Abstract:
An optical semiconductor device containing a photodiode, includes a first semiconductor layer of a first conductive type; and a channel layer of a second conductive type formed from a surface portion of the first semiconductor layer in a light receiving region. The channel layer and the first semiconductor layer in the light receiving region form a p-n junction region.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an optical semiconductor device, and especially relates to an optical semiconductor device in which sensitivity is improved. 
         [0003]    2. Description of Related Art 
         [0004]    Development of a light receiving element for light of shorter wavelength such as blue laser is advanced in order to realize high density recording in a recording apparatus. On the other hand, when an oscillating central wavelength of a semiconductor laser device used as a light source of light signals is short, an optical-absorption coefficient of semiconductor for receiving the laser light becomes large and an invasive length in the semiconductor becomes short. As a result, it is required to prevent reduction of quantum efficiency according to recombination of photo carriers in the vicinity of surface of the semiconductor in order to take out the photo carriers efficiently. 
         [0005]    As a technique aiming to obtain a light receiving element having a high sensitivity and high speed response in response to light of short wavelength, a light receiving element and a manufacturing method thereof and a circuit built-in type light receiving element are disclosed in Japanese Laid Open Patent Publication (JP-P2004-87979A).  FIG. 1  is a cross sectional view showing a structure of a circuit built-in type light receiving element in the related art. This circuit built-in type light receiving element  120  has a P-type semiconductor substrate  101  made of silicon, having the resistivity of approximately 40 Ωcm, and a P-type high concentration embedding diffusion layer  102 , a P-type high specific resistance epitaxial layer  103  having the resistivity of 100 Ωcm or more, and an N-type epitaxial layer  106  having the resistivity of approximately 1 to 5 Ωcm are laminated in this order on a P-type semiconductor substrate  101 . The circuit built-in type light receiving element  120  has a photodiode region and a bipolar element region adjacent thereto. The photodiode region (left side in the figure) and the transistor element region (right side in the figure) are separated by a P-type embedding separating diffusion layer  104  formed in the P-type high resistivity epitaxial layer  103  to reach a boundary with the N-type epitaxial layer  106  from a boundary with P-type high concentration embedding diffusion layer  102  and by a P-type separation diffusion layer  107  formed to reach the P-type embedding separation diffusion layer  104  from a surface of the boundary with the N-type epitaxial layer  106 . 
         [0006]    An N-type impurities diffusion layer  108  is formed in the photodiode region with diffusion of N-type impurities in depth of equal to or less than 0.3 μm (for example, 0.3 μm) from a surface of the N-type epitaxial layer  106  and at a concentration in which a peak concentration of the impurities is lower than 1×10 20  cm −3 , e.g., about 8×10 19  cm −3 . 
         [0007]    An N-type embedding diffusion layer  105  is formed in the transistor element region to be embedded into the surface of the P-type high resistivity epitaxial layer  103 . An N-type well diffusion layer  109  and an N-type impurity diffusion layer  108  as a collector layer are formed on the N-type embedding diffusion layer  105  adjacently each other. A P +  based diffusion layer  111  formed to be adjacent with a P-based diffusion layer  110  and with both sides of the P-based diffusion layer  110  is provided in the N-type well diffusion layer  109 . An N-type emitter diffusion layer  112  is formed in a region of the P-based diffusion layer  110 . 
         [0008]    As described above, an insulating film for surface protection is formed all over the surface of the N-type epitaxial layer  106  where each layer in the photodiode region and each layer in the transistor element region are formed respectively. The insulating film for surface protection has openings respectively on the N-type impurities diffusion layer  108  and on the P-type separating diffusion layer  107  in the photodiode region, on the N-type impurities diffusion layer  108  that is a collector pull-up layer of the bipolar transistor region, on the P +  based diffusion layer  111 , and the N-type emitter diffusion layer  112 . A hard-wiring (electrode) metal layer  114  is provided on respective openings. 
         [0009]    The photo carriers generated in the N-type impurities diffusion layer  108  are moved to a depletion layer by an internal electric field generated on the basis of concentration gradient of N-type impurities, and thus a photoelectric current flows. However, when concentration of impurities in the N-type impurity diffusion layer  108  is high, a lifetime of photo carriers becomes short and the photo carriers recombine before reaching the depletion layer to disappear. For this reason, the photo carriers cannot contribute to the generation of the photoelectric current, and therefore quantum efficiency of the light receiving element decreases. As described above, when an absorption coefficient of light increases in accordance with shortening the wavelength of light and an invasive length of the light in semiconductor layers becomes short, the photo carriers generated in the N-type impurity diffusion layer  108  increase. As a result, when concentration of impurities in the N-type impurity diffusion layer  108  is high, reduction of a quantum efficiency of the light receiving element is apparently shown. In order to prevent reduction of the quantum efficiency on the basis of recombination of the photo carriers in a high concentration impurities and a diffusion layer, a profile of concentration is optimized. 
         [0010]    As a first method for optimizing, the internal electric field is strengthened by shallowly forming the N-type impurity diffusion layer  108  to set a steep diffusion profile when concentration of impurities in the N-type impurity diffusion layer  108  is high. As a result, a moving speed at which the photo carriers move to a depletion layer becomes higher in comparison with a case of deeply forming the N-type impurities diffusion layer  108 , therefore it is possible for the photo carriers to be moved into the depletion layer before recombination. 
         [0011]    As a second method for optimizing, when the N-type impurity diffusion layer  108  is deeply formed, a lifetime of photo carriers can be prolonged by lowering the concentration of impurities. As a result, it is possible for the photo carriers to move into the depletion layer without recombination. 
         [0012]    In each of the first and the second methods mentioned above, it is required for a peak position of an impurity concentration to be set on the surface of a semiconductor or near the surface as possible in order to restraining recombination of carriers in the N-type impurity diffusion layer  108 . To realize this, the ion-implantation method through an oxide film is necessarily required. However, a production cost increases because lead time is prolonged in accordance with forming of the oxide film. In addition, since heat treatment at a high temperature to regions other than a light receiving section such as a circuit element is required, it is difficult to precisely control a diffusion layer in the light receiving section. Therefore, a technique is desired, which can manufacture a light receiving element in a simple way and low cost process, and the light receiving element can receive light of short wavelength such as a blue laser in a high sensitivity and in high-speed response. 
         [0013]    In conjunction with the above description, a divisional light receiving element, a circuit built-in light receiving element, and an optical disk device are disclosed in Japanese Laid Open Patent Publication (JP-P2003-92424A). The divisional light receiving element includes a plurality of second conductivity type diffusion layers formed on a first conductivity type semiconductor layer while keeping a predetermined interval from each other; a leak prevention layer which is formed at least between a plurality of second conductivity type diffusion layers on the first conductivity type semiconductor layer and which prevents leakage occurring between the second conductivity type diffusion layers; and a dielectric film formed at least in a region to which light enters on the first conductivity type semiconductor layer including the second conductivity type diffusion layers and the leakage prevention layer. 
         [0014]    A solid imaging device and a light receiving element are disclosed in Japanese Laid Open Patent Publication (JP-A-Heisei 11-214668). The solid imaging device includes a first conductivity type semiconductor substrate; a plurality of second conductivity type accumulation layers for accumulating signal electric charge by incidence of light, which are formed on the semiconductor substrate; an insulation layer formed on upper sides of the accumulation layers; an optical transparent electrode formed on an upper side of the insulation layer; voltage supplying means for applying electric potential to the optical transparent electrode and for forming a reversal layer on surfaces of the accumulation layers located downward; and signal transmission means for scanning the signal electric charge accumulated in the accumulation layers and for outputting it as image signals to outside. 
       SUMMARY 
       [0015]    In one embodiment of the present invention, an optical semiconductor device containing a photodiode, includes a first semiconductor layer of a first conductive type; and a channel layer of a second conductive type formed from a surface portion of the first semiconductor layer in a light receiving region. The channel layer and the first semiconductor layer in the light receiving region form a p-n junction region. 
         [0016]    In another embodiment of the present invention, an operation method of an optical semiconductor device, the optical semiconductor device is provided which includes a first semiconductor layer of a first conductive type; a second semiconductor layer of a second conductive type formed in a surface portion on the semiconductor layer; a third semiconductor layer of the first conductive type formed under the first semiconductor layer; a fourth semiconductor layer of the first conductive type formed to pass through the first semiconductor layer to the third semiconductor layer; a first electrode provided on the second semiconductor layer; and a second electrode provided on the fourth semiconductor layer. The operation method of the optical semiconductor device is achieved by applying a reverse bias voltage between the first semiconductor layer and the second semiconductor layer to form a channel layer of the second conductive type in a surface portion of the first semiconductor layer; and by detecting a photoelectric current generated when a light is inputted in a light receiving region, in which a pn junction region is formed. 
         [0017]    According to the embodiments of the present invention, an optical semiconductor device with a high sensitivity and with high-speed response can be provided as a receiving element for short wavelength such as the blue laser. An optical semiconductor device with the high sensitivity and in high-speed response as the receiving element for short wavelength light, which can be manufactured in a simple way and low cost process can be provided. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The above and other objects, advantages and features of the present invention will be more apparent from the following description of embodiments taken in conjunction with the accompanying drawings, in which: 
           [0019]      FIG. 1  is a cross sectional view showing a structure of a conventional circuit built-in type light receiving element; 
           [0020]      FIG. 2  is a cross sectional view showing a structure of an optical semiconductor device according to an embodiment of the present invention; 
           [0021]      FIG. 3  is a plan view showing a structure of an optical semiconductor device according to the embodiment of the present invention; 
           [0022]      FIG. 4  is a cross sectional view showing an operation principle of the optical semiconductor device according to the embodiment of the present invention; 
           [0023]      FIGS. 5A to 5L  are cross sectional views showing a manufacturing method of the optical semiconductor device in the embodiment of the present invention; and 
           [0024]      FIG. 6  is a flowchart showing an operation of the optical semiconductor in the embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    Hereinafter, an optical semiconductor device of the present invention will be described with reference to the attached drawings.  FIG. 2  is a cross sectional view showing the optical semiconductor device in which a photodiode and a MOS transistor are formed on a same semiconductor substrate, as an example of a configuration of the optical semiconductor device according to the embodiment of the present invention. Referring to  FIG. 2 , a photodiode region  20  includes a P-type semiconductor substrate  1 , a P +  type embedded layer  2 , a P-type epitaxial layer  3 , a P +  type diffusion layer  4 , an N +  type diffusion layer  5 , a reflection preventing film  6 , a field film  7 , an anode electrode  8 , and a cathode electrode  9 . On the other hand, a MOS transistor region  40  includes an N well diffusion layer  33 , a P +  type diffusion layer  34 , a gate oxidized film+polysilicon gate  35 , a protective insulating film  36 , and drain/source electrodes  37 . Furthermore, a LOCOS (LOCal Oxidation of Silicon) is provided between the photodiode region and the MOS transistor. 
         [0026]    The P-type semiconductor substrate  1  is such as a P-type silicon substrate. The P +  type embedded layer  2  is provided to cover the P-type semiconductor substrate  1 . The P +  type embedded layer  2  is exemplified by a P-type silicon layer with high concentration of impurities. The P-type epitaxial layer  3  is provided to cover the P +  type embedded layer  2 . The P-type epitaxial layer  3  is exemplified by a silicon layer having a high resistivity more than  100  Ωcm with a low concentration of impurities. The P +  type diffusion layer  4  is provided to penetrate from a surface of the P-type epitaxial layer  3  to a surface of the P +  type embedded layer  2  on a predetermined position outside a light receiving region. An impurities concentration of the P-type epitaxial layer  3  is lower than those of the P +  type embedded layer  2  and the P +  type diffusion layer  4 . The P-type epitaxial layer  3  is exemplified by a P-type silicon layer with a high concentration of impurities. The N +  type diffusion layer  5  is shallowly embedded in a surface of the P-type epitaxial layer  3  on a predetermined position outside the light receiving region. The N +  type diffusion layer  5  is exemplified by an N-type silicon layer with a high concentration of impurities. The reflection preventing film  6  is provided to cover a surface (a surface of a channel region  30 ) of the P-type epitaxial layer  3  in the light receiving region. The reflection preventing film  6  includes an oxide film  6   b  such as a silicon oxide film and a nitride film  6   a  such as a silicon nitride film. Thicknesses of the films are set to prevent reflection of light in accordance with wavelengths of received lights. The field film  7  is provided to cover a surface of the P-type epitaxial layer  3  other than the light receiving region. The field film  7  includes an oxide film  7   b  such as a silicon oxide film and a nitride film  7   a  such as a silicon nitride film. The anode electrode  8  is provided to fill an opening of the field film  7  on the P +  type diffusion layer  4  and to reach the P +  type diffusion layer  4 . The cathode electrode  9  is provided to fill an opening of the field film  7  on the N +  type diffusion layer  5  and to reach the N+type diffusion layer  5 . 
         [0027]    The N-type well diffusion layer  33  is provided in the P-type semiconductor substrate  1 . The P +  type diffusion layers  34  are provided in the N-type well diffusion layer  33 . The gate oxide film+polysilicon gate  35  is provided on the P-type semiconductor substrate  1  and is embedded with the protective insulating film  36 . The drain/source electrodes  37  are provided to fill openings on the P +  type diffusion layers  34  and to reach the P +  type diffusion layer  34 . The LOCOS  32  is formed of silicon oxide to insulate between the photodiode region  20  and the MOS transistor and between the MOS transistors. 
         [0028]      FIG. 3  is a plan view showing the photodiode region  20  in the configuration of the optical semiconductor device according to the embodiment of the present invention. In the photodiode region  20 , the cathode electrode  9  and the anode electrode  8  are provided to surround the light receiving region provided with the reflection preventing film  6 . Insulation among the surface of the P-type epitaxial layer  3 , the cathode electrode  9 , and the anode electrode  8  is accomplished with the field film  7 . 
         [0029]    An operation of the optical semiconductor device according to the embodiment of the present invention will be described.  FIG. 4  is a cross sectional view showing an operation principle of the optical semiconductor device in the embodiment of the present invention. In the present embodiment, a diffusion layer is not formed in a light receiving region, and a very shallow inversion layer  10  is formed by applying a bias of a reverse polarity between the anode electrode  8  and the cathode electrode  9  just under the field film  7  and the reflection preventing film  6 , and the inversion layer is used as a cathode diffusion layer. More detailed description will be made below. 
         [0030]    A silicon film with a high resistivity (more than 100 Ωcm) is used for the P-type epitaxial layer  3 , and the reverse bias E over a predetermined voltage is applied between the anode electrode  8  and the cathode electrode  9 . As a result, a very shallow inversion layer (N +  channel)  10  is formed on a surface (channel region  30 ) of the P-type epitaxial layer  3  just under the field film  7  composed of the oxide film  7   b  and the nitride film  7   a  and the reflection preventing film  6  composed of the oxide film  6   b  and the nitride film  6   a . The very shallow inversion layer  10  functions as a cathode diffusion layer. That is to say, since photo carriers are generated inside the inversion layer  10  upon reception of light and the generated photo carriers are moved into a depletion layer  11  by an internal electric field produced on the basis of concentration gradient, a photoelectric current flows between the P +  type embedded layer  2  and P +  type diffusion layer  4  and the inversion layer (N +  channel)  10  through the p-n junction. At this time, the depletion layer  11  is extended to a side of a high resistivity layer when the reverse bias is applied. Therefore, a capacity of the photodiode (the P +  type embedded layer  2  and P +  type diffusion layer  4  and the inversion layer (N +  channel)  10 ) is reduced so that a high speed response is made possible at the same time. 
         [0031]    It is known that charges of “+” are likely to present in the oxide films (SiO 2 )  7   b  and  6   b  of the field film  7  and the reflection preventing film  6 . For this reason, there is a case that holes are lack on the surface of the P-type epitaxial layer  3  in the vicinity of boundary between the P-type epitaxial layer  3  and the oxide films  7   b  and  6   b  because the holes are pushed by “+” charge in the oxide films  7   b  and  6   b . Consequently, a layer in whose hole concentration is quite low is formed along the boundary. In such a situation, when the reverse bias E is applied between the anode electrode  8  and the cathode electrode  9 , a P −  region of the P-type epitaxial layer  3  in the vicinity of the boundary becomes a depletion layer because of the+voltage applied via the oxide films  7   b  and  6   b . Furthermore, when a high-intensity reverse bias is applied, the N-type inversion layer  10  appears by electrons gathering in the boundary. Thus, a p-n junction is realized between the P +  type embedded layer  2  and P +  type diffusion layer  4  and the inversion layer  10 . When the inversion layer  10  is used as a cathode diffusion layer of a light receiving element, it is not required to form a cathode diffusion layer by a method such as ion-implantation. In addition, since the inversion layer  10  is very shallowly formed, reduction of quantum efficiency due to recombination on the surface of the P-type epitaxial layer  3  can be prevented so that the light receiving element of high sensitivity can be produced. As a result, it is not required to form a very shallow diffusion layer in the light receiving region. Since a diffusion layer is not formed, influence of heat treatment at high temperature in forming components other than the light receiving section can be prevented. 
         [0032]      FIGS. 5A to 5L  are cross sectional views showing a manufacturing method of the optical semiconductor device in the embodiment of the present invention. Referring to  FIG. 5A , the P-type semiconductor substrate  1  of p type silicon whose resistivity is approximately 30 Ωcm (concentration of impurities: 4.44×10 14  cm 3 ) is prepared first. And then, the P +  type embedded layer  2  is formed by an impurity diffusion method to cover the P-type semiconductor substrate  1 . A sheet resistance of the P +  type embedded layer  2  is approximately 100 Ω/□. After that, the P-type epitaxial layer  3  is formed by an epitaxial growth method to cover the P+type embedded layer  2 . At this moment, a resistivity of the P-type epitaxial layer  3  is 100 Ωcm or more (concentration of impurities: less than 1.33×10 14  cm −3 ). After that, an oxide film  12  of silicon oxide is formed through thermal oxidation on the surface of the P-type epitaxial layer  3 . 
         [0033]    Referring to  FIG. 5B , a photoresist layer  13  is formed to cover the oxide film  12 , and a patterning is carried out to the photoresist  13  through a lithography process. The P +  type diffusion layer  4  is formed to extend from a surface of the P-type epitaxial layer  3  toward the P-type semiconductor substrate  1  in openings of the photoresist  13  by the ion-implantation method or the impurity diffusion method. At this time, a concentration of impurities of the P +  type diffusion layer  4  is approximately 2.0×10 19  cm −3 . 
         [0034]    Referring to  FIG. 5C , after removing the photoresist  13  and the oxide film  12 , p type impurities of the P +  type diffusion layer  4  and p type impurities of the P +  type embedded layer  2  mutually diffuse and are activated through a heat treatment process. As a result, the P +  type diffusion layer  4  and the P +  type embedded layer  2  are connected. 
         [0035]    Referring to  FIG. 5D , an oxide film  14  of silicon oxide is formed by thermally oxidizing the surface of the P-type epitaxial layer  3  and the P +  type diffusion layer  4 . Referring to  FIG. 5E , a photoresist layer  15  is formed to cover the oxide film  14 , and a patterning is carried out to the photoresist  15  through the lithography process. Through this process, openings  16  are formed in the photoresist  15 . With referring to  FIG. 5F , the N +  type diffusion layers  5  are formed in a surface region of the P-type epitaxial layer  3  in the openings  16  of the photoresist  15  by the ion-implantation method or the impurity diffusion method. At this time, a concentration of impurities of the N +  type diffusion layer  5  is approximately 2.0×10 19  cm −3 . 
         [0036]    Referring to  FIG. 5G , subsequently, after removing the photoresist  15  and the oxide film  14 , n type impurities in the N +  type diffusion layer  5  diffuse and are activated through a heat treatment process. After that, the oxide film  7   b  of silicon oxide is formed by thermally oxidizing the surfaces of the P-type epitaxial layer  3  and the P +  type diffusion layers  4 . At this time, a film thickness of the oxide film  7   b  is approximately 50 nm. The oxidization is carried out in an ISSG (In Situ Steam Generation) method of atmosphere at 1050° C. and in 5% H 2 . Subsequently, the nitride film  7   a  of silicon nitride is formed through a CVD method to cover the oxide film  7   b . At this time, a film thickness of the nitriding film  7   a  is approximately 180 nm. 
         [0037]    Referring to  FIG. 5H  a photoresist layer  17  is formed to cover the nitriding film  7   a , and a patterning is carried out to the photoresist layer  17  through the lithography process. The nitride film  7   a  and the oxide film  7   b  are removed in an opening  18  of the photoresist layer  17  by using a dry etching. As a result, in the opening  18 , the surface of the P-type epitaxial layer  3  is exposed. The exposed region is a light receiving region. In addition, the field film  7  is formed as a laminated film of the oxide film  7   b  and the nitride film  7   a.    
         [0038]    Referring to  FIG. 5I , after that, the oxide film  6   b  of silicon oxide is formed by the CVD method to cover the exposed region of the P-type epitaxial layer  3 . At this time, a film thickness of the oxide film  6   b  is approximately 10 nm. Subsequently, the nitride film  6   a  of silicon nitride is formed by the CVD method to cover the oxide film  6   b . At this time, a film thickness of the nitride film  7   a  is approximately 40 nm. After that, by removing the photoresist  17 , the oxide film  6   b  and the nitride film  6   a  on the photoresist layer  17  are also removed. As a result, the reflection preventing film  6  as the laminated film of the oxide film  6   b  and the nitride film  6   a  is formed in the light receiving region. Film thicknesses of the oxide film  6   b  and the nitriding film  6   a  are preliminarily determined on the basis of a wavelength of light to be received. 
         [0039]    Referring to  FIG. 5J , a photoresist layer  19  is formed to cover the field film  7  and the reflection preventing film  6 , and a patterning is carried out to the photoresist  19  through the lithography process. Thus, openings  24  and  21  are formed in the photoresist layer  19 . Referring to  FIG. 5K , subsequently, in the openings  24  and  21  of the photoresist layer  19 , through-holes  22  and  23  are formed in the field film  7  by the dry etching. At this time, the through-hole  22  is formed on the N +  type diffusion layer  5  so that the N +  type diffusion layers  5  can be exposed. The through-hole  23  is formed on the P +  type diffusion layer  4  so that the P +  type diffusion layers  4  can be exposed. Referring to  FIG. 5L , after that, through the lithography process, the metal film formation process, and the photoresist removing process, the cathode electrode  9  is provided to fill the opening part  22  and to reach a surface of the N +  type diffusion layer  5 , and the anode electrode  8  is provided to fill the opening part  23  and to reach the surface of the P +  type diffusion layer  4 . 
         [0040]    Through the processes described above, the photodiode region  20  is manufactured. 
         [0041]    An operation method of the optical semiconductor device of the present invention will be described.  FIG. 6  is a flowchart showing an operation of the optical semiconductor device in the embodiment of the present invention. At first, the optical semiconductor device ( FIG. 2 ) is prepared (step S 01 ). Then, by applying the reverse bias E between the anode electrode  8  and the cathode electrode  9 , the optical semiconductor device ( FIG. 4 ) is set to a state ready to measure (step S 02 ). Thus, the very shallow inversion layer (N +  channel)  10  is formed on the surface of the P-type epitaxial layer  3  just under the field film  7  and the reflection preventing film  6 . The very shallow inversion layer  10  operates as a cathode diffusion layer. By radiating light to be measured, the optical semiconductor device ( FIG. 4 ) receives the light (step S 03 ). When the light reaches the surface of the P-type epitaxial layer  3 , photo carriers are generated inside the inversion layer  10  because of receiving of the light, and photoelectric current flows through the p-n junction between the P +  type embedded layer  2  and P +  type diffusion layer  4  and the inversion layer (N +  channel)  10  since the photocarriers are moved into the depletion layer  11  by an internal electric field generated on the basis of concentration gradient. The measurement is performed in a manner that the photoelectric current is taken from the anode electrode  8  via the P +  type embedded layer  2  and P +  type diffusion layer  4  and is measured. 
         [0042]    In the present invention, the very shallow inversion layer (N +  channel)  10  is formed under the field film and the reflection preventing film by using silicon with a high resistance for the P-type epitaxial layer  3  and by applying the reverse bias E between the anode electrode  8  and the cathode electrode  9 . The formed N +  channel (the inversion layer  10 ) operates as a light receiving region through the p-n junction with P-type diffusion layers (P +  type embedded layer  2  and P +  type diffusion layer  4 ). Thus, even when an invasive length of incidence light of short wavelength in the semiconductor is short, photo carriers are converted into a photo electric current with high efficiency since the formed very shallow N +  channel operates as a cathode diffusion layer to prevent lowering of quantum efficiency through recombination of the photocarriers. 
         [0043]    According to the present invention, light of short wavelength such as blue laser can be received with a high sensitivity and high speed response. In addition, a light receiving element that is able to receive light of short wavelength such as a blue laser with the high sensitivity and high-speed response can be manufactured in a simple way and low cost process. 
         [0044]    Although the present invention has been described above in connection with several embodiments thereof, it will be apparent to those skilled in the art that those embodiments are provided solely for illustrating the invention, and should not be relied upon to construe the appended claims in a limiting sense.