on the lower right side has an area sufficient to cover substantially the entire third aperture, and the trilobate leaf-shaped fourth light-receiving terminal-portion on the upper right side has an area sufficient to cover substantially the entire fourth aperture. 
     As in the case of the planar patterns shown in  FIG. 20  and  FIG. 24 , the charge-modulation arrangement-region in the central portion of the surface-buried region  25   r  has a polygonal shape including fishbone-shaped branches extending in the vertical direction, instead of a rectangular shape extending straight in the horizontal direction. In other words, the surface-buried region  25   r  including the charge-modulation arrangement-region in the central portion is provided with first and second branches extending downward in parallel on the lower side of the charge-modulation arrangement-region, and a third branch extending upward on the upper side of the charge-modulation arrangement-region on the opposite side of the first and second branches. 
     In the central portion of the charge-modulation arrangement-region, a first charge-accumulation region  24   h  and a second charge-accumulation region  24   i  of n-type conductivity having a higher impurity concentration than the first surface-buried region  25   r  are connected to the tips of the branches of the first and second branches, respectively, and a third charge-accumulation region  24   l  of n-type conductivity having a higher impurity concentration than the first surface-buried region  25   r  is connected to the tip of the branch of the third branch. The range sensor according to modified example 3 of the fifth embodiment includes a first transfer-control mechanism ( 31 ,  42   h ), a second transfer-control mechanism ( 31 ,  44   h ) and a third transfer-control mechanism ( 31 ,  43   h ) provided adjacent to the first charge-accumulation region  24   h , the second charge-accumulation region  24   i  and the third charge-accumulation region  24   l,  respectively, to control a transfer of signal charges to the first charge-accumulation region  24   h , the second charge-accumulation region  24   i  and the third charge-accumulation region  24   l.    
     The range sensor according to modified example 3 of the fifth embodiment further includes a guide region  26   g  buried in a part of the surface-buried region  25   r  on the upper side, having a four-legged (bronze vessel) shape including Z-shaped first and second protruding-edges connecting the first and second photodiodes in the vertical direction on the left side in  FIG. 25  and Z-shaped third and fourth protruding-edges connecting the third and fourth photodiodes in the vertical direction on the right side in  FIG. 25 , and having a higher impurity concentration than the surface-buried region  25   r  and a lower impurity concentration than the first charge-accumulation region  24   h , the second charge-accumulation region  24   i  and the third charge-accumulation region  24   l.  The structure of the guide region  26   g  is the same as described with reference to  FIG. 24 , and overlapping explanations are not repeated below. 
     As in the case shown in  FIG. 24 , the range sensor according to modified example 3 of the fifth embodiment further includes a first extraction control mechanism ( 31 ,  41   q ) provided adjacent to the first extraction region  24   g  and configured to control extraction of charges toward the first extraction region  24   g  via the branch extending in the lower direction of the surface-buried region  25   r , a second extraction control mechanism ( 31 ,  41   p ) provided adjacent to the second extraction region  24   f  and configured to control extraction of charges toward the second extraction region  24   f  via the branch extending in the upper direction of the surface-buried region  25   r , a third extraction control mechanism ( 31 ,  41   r ) provided adjacent to the third extraction region  24   j  and configured to control extraction of charges toward the third extraction region  24   j  via the branch extending in the lower direction of the surface-buried region  25   r , and a fourth extraction control mechanism ( 31 ,  41   s ) provided adjacent to the fourth extraction region  24   k  and configured to control extraction of charges toward the fourth extraction region  24   k  via the branch extending in the upper direction of the surface-buried region  25   r . Accordingly, the range sensor according to modified example 3 of the fifth embodiment can switch the operation between the extraction of photoelectrons toward the first extraction region  24   g  and the second extraction region  24   f  or the third extraction region  24   j  and the fourth extraction region  24   k  and the transport of the photoelectrons toward the charge-modulation portion including the first transfer-control mechanism ( 31 ,  42   h ), the second transfer-control mechanism ( 31 ,  44   h ) and the third transfer-control mechanism ( 31 ,  43   h ). 
     The range sensor according to modified example 3 of the fifth embodiment includes a plurality of trilobate leaf-shaped photodiodes provided radially around the pixel and the merged common part in the central portion of the pixel, so as to exhibit a more rapid response and higher-speed performance (improve the efficiency of charge collection more) than the range sensor according to the first embodiment. The range sensor according to modified example 3 of the fifth embodiment can serve as a pixel (lock-in pixel) of a solid-state imaging device to achieve a time-of-flight range image sensor capable of rapid transfer of signal charges with high sensitivity at a low dark current even when a plurality of pixels each having a large area of which a pixel size is five micrometers square or greater are provided on a single semiconductor chip. 
     (Sixth Embodiment) 
     A range sensor according to the sixth embodiment of the present invention is a lock-in pixel including a shielding plate  51  having four apertures: a first aperture as indicated by the dashed-dotted line on the upper left side, a second aperture as indicated by the dashed-dotted line on the lower left side, a third aperture as indicated by the dashed-dotted line on the lower right side, and a fourth aperture as indicated by the dashed-dotted line on the upper right side, to define four light-receiving areas in one pixel, as shown in the schematic plan view from above of the principal part in  FIG. 26 . The lock-in pixel shown in  FIG. 26  also includes four photodiodes provided in the respective four light-receiving areas to receive light collected in the respective independent microlenses  53   p ,  53   q ,  53   r  and  53   s , as in the case of the lock-in pixel shown in  FIG. 20 ,  FIG. 24  and  FIG. 25 . 
     As in the case shown in the cross-sectional views of  FIG. 2  to  FIG. 4 , the first light-receiving area defined by the first aperture on the upper left side in  FIG. 26  is provided with a pixel layer  22  made of a semiconductor of a first conductivity type (p-type), and a light-receiving terminal-portion (first light-receiving terminal-portion)  25   s   1  of a surface-buried region of a second conductivity type (n-type) to implement a first photodiode by forming a junction with the pixel layer  22  into a trilobate leaf shape. The second light-receiving area defined by the second aperture on the lower left side is provided with the pixel layer  22 , and a light-receiving terminal-portion (second light-receiving terminal-portion)  25   s   2  of the surface-buried region of n-type conductivity to implement a second photodiode by forming a junction with the pixel layer  22  into a trilobate leaf shape. 
     The third light-receiving area defined by the third aperture on the lower right side is provided with the pixel layer  22 , and a light-receiving terminal-portion (third light-receiving terminal-portion)  25   s   3  of the surface-buried region of n-type conductivity to implement a third photodiode by forming a junction with the pixel layer  22  into a trilobate leaf shape, and the fourth light-receiving area defined by the fourth aperture on the upper right side is provided with the pixel layer  22 , and a light-receiving terminal-portion (fourth light-receiving terminal-portion)  25   s   4  of the surface-buried region of n-type conductivity to implement a fourth photodiode by forming a junction with the pixel layer  22  into a trilobate leaf shape. The trilobate leaf-shaped first light-receiving terminal-portion  25   s   4 , second light-receiving terminal-portion  25   s   2 , third light-receiving terminal-portion  25   s   3  and fourth light-receiving terminal-portion  25   s   4  are not independent semiconductor regions but are integrated together via a charge-modulation arrangement-region buried into an X-shape in a planar pattern in the upper portion of the common pixel layer  22 . Namely, the surface-buried region ( 25   s   2 ,  25   s   3 ,  25   s   4 ) in the range sensor according to the sixth embodiment has a planar pattern in which the trilobate leaf-shaped first to fourth light-receiving terminal-portions  25   s   1 ,  25   s   2 ,  25   s   3  and  25   s   4  are provided radially in the first to fourth light-receiving areas into a tetracyclic flower shape as shown in  FIG. 26 . 
     As shown in  FIG. 26 , the trilobate leaf-shaped first light-receiving terminal-portion  25   s   1  on the upper left side has an area sufficient to cover substantially the entire first aperture, the trilobate leaf-shaped second light-receiving terminal-portion  25   s   2  on the lower left side has an area sufficient to cover substantially the entire second aperture, the trilobate leaf-shaped third light-receiving terminal-portion  25   s   3  on the lower right side has an area sufficient to cover substantially the entire third aperture, and the trilobate leaf-shaped fourth light-receiving terminal-portion  25   s   4  on the upper right side has an area sufficient to cover substantially the entire fourth aperture. 
     The charge-modulation arrangement-region in the central portion of the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) has a complicated shape in a planar pattern having bipinnate compound leaf-shaped branches (not shown), instead of a simple X-shape. As used herein, the “bipinnate compound leaf shape” is a fractal figure having branches in a pinnate pattern such as angelica tree. More particularly, the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) has a topology in which concentric large and small X-shapes are oriented at 45 degrees to each other. The small X-shape is shifted by 45 degrees with respect to the large X-shape to have a plus sign shape with four branches. A first charge-accumulation region  24   r  of n-type conductivity having a higher impurity concentration than the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) is connected to a vertical branch extending on the upper side of the plus sign shape, and a third charge-accumulation region  24   p  of n-type conductivity having a higher impurity concentration than the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) is connected to a vertical branch extending on the lower side of the plus sign shape. A second charge-accumulation region  24   o  of n-type conductivity having a higher impurity concentration than the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) is connected to a lateral branch extending on the left side of the plus sign shape. A central extraction region  24   q  of n-type conductivity having a higher impurity concentration than the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) is connected to a lateral branch extending on the right side of the plus sign shape. 
     The large X-shape of the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) is provided toward the first light-receiving terminal-portion  25   s   1  with branches on both sides of the charge-modulation arrangement-region in the direction perpendicular to the charge-modulation arrangement-region extending in the upper left direction, and a first upper extraction region  24   m   1   a  and a first lower extraction region  24   m   1   b  are connected to the tips of the respective branches. The large X-shape of the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) is provided toward the second light-receiving terminal-portion  25 s 2  with branches on both sides of the charge-modulation arrangement-region in the direction perpendicular to the charge-modulation arrangement-region extending in the lower left direction, and a second upper extraction region  24   m   2   a  and a second lower extraction region  24   m   2   b  are connected to the tips of the respective branches. The large X-shape of the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) is provided toward the third light-receiving terminal-portion  25   s   3  with branches on both sides of the charge-modulation arrangement-region in the direction perpendicular to the charge-modulation arrangement-region extending in the lower right direction, and a third lower extraction region  24   m   3   a  and a third upper extraction region  24   m   3   b  are connected to the tips of the respective branches. The large X-shape of the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) is provided toward the fourth light-receiving terminal-portion  25   s   4  with branches on both sides of the charge-modulation arrangement-region in the direction perpendicular to the charge-modulation arrangement-region extending in the upper right direction, and a fourth lower extraction region  24   m   4   a  and a fourth upper extraction region  24   m   4   b  are connected to the tips of the respective branches. 
     The range sensor according to the sixth embodiment includes a first transfer-control mechanism ( 31 ,  48 ), a second transfer-control mechanism ( 31 ,  45 ) and a third transfer-control mechanism ( 31 ,  46 ) provided adjacent to the first charge-accumulation region  24   r , the second charge-accumulation region  24   o  and the third charge-accumulation region  24   p , respectively, to control a transfer of signal charges to the first charge-accumulation region  24   r , the second charge-accumulation region  24   o  and the third charge-accumulation region  24   p.    
     The range sensor according to the sixth embodiment further includes a guide region  26   h  having concentric large and small X-shapes oriented at 45 degrees to each other as shown in  FIG. 26  and having a higher impurity concentration than the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) and a lower impurity concentration than the first charge-accumulation region  24   r , the second charge-accumulation region  24   o  and the third charge-accumulation region  24   p . The guide region  26   h  is buried in a part of the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) on the upper side (not shown), as in the case shown in the cross-sectional views of  FIG. 2A ,  FIG. 3A ,  FIG. 4A  and  FIG. 5A . The oblique branch of the large X-shape in the guide region  26   h  extendtending in the upper left direction has a stepped shape gradually increased in width from the first light-receiving terminal-portion  25   s   1  on the outer side toward the center of the large X-shape. The oblique branch of the large X-shape in the guide region  26   h  extending in the lower left direction has a stepped shape gradually increased in width from the second light-receiving terminal-portion  25   s   2  on the outer side toward the center of the large X-shape. The oblique branch of the large X-shape in the guide region  26   h  extending in the lower right direction has a stepped shape gradually increased in width from the third light-receiving terminal-portion  25   s   3  on the outer side toward the center of the large X-shape. The oblique branch of the large X-shape in the guide region  26   h  extending in the upper right direction has a stepped shape gradually increased in width from the fourth light-receiving terminal-portion  25   s   4  on the outer side toward the central portion of the large X-shape. 
     The range sensor according to the sixth embodiment generates a high drift field in the entire regions of the respective depleted oblique branches of the X-shape gradually increased in width toward the central portion of the X-shape, so as to rapidly transfer the photoelectrons as signal charges along the oblique branches of the X-shape in the longitudinal direction from the outer side to the central portion of the guide region  26   h  extendven when the light-receiving area has a large area of which a pixel size is five micrometers square or greater. 
     The range sensor according to the sixth embodiment includes a first upper extraction control mechanism ( 31 ,  41   t   1   a ) provided adjacent to the first upper extraction region  24   m   1   a  along the charge-modulation arrangement-region extending in the upper left direction of the X-shape of the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) and configured to control extraction of charges toward the first upper extraction region  24   m   1   a  via the branch extending in the upper right direction from the charge-modulation arrangement-region, and a first lower extraction control mechanism ( 31 ,  41   t   1   b ) provided adjacent to the first lower extraction region  24   m   1   b  along the charge-modulation arrangement-region extending in the upper left direction of the X-shape of the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) and configured to control extraction of charges toward the first lower extraction region  24   m   1   b  via the branch extending in the lower left direction from the charge-modulation arrangement-region. 
     The range sensor according to the sixth embodiment further includes a second upper extraction control mechanism ( 31 ,  41   t   2   a ) provided adjacent to the second upper extraction region  24   m   2   a  along the charge-modulation arrangement-region extending in the lower left direction of the X-shape of the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) and configured to control extraction of charges toward the second upper extraction region  24   m   2   a  via the branch extending in the upper left direction from the charge-modulation arrangement-region, and a second lower extraction control mechanism ( 31 ,  41   t   2   b ) provided adjacent to the second lower extraction region  24   m   2   b  along the charge-modulation arrangement-region extending in the lower left direction of the X-shape of the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) and configured to control extraction of charges toward the second upper extraction region  24   m   2   a  via the branch extending in the lower right direction from the charge-modulation arrangement-region. 
     The range sensor according to the sixth embodiment further includes a third lower extraction control mechanism ( 31 ,  41   t   3   a ) provided adjacent to the third lower extraction region  24   m   3   a  along the charge-modulation arrangement-region extending in the lower right direction of the X-shape of the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) and configured to control extraction of charges toward the third lower extraction region  24   m   3   a  via the branch extending in the lower left direction from the charge-modulation arrangement-region, and a third upper extraction control mechanism ( 31 ,  41   t   3   b ) provided adjacent to the third upper extraction region  24   m   3   b  along the charge-modulation arrangement-region extending in the lower right direction of the X-shape of the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) and configured to control extraction of charges toward the third upper extraction region  24   m   3   a  via the branch extending in the upper right direction from the charge-modulation arrangement-region. 
     The range sensor according to the sixth embodiment further includes a fourth lower extraction control mechanism ( 31 ,  41   t   4   a ) provided adjacent to the fourth lower extraction region  24   m   4   a  along the charge-modulation arrangement-region extending in the upper right direction of the X-shape of the surface-buried region ( 25   s   t ,  25   s   2 ,  25   s   3 ,  25   s   4 ) and configured to control extraction of charges toward the fourth lower extraction region  24   m   4   a  via the branch extending in the lower right direction from the charge-modulation arrangement-region, and a fourth upper extraction control mechanism ( 31 ,  41   t   4   b ) provided adjacent to the fourth upper extraction region  24   m   4   b  along the charge-modulation arrangement-region extending in the upper right direction of the X-shape of the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) and configured to control extraction of charges toward the fourth upper extraction region  24   m   4   b  via the branch extending in the upper left direction from the charge-modulation arrangement-region. 
     The range sensor according to the sixth embodiment further includes a central extraction control mechanism ( 31 ,  47 ) provided adjacent to the central extraction region  24   q  provided at the lateral branch extending on the right side of the plus sign shape and configured to control extraction of charges toward the central extraction region  24   q  via the lateral branch of the plus sign shape from the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ). 
     The range sensor according to the sixth embodiment can switch the operation between the extraction of photoelectrons toward the first upper extraction region  24   m   1   a , the first lower extraction region  24   m   1   b , the second upper extraction region  24   m   2   a , the second lower extraction region  24   m   2   b , the third lower extraction region  24   m   3   a , the third upper extraction region  24   m   3   b , the fourth lower extraction region  24   m   4   a , the fourth upper extraction region  24   m   4   b  and the central extraction region  24   q  due to the first upper extraction control mechanism ( 31 ,  41   t   1   a ), the first lower extraction control mechanism ( 31 ,  41   t   1   b ), the second upper extraction control mechanism ( 31 ,  41   t   2   a ), the second lower extraction control mechanism ( 31 ,  41   t   2   b ), the third lower extraction control mechanism ( 31 ,  41   t   3   a ), the third upper extraction control mechanism ( 31 ,  41   t   3   b ), the fourth lower extraction control mechanism ( 31 ,  41   t   4   a ), the fourth upper extraction control mechanism ( 31 ,  41   t   4   b ) and central extraction control mechanism ( 31 ,  47 ), and the transport of the photoelectrons toward the first charge-accumulation region  24   r , the second charge-accumulation region  24   o  and the third charge-accumulation region  24   p  due to the first transfer-control mechanism ( 31 ,  48 ), the second transfer-control mechanism ( 31 ,  45 ) and the third transfer-control mechanism ( 31 ,  46 ). 
       FIG. 27  illustrates an equivalent circuit of the range sensor according to the sixth embodiment including four photodiodes in one pixel: a first photodiode D ij1 , a second photodiode D ij2 , a third photodiode D ij3  and a fourth photodiode D ij4 . The charge-modulation portion common to the four photodiodes in the range sensor according to the sixth embodiment is implemented by three transistors arranged in parallel: a first transfer transistor Q 1T  as the first transfer-control mechanism ( 31 ,  48 ), a second transfer transistor Q 2T  as the second transfer-control mechanism ( 31 ,  45 ), and a third transfer transistor Q 3T  as the third left transfer-control mechanism ( 31 ,  46 ), as indicated in the middle in  FIG. 27 . 
     The static induction channels from the respective first photodiode D ij1 , second photodiode D ij2 , third photodiode D ij3  and fourth photodiode D ij4  indicated on the left side to the “common charge-modulation portion” are shown in a circuit indicated by the broken lines in  FIG. 27 . 
     The static induction channel is indicated on the uppermost left side by two first junction field-effect transistors Q P11  and Q P12  , of which own gates are grounded. A source terminal of a first charge-extraction MOS transistor Q D1  for charge extraction is connected to a center tap of the two first junction field-effect transistors Q P11  and Q P12  connected in series, and a drain terminal of the first charge-extraction MOS transistor Q D1  is connected to a power source V DD  having a high potential. The broken lines in  FIG. 27  on the uppermost left side indicate paths in which electrons flow rapidly due to a depletion electric field because of the connection in the semiconductor region in  FIG. 27 , photoelectrons generated in the first photodiode D ij1  immediately reach the common charge-modulation portion as signal charges when a voltage G D1  applied to each of an extraction gate electrode  41   t   1   a  and an extraction gate electrode  41   t   1   b  respectively implementing the first upper extraction control mechanism ( 31 ,  41   t   1   a ) and the first lower extraction control mechanism ( 31 ,  41   t   1   b ) is set at a low potential (L). 
     The static induction channel is indicated on the second stage on the left side by two second junction field-effect transistors Q p21  and Q p22  of which own gates are grounded. A source terminal of a second charge-extraction MOS transistor Q D2  for charge extraction is connected to a center tap of the two second junction field-effect transistors Q p21  and Q p22  connected in series, and a drain terminal of the second charge-extraction MOS transistor Q D2  is connected to a power source V DD  having a high potential. The broken lines in  FIG. 27  on the second stage on the left side indicate paths in which electrons flow rapidly due to a depletion electric field because of the connection in the semiconductor region in  FIG. 27 , photoelectrons generated in the second photodiode D ij2  immediately reach the common charge-modulation portion as signal charges when a voltage G D2  applied to each of an extraction gate electrode  41   t   2   a  and an extraction gate electrode  41   t   2   b  respectively implementing the second upper extraction control mechanism ( 31 ,  41   t   2   a ) and the second lower extraction control mechanism ( 31 ,  41   t   2   b ) is set at a low potential (L). 
     The static induction channel is indicated on the third stage on the left side by two third junction field-effect transistors Q P31  and Q P32  of which own gates are grounded. A source terminal of a third charge-extraction MOS transistor Q D3  for charge extraction is connected to a center tap of the two third junction field-effect transistors Q P31  and Q P32  connected in series, and a drain terminal of the third charge-extraction MOS transistor Q D3  is connected to a power source V DD  having a high potential. The broken lines in  FIG. 27  on the third stage on the left side indicate paths in which electrons flow rapidly due to a depletion electric field because of the connection in the semiconductor region in  FIG. 27 , photoelectrons generated in the third photodiode D ij3  immediately reach the common charge-modulation portion as signal charges when a voltage G D3  applied to each of an extraction gate electrode  41   t   3   a  and an extraction gate electrode  41   t   3   b  respectively implementing the third lower extraction control mechanism ( 31 ,  41   t   3   a ) and the third upper extraction control mechanism ( 31 ,  41   t   3   b ) is set at a low potential (L). 
     The static induction channel is indicated on the lowermost left side by two fourth junction field-effect transistors Q P41  and Q P42  of which own gates are grounded. A source terminal of a fourth charge-extraction MOS transistor Q D4  for charge extraction is connected to a center tap of the two fourth junction field-effect transistors Q P41  and Q P42  connected in series, and a drain terminal of the fourth charge-extraction MOS transistor Q D4  is connected to a power source V DD having a high potential. The broken lines in  FIG. 27  on the lowermost left side indicate paths in which electrons flow rapidly due to a depletion electric field because of the connection in the semiconductor region in  FIG. 27 , photoelectrons generated in the fourth photodiode D ij4  immediately reach the common charge-modulation portion as signal charges when a voltage G D4  applied to each of an extraction gate electrode  41   t   4   a  and an extraction gate electrode  41   t   4   b  respectively implementing the fourth lower extraction control mechanism ( 31 ,  41   t   4   a ) and the fourth upper extraction control mechanism ( 31 ,  41   t   4   b ) is set at a low potential (L). 
     The equivalent circuit is illustrated with the case in which one end of each of the first transfer transistor Q 1T , the second transfer transistor Q 2T  and the third transfer transistor Q 3T  implementing the common charge-modulation portion is connected to the first junction field-effect transistor Q P12 , the second junction field-effect transistor Q P22 , the third junction field-effect transistor Q P3  and the fourth junction field-effect transistor Q P42  arranged in parallel. 
     Namely, the output terminals of the first junction field-effect transistor Q P12 , the second junction field-effect transistor Q P22 , the third junction field-effect transistor Q P32  and the fourth junction field-effect transistor Q P42  are connected to the input terminal of each of the first transfer transistor Q 1T , the second transfer transistor Q 2T , the third transfer transistor Q 3T  and a central drain transistor Q DD  arranged in parallel. 
     In the circuit diagram, the other ends of the first transfer transistor Q 1T , the second transfer transistor Q 2T  and the third transfer transistor Q 3T  are connected to the first charge-accumulation region  24   r  serving as node D 1 , the second charge-accumulation region  24   o  serving as node D 2  and the third charge-accumulation region  24   p  serving as node D 3 , respectively. When the voltage at the intermediate potential (M) is applied to one of the first transfer gate electrode  48 , the second transfer gate electrode  45  and the third transfer gate electrode  46 , and the voltage at the low potential (L) is applied to the other two electrodes, the photoelectrons are transferred to one of the first charge-accumulation region  24   r , the second charge-accumulation region  24   o  and the third charge-accumulation region  24   p.    
     The three common nodes D 1 , D 2  and D 3  shown in  FIG. 27  are connected to capacitors C 1 , C 2  and C 3  for charge-accumulation. The capacitors C 1 , C 2  and C 3  are preferably depletion-mode MOS capacitors in which a threshold voltage is set at a negative voltage so as to reduce voltage dependence. The first node D 1  is connected to a gate terminal of a first amplification transistor Q 1A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the first node D 1 . The first node D 1  is further connected to a first reset transistor Q 1R  for initializing signal charges after signal readout. A source terminal of the first amplification transistor Q 1A  is connected to a first selection transistor Q 1S , serving as a switch for readout image selection. An output of the first selection transistor Q 1S  is connected to a signal readout line extending in the vertical direction. 
     The second node D 2  is connected to a gate terminal of a second amplification transistor Q 2A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the second node D 2 . The second node D 2  is further connected to a second reset transistor Q 2R  for initializing s signal charges after signal readout. A source terminal of the second amplification transistor Q 2A  is connected to a second selection transistor Q 2S , serving as a switch for readout image selection. An output of the second selection transistor Q 2S  is connected to a signal readout line extending in the vertical direction. The third node D 3  is connected to a gate terminal of a third amplification transistor Q 3A  implementing a source follower circuit for reading out a potential change in association with a change in the charge amount of the third node D 3 . The third node D 3  is further connected to a third reset transistor Q 3R  for initializing signal charges after signal readout. A source terminal of the third amplification transistor Q 3A  is connected to a third selection transistor Q 3S , serving as a switch for readout image selection. An output of the third selection transistor Q 3S  is connected to a signal readout line extending in the vertical direction. 
     The signal readout method in the range sensor according to the sixth embodiment may be either a method of reading out a signal from each of three signal readout lines arranged in parallel as shown in  FIG. 27 , or a method of turning on a switch sequentially by selection signals SL 1 , SL 2  and SL 3  to read out each signal as a time-series signal from a single signal readout line as shown in  FIG. 11 . 
     In the range sensor and the solid-state imaging device according to the sixth embodiment, the first photodiode D ij1 , the second photodiode D ij2 , the third photodiode D ij3  and the fourth photodiode D ij4  each receive light and accumulate signals as charges after subjected to charge modulation in the common charge-modulation portion provided in the central portion of the pixel so as to amplify the signals. 
     When a solid-state imaging device (image sensor) having a large pixel area and using a single photodiode cannot ensure a sufficiently rapid response, a plurality of photodiodes, each based on the structure according to the first embodiment, may be provided around the pixel as illustrated in the sixth embodiment to merge the common part in the central portion of the pixel, so as to strengthen the function of the range sensor according to the first embodiment and exhibit a more rapid response and higher-speed performance (improve the efficiency of charge collection). 
     The range sensor according to the sixth embodiment has the function suitable for the time-of-flight measurement by use of the Eq. (4) or the Eq. (8) as described in the first embodiment. The range sensor according to the sixth embodiment thus can serve as a pixel (lock-in pixel) of a solid-state imaging device to achieve a time-of-flight range image sensor capable of rapid transfer of signal charges with high sensitivity at a low dark current even when a plurality of pixels each having a large area of which a pixel size is five micrometers square or greater are provided on a single semiconductor chip. 
     As in the case of the range sensor according to the respective first to fifth embodiments, the range sensor according to the sixth embodiment is provided with the static induction channel having a sufficient length from the respective four ends of the X-shaped charge-modulation arrangement-region to the central portion of the charge-modulation portion, which is shielded by the shielding plate  51 , so as to achieve remarkable effects upon reducing the influence of background light while using a pulse at short intervals. 
     Although the range sensor according to the sixth embodiment has a structure for illustration purposes in which the four photodiodes are provided around the pixel to each receive light to accumulate signals as charges after subjected to charge modulation in the common charge-modulation portion provided in the central portion of the pixel to amplify the signals, the number of photodiodes provided around the pixel may be two or the other number such as six or eight, depending on the size of the pixel area or the response speed or sensitivity to be required. 
     The range sensor according to the sixth embodiment includes a plurality of trilobate leaf-shaped photodiodes provided radially around the pixel and the merged common part in the central portion of the pixel, so as to exhibit a more rapid response and higher-speed performance (improve the efficiency of charge collection more) than the range sensor according to the first embodiment. The range sensor according to the sixth embodiment can serve as a pixel (lock-in pixel) of a solid-state imaging device to achieve a time-of-flight range image sensor capable of rapid transfer of signal charges with high sensitivity at a low dark current even when a plurality of pixels each having a large area of which a pixel size is five micrometers square or greater are provided on a single semiconductor chip. 
     (Seventh Embodiment) 
     A range sensor according to the seventh embodiment of the present invention is a lock-in pixel including a shielding plate  51  having an aperture indicated by the dashed-dotted line in  FIG. 28  defining a light-receiving area. As shown in the plan view of  FIG. 28  and the cross-sectional views of  FIG. 29A  and  FIG. 30A , the range sensor according to the seventh embodiment fundamentally has the same structure as the range sensor according to the first embodiment and serves as a lock-in pixel including: a pixel layer  22  made of a semiconductor of a first conductivity type (p-type); the shielding plate  51  having the aperture and provided above the pixel layer  22  to define the light-receiving area on the pixel layer  22  below the aperture; a surface-buried region  62  of a second conductivity type (n.-type) selectively buried in the upper portion of the pixel layer  22  to implement a photodiode in the light-receiving area by forming a junction with the pixel layer  22 , and extending from the light-receiving area toward plural portions shielded by the shielding plate  51  along the upper portion of the pixel layer  22  so as to delineate a plurality of branches, implementing a T-shaped configuration on the end side; a first charge-accumulation region  24   b , a second charge-accumulation region  24   d  and a third charge-accumulation region  24   c  of n-type conductivity, which is connected to the tips of the branches and having a higher impurity concentration than the surface-buried region  62 ;  a  first transfer-control mechanism ( 31 ,  42 ), a second transfer-control mechanism ( 31 ,  44 ) and a third transfer-control mechanism ( 31 ,  43 ) provided at the branches adjacent to the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c , respectively, to control a transfer of signal charges to the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c;  and a guide region ( 63 ,  64 ) of n-type conductivity, which is buried in a part of the upper portion of the surface-buried region  62  with one end provided below a part of the aperture and other ends extending to at least a part of the respective transfer-control mechanisms, and having a higher impurity concentration than the surface-buried region  62  and a lower impurity concentration than the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c.    
     The range sensor according to the seventh embodiment differs from the range sensor according to the first embodiment in that the surface-buried region  62  is locally buried in the light-receiving area into a reverse U-shape to surround the guide region ( 63 ,  64 ) in a planar pattern, while the surface-buried region  25  in the first embodiment has a fork-like shape in the light-receiving area (in the seventh embodiment, the “fork-like shape” is also referred to as an E-shape). The surface-buried region  62  has a stepped shape of which the width measured in the direction perpendicular to the vertical direction of  FIG. 28  is gradually increased from top to bottom. The outline of the surface-buried region  62  in the planar pattern is an envelope curve approximate to a reverse U-shape. 
     As shown in  FIG. 28 ,  FIG. 29A  and  FIG. 30A , the range sensor according to the seventh embodiment differs from the range sensor according to the first embodiment in that the guide region ( 63 ,  64 ) has a double-layer structure (a complex structure) including an auxiliary guide region  63  and a main guide region  64  buried in the auxiliary guide region  63 , while the guide region  26   a  according to the first embodiment has a single-layer structure. The auxiliary guide region  63  of the guide region ( 63 ,  64 ) in the range sensor according to the seventh embodiment is a semiconductor region of n-type conductivity (n 2 -type) having one end provided below a part of the aperture and other ends extending toward the first transfer-control mechanism ( 31 ,  42 ), the second transfer-control mechanism ( 31 ,  44 ) and the third transfer-control mechanism ( 31 ,  43 ), and having a higher impurity concentration than the surface-buried region. The main guide region  64  is a semiconductor region of n-type conductivity (n 3 -type) having one end provided in a part of the auxiliary guide region  63  and other protruding-edges extending to at least a part of the respective transfer-control mechanisms, and having a higher impurity concentration than the auxiliary guide region  63 . The main guide region  64  is a semiconductor region having a lower impurity concentration than the first charge-accumulation region  24   b , the second charge-accumulation regions  24   d  and the third charge-accumulation region  24   c.    
     Namely, the range sensor according to the seventh embodiment includes the auxiliary guide region  63  buried in the central portion of the surface-buried region  62  and having a higher impurity concentration than the surface-buried region  62 . The auxiliary guide region  63  guides signal charges from the surface-buried region  62  to the main guide region  64 , so as to facilitate the transfer of the signal charges in the surface-buried region  62  toward the main guide region  64 . The outline of the auxiliary, guide region  63  in the planar pattern is an envelope curve having a stepped shape approximate to a reverse V-shape of which the width measured in the direction perpendicular to the vertical direction of  FIG. 28  is gradually increased from top to bottom. 
     The main guide region  64  is a semiconductor region for guiding the photoelectrons guided by the auxiliary guide region  63  further to the respective narrow transfer channels in the charge-modulation portion. As shown in  FIG. 28 , the main guide region  64  has a stepped shape of which the width in the direction perpendicular to the longitudinal direction (the vertical direction in  FIG. 28 ) of the main guide region  64  is gradually increased from upper-side to lower-side in the drawing of  FIG. 28 . 
     The triple-layered semiconductor regions of n-type conductivity having different impurity concentrations as shown in  FIG. 28  have a large potential gradient in every region even in a photodiode having a significantly large area of which a pixel size is 10 micrometers square or greater, so as to increase the electric field to lead to high-speed performance. As shown in the plan view of  FIG. 28  and the potential profile in  FIG. 29B , the tip of the auxiliary guide region  63  corresponds to the bottom of the potential profile of the surface-buried region  62  to which carriers generated are transferred. The tip of the main guide region  64  having a topology in which the width is narrower than that of the auxiliary guide region  63  is provided in the auxiliary guide region  63 . Since the main guide region  64  is buried into the region, around the tip of the auxiliary guide region  63  at a predetermined gap, the potential gradient increases in every region, so as to increase the electric field to achieve higher-speed performance of the range sensor. 
     The auxiliary guide region  63  may be buried as a semiconductor region having a higher impurity concentration than the surface-buried region  62  such that a part of an ion-implanted area to bury the surface-buried region  62  is doubly ion-implanted in the planar pattern shown in  FIG. 28 . The main guide region  64  may be buried as a semiconductor region having a higher impurity concentration than the auxiliary guide region  63  such that a part of the region implanted with ions to bury the surface-buried region  62  is triply implanted with ions in the planar pattern shown in  FIG. 28 . 
       FIG. 29A  is a cross-sectional view of the range sensor according to the seventh embodiment as viewed from direction XXIX-XXIX in  FIG. 28 , and  FIG. 29B  is a view illustrating a potential profile indicated by the thin solid line corresponding to the cross section along the dashed-dotted line in  FIG. 29A .  FIG. 29C  is a cross-sectional view which is the same as shown in  FIG. 5  in order to compare the range sensor according to the seventh embodiment and the range sensor according to the first embodiment.  FIG. 29B  further indicates a potential profile by the thick solid line corresponding to the cross section along the dashed-dotted line in  FIG. 29C  to compare the potential profile of the range sensor according to the seventh embodiment and the potential profile of the range sensor according to the first embodiment. 
     In the range sensor according to the first embodiment, as shown in the potential profile in  FIG. 5 , the narrow tip of the guide region  26   a  is in contact with the surface-buried region  25  to which carriers generated are transferred at a position corresponding to the bottom of the potential profile. The potential profile shows a flat pattern in the region other than the narrow tip of the guide region  26   a . In the range sensor according to the seventh embodiment, the potential profile shown in  FIG. 29B  shows a substantially V-shaped steep gradient tapered to the bottom of the potential profile of the surface-buried region  62 . 
       FIG. 30A  is a cross-sectional view as viewed from direction XXX-XXX in  FIG. 28 , and  FIG. 30B  is a view illustrating a potential profile indicated by the thin solid line corresponding to the cross section in  FIG. 30A  when gate signal G 1  of an intermediate potential (M) is applied to the first transfer gate electrode  42 .  FIG. 30C  is a view corresponding to the cross-sectional view shown in  FIG. 3  of which the right and left sides are reversed in order to compare the range sensor according to the seventh embodiment and the range sensor according to the first embodiment.  FIG. 30B  further indicates a potential profile by the thick solid line corresponding to the cross section of  FIG. 30C  to compare the potential profile of the range sensor according to the seventh embodiment and the potential profile of the range sensor according to the first embodiment. 
       FIG. 31A  is a cross-sectional view as viewed from direction XXX-XXX iii  FIG. 28 , and  FIG. 31B  is a view illustrating a potential profile indicated by the thin solid line corresponding to the cross section in  FIG. 31A  when gate signal G 3  of an intermediate potential (M) is applied to the third transfer gate electrode  43 .  FIG. 31C  is a view corresponding to the cross-sectional view shown in  FIG. 3  of which the right and left sides are reversed, as in the case of  FIG. 30C , in order to compare the range sensor according to the seventh embodiment and the range sensor according to the first embodiment.  FIG. 31B  further indicates a potential profile by the thick solid line corresponding to the cross section of  FIG. 31C  to compare the potential profile of the range sensor according to the seventh embodiment and the potential profile of the range sensor according to the first embodiment. 
     Although the both potential profiles in  FIG. 30B  and  FIG. 31B  show no potential barrier in the static induction channel from the trunk path toward the third charge-accumulation region  24   c  of the charge-modulation portion, the range sensor according to the seventh embodiment shows a profile having a steeper gradient of the potential in the static induction channel than the profile of the range sensor according to the first embodiment. 
     The range sensor according to the seventh embodiment with the profile having a steeper gradient of the potential in the static induction channel than the range sensor according to the first embodiment can transfer the photoelectrons generated in the photodiode as signal charges to the charge-modulation portion via the V-shaped potential channel formed in the static induction channel more rapidly than the range sensor according to the first embodiment. 
     As shown in  FIG. 28 , the range sensor according to the seventh embodiment includes the first transfer gate electrode  42 , the second transfer gate electrode  44  and the third transfer gate electrode  43  in the charge-modulation portion. When the gate signal G 1  of the intermediate potential (M) is applied to the first transfer gate electrode  42 , the potential profile as shown in  FIG. 32  appears. When the gate signal G 2  of the intermediate potential (M) is applied to the second transfer gate electrode  44 , the potential profile as shown in  FIG. 33  appears. When the gate signal G 3  of the intermediate potential (M) is applied to the third transfer gate electrode  43 , the potential profile as shown in  FIG. 34  appears. In  FIG. 32  to  FIG. 34 , the thick solid line indicates an equipotential line in which a potential is −0.2 V (indicated by “−0.2 V” in  FIG. 32  to  FIG. 34 ), the dashed-dotted line in indicates an equipotential line in which a potential is 0 V or greater and less than 1 V (indicated by “0 V” in  FIG. 32  to  FIG. 34 ) (at 0.25 V intervals from 0 V), the thin solid line indicates an equipotential line in which a potential is 1 V or greater and less than 2 V (indicated by “1 V” in  FIG. 32  to  FIG. 34 ) (at 0.25 V intervals from 1 V), the fine broken line indicates an equipotential line in which a potential is 2 V or greater and less than 3 V (indicated by “2 V” in  FIG. 32  to  FIG. 34 ) (at 0.25 V intervals from 2 V), and the long broken line indicates an equipotential line in which a potential is 3 V or greater and less than 4 V (indicated by “3 V” in  FIG. 32  to  FIG. 34 ) (at 0.25 V intervals from 3 V). As described above, the gate signal of the intermediate potential (M) is applied to each of the first transfer gate electrode  42 , the second transfer gate electrode  44  and the third transfer gate electrode  43 , so as to rapidly transfer the signal charges to the corresponding first charge-accumulation region  24   b , second charge-accumulation regions  24   d  and third charge-accumulation region  24   c  along the respective paths as indicated by the thick solid lines in  FIG. 32  to  FIG. 34 . This is the principal operation of the range sensor for detecting photoelectrons in synchronization with a light pulse. 
     When gate signal G D  of a high potential (H) higher than the intermediate potential (M) is applied to the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  in the range sensor according to the seventh embodiment, the potential profile indicated by equipotential lines as shown in  FIG. 35  appears. Electrons reached the static induction channel formed in the trunk path are directed to the first extraction region  24   a  to be extracted along the path as indicated by the very thick solid line shown in  FIG. 35 . In  FIG. 35 , the thick solid line indicates an equipotential line in which a potential is −0.2 V (indicated by “−0.2 V” in  FIG. 35 ), the dashed-dotted line indicates an equipotential line in which a potential is 0 V or greater and less than 1 V (indicated by “0 V” in  FIG. 35 ) (at 0.25 V intervals from 0 V), the thin solid line indicates an equipotential line in which a potential is 1 V or greater and less than 2 V (indicated by “1 V” in  FIG. 35 ) (at 0.25 V intervals from 1 V), the fine broken line indicates an equipotential line in which a potential is 2 V or greater and less than 3 V (indicated by “2 V” in  FIG. 35 ) (at 0.25 V intervals from 2 V), and the long broken line indicates an equipotential line in which a potential is 3 V or greater and less than 4 V (indicated by “3 V” in  FIG. 35 ) (at 0.25 V intervals from 3 V). When the gate signal G D  of the high potential (H) is applied to the second extraction gate electrode  41   b , although the potential profile is not shown in the drawing, electrons reached the static induction channel formed in the trunk path are directed to the second extraction region  24   e  to be extracted. 
     The time of transfer of charges to the respective charge-accumulation regions at positions “a” to “e” in each of the X-Y coordinate system of the range sensor according to the seventh embodiment having the reverse U-shaped planar pattern as shown in  FIG. 32  to  FIG. 35  and the X-Y coordinate system of the range sensor according to the first embodiment having the E-shaped planar pattern as shown in  FIG. 6  to  FIG. 9  was measured through simulations. 
     The position “a” of charges was allotted to the coordinate in which X=5.4 μm, Y=13 μm and Z (depth)=3 μm. The position “b” of charges was allotted to the coordinate in which X=8.4 μm, Y=13 μm and Z (depth)=3 μm. The position “c” of charges was allotted to the coordinate in which X=12 μm, Y=12 μm and Z (depth)=3 μm. The position “d” of charges was allotted to the coordinate in which X =8.4 μm, Y=5.5 μm and Z (depth) =0.2 μm. The position “e” of charges was allotted to the coordinate in which X=11 μm, Y=3 μm and Z (depth)=3 μm. Table 1 summarizes the measurement results. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 G 1   
                 G 2   
                 G 3   
                 G D   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Charge 
                   
                 Reverse 
                   
                 Reverse 
                   
                 Reverse 
                   
                 Reverse 
               
               
                 position 
                 E-shape 
                 U-shape 
                 E-shape 
                 U-shape 
                 E-shape 
                 U-shape 
                 E-shape 
                 U-shape 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 a 
                 1.51 
                 0.56 
                 1.51 
                 0.56 
                 1.56 
                 0.57 
                 1.15 
                 0.46 
               
               
                 b 
                 0.46 
                 0.35 
                 0.46 
                 0.35 
                 0.49 
                 0.37 
                 0.7 
                 0.26 
               
               
                 c 
                 1.29 
                 0.60 
                 1.29 
                 0.60 
                 1.33 
                 0.61 
                 0.93 
                 0.49 
               
               
                 d 
                 0.008 
                 0.004 
                 0.007 
                 0.003 
                 — 
                 — 
                 0.42 
                 — 
               
               
                 e 
                 — 
                 0.32 
                 0.58 
                 0.31 
                 — 
                 0.34 
                 — 
                 0.12 
               
               
                   
               
               
                 Unit: ns 
               
            
           
         
       
     
     The section “G 1  ” in Table 1 indicates the case regarding the range sensor according to the first embodiment having the E-shaped planar pattern in which the gate signal G 1  applied to the first transfer gate electrode  42  is turned on (3.3 V), while the gate signal G 2  applied to the second transfer gate electrode  44 , the gate signal G 3  applied to the third transfer gate electrode  43  and the gate signal G D  applied to the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  are turned off. (1.5 V), and the case regarding the range sensor according to the seventh embodiment having the reverse U-shaped planar pattern in which the gate signal G 1  applied to the first transfer gate electrode  42  is turned on (3.3 V), while the gate signal G 2  applied to the second transfer gate electrode  44 , the gate signal G 3  applied to the third transfer gate electrode  43  and the gate signal G D  applied to the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  are turned off (0 V), 
     The section “G 2 ” in Table 1 indicates the case regarding the range sensor according to the first embodiment having the E-shaped planar pattern in which the gate signal G 2  applied to the second transfer gate electrode  44  is turned on (3.3 V), while the gate signal G 1  applied to the first transfer gate electrode  42 , the gate signal G 3  applied to the third transfer gate electrode  43  and the gate signal G D  applied to the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  are turned off (1.5 V), and the case regarding the range sensor according to the seventh embodiment having the reverse U-shaped planar pattern in which the gate signal G 2  applied to the second transfer gate electrode  44  is turned on (3.3 V), while the gate signal G 1  applied to the first transfer gate electrode  42 , the gate signal G 3  applied to the third transfer gate electrode  43  and the gate signal G D  applied to the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  are turned off (0 V). 
     The section “G 3 ” in Table 1 indicates the case regarding the range sensor according to the first embodiment having the E-shaped planar pattern in which the gate signal G 3  applied to the third transfer gate electrode  43  is turned on (3.3 V), while the gate signal G 1  applied to the first transfer gate electrode  42 , the gate signal G applied to the second transfer gate electrode  44  and the gate signal G D  applied to the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  are turned off (1.5 V), and the case regarding the range sensor according to the seventh embodiment having the reverse U-shaped planar pattern in which the gate signal G 3  applied to the third transfer gate electrode  43  is turned on (3.3 V), while the gate signal G 1  applied to the first transfer gate electrode  42 , the gate signal G 2  applied to the second transfer gate electrode  44  and the gate signal G D  applied to the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  are turned off (0 V). 
     The section “G D ” in Table 1 indicates the case regarding the range sensor according to the first embodiment having the E-shaped planar pattern and the range sensor according to the seventh embodiment having the reverse U-shaped planar pattern in which the gate signal G D  applied to the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  is turned on (3.3 V), while the gate signal G 1  applied to the first transfer gate electrode  42 , the gate signal G 2  applied to the second transfer gate electrode  44  and the gate signal G 3  applied to the third transfer gate electrode  43  are turned off (0 V). 
     The transfer time in the range sensor according to the seventh embodiment having the reverse U-shaped planar pattern is approximately half of or a third of the transfer time in the range sensor according to the first embodiment having the E-shaped planar pattern. Thus, the range sensor according to the seventh embodiment can exhibit a more rapid charge transfer than the range sensor according to the first embodiment. 
     The range sensor according to the seventh embodiment also has the function suitable for the time-of-flight measurement by use of the Eq. (4) or the Eq. (8). The range sensor according to the seventh embodiment thus can serve as a pixel (lock-in pixel) of a solid-state imaging device to achieve a time-of-flight range image sensor capable of rapid transfer of signal charges with high sensitivity at a low dark current even when a plurality of pixels each having a large area of which a pixel size is five micrometers square or greater are provided on a single semiconductor chip. 
     As in the case of the range sensor according to the first embodiment, the range sensor according to the seventh embodiment is provided with the static induction channel having a sufficient length from the trunk path adjacent to the light-receiving area to the charge-modulation portion, which is shielded by the shielding plate  51 , so as to achieve remarkable effects upon reducing the influence of background light while using a pulse at short intervals. 
     The range sensor according to the seventh embodiment has a profile having a steeper potential gradient in the surface-buried region  62  implementing the photodiode and having the reverse U-shaped planar pattern than the profile of the surface-buried region in the range sensor according to the first embodiment having the E-shaped planar pattern, so as to exhibit a more rapid charge transfer than the range sensor according to the first embodiment. 
     Although  FIG. 28  is illustrated with the case of the triple-layered semiconductor regions of n-type conductivity having different impurity concentrations and including the double-layered guide regions, the range sensor may include semiconductor regions of n-type conductivity having different impurity concentrations and including multiple (three or more) guide regions of n-type conductivity, including a third, fourth, . . . guide regions, so as to have a larger potential gradient in every region in a photodiode having a larger pixel size to ensure higher-speed performance. The range sensor according to the respective second to seventh embodiments may also include multiple (three or more) semiconductor regions of n-type conductivity, so as to achieve the effects of having a larger potential gradient in every region in a photodiode having a larger pixel size, so as to ensure higher-speed performance. 
     (Other Embodiments) 
     While the present invention has been described above by reference to the first to seventh embodiments, it should be understood that the present invention is not intended to be limited to the description and the drawings composing part of this disclosure. Various alternative embodiments, examples, and technical applications will be apparent to those skilled in the art according to this disclosure. 
     While the first to seventh embodiments have exemplified the case in which the first conductivity type is assigned as n-type, and the second conductivity type is assigned as p-type, it will be evident to those skilled in the art that the present invention may also be applicable to the case in which the first conductivity type is p-type and the second conductivity type is n-type to achieve similar effects when the electrical polarities are set to be reverse. 
     While the first to seventh embodiments have exemplified the case in which the signal charges which are subject to the processes of transportation and accumulation or the like are defined as electrons, and in the potential diagrams, the downward direction (depth direction) is defined as the positive direction of the potential. In a case where electrical polarities are set to be inverse, since the charges which are subject to processes are holes, in a potential shape representing a potential barrier, a potential valley, a potential well, or the like in the charge modulation element, the downward direction (depth direction) of the drawings is represented by the negative direction of the potential. 
     While the range sensor according to the first embodiment has been illustrated with the case in which the pixel layer  22  is deposited on the semiconductor substrate  21  of p-type conductivity as shown in the cross-sectional views of  FIG. 2A ,  FIG. 3A  and  FIG. 4A , the semiconductor substrate may be a semiconductor substrate  61  of n-type conductivity as shown in  FIG. 36A ,  FIG. 37A  and  FIG. 38A .  FIG. 36A  is a cross-sectional view as viewed from direction II-II in  FIG. 1 , and  FIG. 36B  is a diagram illustrating a potential profile corresponding to the cross section of  FIG. 36A .  FIG. 37A  is a cross-sectional view as viewed from direction III-III in  FIG. 1 , and  FIG. 37B  is a diagram illustrating a potential profile corresponding to the cross section of  FIG. 37A .  FIG. 38A  is a cross-sectional view as viewed from direction IV-IV in  FIG. 1 , and  FIG. 38B  is a diagram illustrating a potential profile corresponding to the cross section of  FIG. 38A . 
     As shown in  FIG. 36A ,  FIG. 37A  and  FIG. 38A , while the pixel layer  22  is deposited on the semiconductor substrate  61  of n-type conductivity, the other structures may be completely the same as those in the range sensor according to the first embodiment as shown in  FIG. 2A ,  FIG. 3A  and  FIG. 4A . When the pixel layer  22  is deposited on the semiconductor substrate  61  of n-type conductivity, and the intermediate potential (M) is applied to the first extraction gate electrode  41   a  and the second extraction gate electrode  41   b  arranged symmetrically, a U-shaped potential channel is also formed in the static induction channel of the trunk path extending to the central portion bar of the T shape, while potential barriers remain in the cross-sectional direction as viewed from direction II-II as shown in  FIG. 36B . As shown in the potential profile in  FIG. 37B , no potential barrier appears in the static induction channel in the cross-sectional direction as viewed from direction III-III in  FIG. 1  from the trunk path toward the third charge-accumulation region  24   c  of the charge-modulation portion. The photoelectrons generated in the photodiode are rapidly transferred to the charge-modulation portion as signal charges via the U-shaped potential channel formed in the static induction channel. 
     As shown in  FIG. 38B , the voltage level G 3  of the gate signal applied to the third transfer gate electrode  43  is set at the low potential (L), and the voltage level G 1  of the gate signal applied to the first transfer gate electrode  42  and the voltage level G 2  of the gate signal applied to the second transfer gate electrode  44  are set at either the low potential (L) or the intermediate potential (M) differently from each other, so as to transfer photoelectrons to either the first charge-accumulation region  24   b  or the second charge-accumulation region  24   d , Thus, the range sensor in which the pixel layer  22  is deposited on the semiconductor substrate  61  of n-type conductivity can implement substantially the same operations as the range sensor according to the first embodiment. 
     In the range sensor according to the first embodiment, since the semiconductor substrate  21  is a p + -type region having a high impurity concentration, a dark current (a diffusion current component) generated in a deep position of the semiconductor substrate  21  is decreased. Further, the range sensor according to the first embodiment can allow carriers generated in the deep position of the semiconductor substrate  21  and approaching the surface of the semiconductor substrate  21  slowly as a diffusion component to disappear by Auger recombination in the p + -region having a high impurity concentration so as to exhibit a rapid response as a time-of-flight sensor. 
     As shown in  FIG. 36A ,  FIG. 37A  and  FIG. 38A , when the semiconductor substrate  61  as a semiconductor region of n-type conductivity and the pixel layer  22  as a semiconductor region of p-type conductivity are used to provide a p-n junction, the range sensor can achieve the effects of suppressing a diffusion component of a dark current in the deep side of the semiconductor substrate  61  and preventing a carrier component from approaching the surface of the semiconductor substrate  61  slowly as a diffusion component due to the potential barrier of the p-n junction. 
     The range sensor according to the respective second to seventh embodiments may also use a semiconductor substrate of n-type conductivity so as to suppress a diffusion component of a dark current in the deep side of the semiconductor substrate  61  and preventing a carrier component from approaching the surface of the semiconductor substrate  61  slowly as a diffusion component due to the potential barrier of the p-n junction. 
     As shown in  FIG. 39  and  FIG. 40A , the surface-buried region  62  of n-type conductivity, which implements a photodiode may be formed into a fork-like shape in a planar pattern as in the case of the range sensor according to the first embodiment, and the auxiliary guide region (second guide region)  63  of n-type conductivity having a higher impurity concentration than the surface-buried region  62  is provided in the central portion at the base of the fork-like shape, so as to collect signal charges in the auxiliary guide region  63 .  FIG. 40A  is a cross-sectional view as viewed from direction XXXXIV-XXXXIV in  FIG. 39 , and  FIG. 40B  is a diagram illustrating a potential profile corresponding to the cross section of  FIG. 40A . 
     The auxiliary guide region  63  has a planar pattern of a bird-like shape symmetrically spreading the wings of which a head portion is provided in the central portion at the base of the fork-like shape, as shown in  FIG. 39 . Each of prongs of the fork-like shape has a stepped shape gradually increased in width from upper-side to lower-side in the drawing of  FIG. 39 , which is the same as the planar pattern of the range sensor according to the first embodiment. The range sensor according to another embodiment as shown in  FIG. 39  generates a high drift field in the entire depleted branches of the surface-buried region  62  having a fork-like shape gradually increased in width in the planar pattern in the light-receiving area, so as to rapidly transfer the photoelectrons as signal charges to the auxiliary guide region  63  in the longitudinal direction of the prongs of the fork-like shape even when the light-receiving area has a large area. 
     The range sensor according to another embodiment as shown in  FIG. 39  includes a guide region (a main guide region)  64  of n-type conductivity, which is buried in the central portion of the auxiliary guide region  63  and having a higher impurity concentration than the auxiliary guide region  63 , as shown in  FIG. 40A , so as to collect signal charges to the guide region  64 . The guide region  64  is a semiconductor region for guiding the photoelectrons collected in the auxiliary guide region  63  to the respective narrow transfer channels in the charge-modulation portion. The guide region  64  has a stepped shape of which the width in the direction perpendicular to the longitudinal direction (the vertical direction in  FIG. 39 ) of the guide region  64  is gradually increased from upper-side to lower-side in the drawing of  FIG. 39 . 
     The range sensor according to another embodiment as shown in  FIG. 39  also generates a high drift field in the entire depleted guide region  64  gradually increased in width in the planar pattern. Accordingly, the photoelectrons as signal charges can rapidly be transferred in the longitudinal direction of the guide region  64 . 
     The triple-layered semiconductor regions of extendconductivity having different impurity concentrations as shown in  FIG. 39  have a large potential gradient in every region in a photodiode having a significantly large area of which a pixel size is 10 micrometers square or greater, so as to increase the electric field to lead to high-speed performance. As understood from the plan view of  FIG. 39  and the potential profile of  FIG. 40B , the tip of the auxiliary guide region (the second guide region)  63  corresponds to the bottom of the potential profile of the surface-buried region  62  to which carriers generated are transferred, and the tip of the guide region (the main guide region)  64  is provided in contact with the auxiliary guide region  63 . Accordingly, the potential gradient increases in every region, so as to increase the electric field to achieve high-speed performance of the range sensor. As shown in  FIG. 40B , the potential profile has a profile in which the portion corresponding to the auxiliary guide region  63  is deeper than that shown in  FIG. 5 . 
     The auxiliary guide region  63  may be buried as a semiconductor region having a higher impurity concentration than the surface-buried region  62  such that a part of an ion-implanted area to bury the surface-buried region  62  is doubly ion-implanted in the planar pattern shown in  FIG. 39 . The main guide region  64  may be buried as a semiconductor region having a higher impurity concentration than the auxiliary guide region  63  such that a part of the region implanted with ions to bury the surface-buried region  62  is triply implanted with ions in the planar pattern shown in  FIG. 39 . 
     Although  FIG. 39  is illustrated for a case of the triple-layered semiconductor regions of n-type conductivity, each having different impurity concentrations, and a case which includes the double-layered guide regions, the range sensor may include semiconductor regions of n-type conductivity, each having different impurity concentrations, and a case which includes multiple (three or more) guide regions of n-type conductivity. The multiple guide regions may include a third, fourth, . . . guide regions, so as to have a larger potential gradient in every region in a photodiode having a larger pixel size to ensure higher-speed performance. The range sensor according to the respective second to seventh embodiments may also include multiple (three or more) semiconductor regions of n-type conductivity, so as to achieve the effects of having a larger potential gradient in every region in a photodiode having a larger pixel size, so as to ensure higher-speed performance. 
     When the surface-buried region includes light-receiving terminal-portions having a trilobate leaf shape in a planar pattern, such as the surface-buried region  25   r  according to the fifth embodiment or the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) according to the sixth embodiment, the auxiliary guide region (the second guide region) may have a trilobate leaf shape smaller than the light-receiving terminal-portions so as to be provided in each of the four apertures. 
     In the case of the planar pattern of the fifth embodiment, for example, the range sensor may include four auxiliary guide regions of n-type conductivity having a higher impurity concentration than the surface-buried region  25   r  in which each one end having a trilobate leaf shape is provided below each of the four apertures and the other ends extend to the charge-modulation arrangement-region shielded by the shielding plate  51 . Namely, the guide region  26   g  shown in  FIG. 25  has a double-layer structure including the four auxiliary guide regions and a main guide region of n-type conductivity having protruding-edges extending toward the respective four auxiliary guide regions from the charge-modulation arrangement-region, the main guide region having a higher impurity concentration than the auxiliary guide regions and a lower impurity concentration than the first charge-accumulation region  24   h , the second charge-accumulation region  24   i  and the third charge-accumulation region  24   l.    
     In the case of the sixth embodiment, for example, the range sensor may have a topology including four auxiliary guide regions of n-type conductivity having a higher impurity concentration than the surface-buried region ( 25   s   1 ,  25   s   2 ,  25   s   3 ,  25   s   4 ) in which each one end having a trilobate leaf shape is provided below each of the four apertures and the other ends extend to the charge-modulation arrangement-region shielded by the shielding plate  51 . In this case, the guide region  26   h  shown in  FIG. 26  has a double-layer structure including the four auxiliary guide regions and a main guide region of n-type conductivity having a topology including radial (X-shaped) four protruding-edges extending below the respective four auxiliary guide regions from the charge-modulation arrangement-region, the main guide region having a higher impurity concentration than the auxiliary guide regions and a lower impurity concentration than the first charge-accumulation region  24   r , the second charge-accumulation region  24   o  and the third charge-accumulation region  24   p.    
     Further, as shown in  FIG. 41A , a first block region  65  of p-type conductivity may be selectively buried immediately below the auxiliary guide region (the second guide region)  63 , and a second block region  66  of p-type conductivity having a higher impurity concentration than the first block region  65  may be selectively buried immediately below the guide region  64 , so as to reduce a component introduced without charge modulation.  FIG. 41A  illustrates the first block region  65  partly overlapping with a part of the bottom portion of the surface-buried region  62 . The second block region  66  is buried such that the bottom portion partly overlaps with a part of the upper portion of the first block region  65 , and the upper portion of the second block region  66  is introduced into a part of the bottom portion of each of the surface-buried region  62  and the auxiliary guide region  63 . 
     The range sensor including the first block region  65  and the second block region  66  as shown in  FIG. 41A  can also have a structure in which the auxiliary guide region  63  is buried at a portion corresponding to the bottom of the potential profile of the surface-buried region  62  to which carriers generated are transferred, and the narrow tip of the guide region  64  is provided in contact with the auxiliary guide region  63 , as shown in the potential profile of  FIG. 41B . Accordingly, the potential gradient increases in every region, so as to increase the electric field to achieve high-speed performance of the range sensor. The first block region  65  and the second block region  66  may be buried by double ion implantation. 
     The range sensor according to the respective second to seventh embodiments may also include multiple (three or more) layered semiconductor regions of n-type conductivity in which the block regions are further buried immediately below the respective auxiliary guide region (second guide region) and guide region (main guide region), so as to achieve the effects of reducing a component introduced without charge modulation and increasing the potential gradient in every region to ensure the higher-speed performance when a photodiode has a larger pixel size. 
     The range sensor according to the first embodiment has been illustrated with the case in which each of the branches of the surface-buried region  25  having a fork-like shape has a stepped shape gradually increased in width from upper-side to lower-side in the drawing of  FIG. 1 . When the degree of steps along the surface-buried region  25  are minimized as much as possible, and the number of the steps increases infinitely, the range sensor in which each prong has a width gradually increased from top to bottom in a straight manner as shown in  FIG. 42  may be obtained. 
     The actual topology of the surface-buried region  25  may result in a gently stepped shape as shown in  FIG. 42  because of limitations of the manufacture process in photolithography even when the surface-buried region  25  has a stepped shape as shown in  FIG. 1  in a mask level. The range sensor including the surface-buried region  25  gradually increased in width from top to bottom in a straight manner as shown in  FIG. 42  can also generate a high drift field in the entire depleted prongs of the surface-buried region  25  having a fork-like shape. Accordingly, photoelectrons as signal charges can be transferred rapidly in the longitudinal direction of the prongs of the fork-like shape gradually increased in width from top to bottom in a straight manner, as shown in  FIG. 42 . 
     The range sensor according to the first embodiment having the topology as shown in  FIG. 42 , instead of the topology as shown in  FIG. 1 , can also generate a high drift field in the entire depleted guide region  26   a . Accordingly; photoelectrons as signal charges can be transferred rapidly in the longitudinal direction of the guide region  26   a  having the planar pattern as shown in  FIG. 42 . 
     It will be evident to those skilled in the art according to the explanations made above and the scope of the present invention that the range sensor according to the respective third to seventh embodiments may employ the lateral electric field control effect as described in the second embodiment, so as to transfer electric charges more rapidly than “the transfer gate method” as described in the range sensor according to the respective third to seventh embodiments. 
     The range sensor according to the first embodiment has been illustrated with the case provided with the additional branches extending adjacent to the light-receiving area in the direction perpendicular to the longitudinal direction of the trunk path of the surface-buried region  25 , and the respective tips of the additional branches are connected with the first extraction region  24   a  and the second extraction region  24   e  of n-type conductivity having a higher impurity concentration than the surface-buried region  25 . The range sensor is not necessarily provided with additional branches adjacent to the light-receiving area in the direction perpendicular to the longitudinal direction of the trunk path of the surface-buried region  25 , as shown in  FIG. 43  and  FIG. 44 . In other words, the surface-buried region  25  may be separated from the first extraction region  24   a  and the second extraction region  24   e.    
     The range sensor according to the seventh embodiment has been illustrated with the case in which the reverse U-shaped surface-buried region  25  has a stepped shape gradually increased in width from upper-side to lower-side in the drawing of  FIG. 28 . When the degree of the steps along the surface-buried region  25  are minimized as much as possible, and the number of the steps increases infinitely, the range sensor including the surface-buried region  62  having a trapezoidal shape gradually increased in width in a straight manner as shown in  FIG. 45  or a surface-buried region having a reverse U-shape approximate to a parabolic curve gradually increased in width into an arc-like shape (not shown) may be obtained. The actual topology of the surface-buried region  62  may result in a gently stepped trapezoidal shape as shown in  FIG. 45  because of limitations of the manufacture process in photolithography even when the surface-buried region  62  has a stepped shape as shown in  FIG. 28  in a mask level. The range sensor including the surface-buried region  62  gradually increased in width from top to bottom in a straight manner as shown in  FIG. 45  can also rapidly transfer photoelectrons. When the end of the surface-buried region  62  further extends upward in  FIG. 45 , the outline of the surface-buried region  62  may be an envelope curve approximate to a reverse V-shape (isosceles triangle). 
     Namely, the outline of the surface-buried region  62  or the envelope curve of the outline is only required to have a gradually increased width measured in the direction perpendicular to the transfer direction of signal charges so as to have a trapezoidal shape, a parabolic curve, a reverse U-shape or a reverse V-shape in the light-receiving area to surround the auxiliary guide region  63  of the guide region ( 63 ,  64 ) in a planar pattern. The range sensor may only include the main guide region  64  of the guide region ( 63 ,  64 ) without the auxiliary guide region  63  shown in  FIG. 45 . 
     The semiconductor material implementing the photodiode in the range sensor according to the present invention is not limited to silicon (Si). Range sensors and solid-state imaging devices including various kinds of compound semiconductors such as group III-V compound semiconductor or group II-VI compound semiconductor can employ the structures and technical ideas of the range sensor and the solid-state imaging device according to the respective first to seventh embodiments. 
     It should be noted that the present invention includes various embodiments which are not disclosed herein. Therefore, the scope of the present invention is defined only by the present invention specifying matters according to the claims reasonably derived from the description heretofore. 
     REFERENCE SIGNS LIST 
     
         
           21 ,  61  Semiconductor substrate 
           22  Pixel layer 
           23  Well region 
           24   a ,  24   aa ,  24   ba ,  24   g  First extraction region 
           24   be ,  24   ae ,  24   e ,  24   f  Second extraction region 
           24   j  Third extraction region 
           24   k  Fourth extraction region 
           24 ,  24   ab ,  24   b ,  24   bb ,  24   h  First charge-accumulation region 
           24   ad ,  24   bd ,  24   i ,  24   d  Second charge-accumulation region 
           24   ac ,  24   bc ,  24   l ,  24   c  Third charge-accumulation region 
           25 ,  25   p ,  25   q ,  62  Surface-buried region 
           25   a  First surface-buried region 
           25   b  Second surface-buried region 
           26   a ,  26   b ,  26   f ,  64  Guide region (main guide region) 
           26   d  First guide region 
           26   e  Second guide region 
           27  Pinning layer 
           31  Insulating film 
           32 ,  32 a,  32 b,  32 c Edge of filed insulating film 
           41   a ,  41   aa ,  41   ba ,  41   m  First extraction gate electrode 
           41   ab ,  41   b ,  41   bb ,  41   l  Second extraction gate electrode 
           41   n  Third extraction gate electrode 
           41   o  Fourth extraction gate electrode 
           42   a ,  42 ,  42   b ,  42   h  First transfer gate electrode 
           44   a ,  44 ,  44   b ,  44   h  Second transfer gate electrode 
           43   a ,  43 ,  43   b ,  43   h  Third transfer gate electrode 
           42   p  First field-control electrode 
           44   p  Second field-control electrode 
           43   p  Third field-control electrode 
           51  Shielding plate 
           53   p ,  53   q ,  53   r ,  53   s  Microlens 
           65  First block region 
           66  Second block region