JP6702283B2 - Light receiving element - Google Patents

Description

The technique disclosed in the present specification relates to a light receiving element made of a semiconductor substrate.

A light receiving element made of a semiconductor substrate is known. Patent Documents 1 and 2 exemplify such a light receiving element. The light receiving elements exemplified in Patent Documents 1 and 2 utilize the photovoltaic effect of the Schottky junction between the semiconductor and the metal.

The sensitivity of the light receiving element using the Schottky junction is better as the amount of light received by the Schottky junction is larger. In the light receiving element of Patent Document 1, a light reflection film is provided on the back side of the Schottky junction (the side opposite to the side on which light is incident), and the light incident on the Schottky junction is reflected by the light reflection film, and the Schottky again. The joint is irradiated. Further, in the light receiving element of Patent Document 1, the optical path length between the Schottky junction and the light reflecting film is set to 1/4 of the wavelength of the light to be received. Then, the Schottky junction corresponds to the position of the antinode of the standing wave formed by overlapping the incident light and the reflected light. The antinode of the standing wave has a high light intensity, and since the photovoltaic effect is obtained at that part, the measurement sensitivity of light is improved.

JP, 09-321333, A JP, 09-092872, A

The present specification provides a light-receiving element having higher sensitivity than the light-receiving element of Patent Document 1.

A light receiving element disclosed in the present specification includes a first conductivity type main waveguide formed on a semiconductor substrate, a sub-waveguide that guides light from the outside to the main waveguide, a first electrode, and a second electrode. There is. The first electrode is made of a metal that reflects light, and is ohmic-bonded to both ends of the optical path of the main waveguide. The second electrode is Schottky joined in the middle of the main waveguide. The main waveguide, the first electrode, and the second electrode form a semiconductor element. The length of the main waveguide is (nL)/2 (where L is the wavelength of the light to be received and n is a natural number). The second electrode is arranged at a position at a distance of {L/4+(mL)/2} (m is an integer of 0≦m<n) from one end of the main waveguide.

According to the light receiving element disclosed in this specification, light is reflected at both ends of the main waveguide to form a standing wave. The second electrode that causes the photovoltaic effect is located at the antinode of the standing wave. Therefore, the light receiving sensitivity is increased. In the light receiving element of Patent Document 1, incident light is reflected once to form a standing wave, whereas in the light receiving element disclosed in this specification, light is reflected at both ends of the main waveguide to form a standing wave. To do. Therefore, in the light receiving element disclosed in this specification, the intensity of light in the standing wave is higher than that in the light receiving element disclosed in Patent Document 1, and the light measurement sensitivity is improved.

Specific examples of the semiconductor element formed by the main waveguide, the first electrode, and the second electrode are a diode and a transistor. In the case of a diode, by forming a high-concentration first conductivity type semiconductor region in which the impurity concentration is higher than that in the central portion of the main waveguide, at the portions joined to the first electrodes at both ends of the main waveguide, The first electrode, the second electrode, and the main waveguide form a diode.

In the case of a transistor, the second conductivity type semiconductor region is formed at a portion joined to each of the first electrodes at both ends of the main waveguide. Then, a low-concentration second conductivity type semiconductor region having an impurity concentration lower than that of the second conductivity type semiconductor region is formed at the boundary between one of the second conductivity type semiconductor regions and the first conductivity type region. By forming some semiconductor regions as described above, the first electrode, the second electrode, and the main waveguide form a transistor.

There are the following two specific examples of the sub-waveguide. One is a type that is close to and separated from the main waveguide and guides light to the main waveguide with evanescent light. The other is a type in which the main waveguide is connected at an acute angle.

Details of the technology disclosed in the present specification and further improvements will be described in “Mode for Carrying Out the Invention” below.

It is a top view of the light receiving element of a 1st Example. FIG. 2 is a sectional view taken along line II-II in FIG. 1. It is sectional drawing of the light receiving element of 2nd Example. It is sectional drawing of the light receiving element of 3rd Example. It is a top view of the photo acceptance unit of the 4th example.

(First Embodiment) The light receiving element 2 of the first embodiment will be described with reference to FIGS. FIG. 1 is a plan view of the light receiving element 2, and FIG. 2 is a sectional view taken along line II-II of FIG. The light receiving element 2 is formed on the silicon substrate 3 (semiconductor substrate). The entire light receiving element is covered with a silicon oxide (SiO ₂ ) layer (upper silicon oxide layer 8 and lower silicon oxide layer 9) which is an insulator. Therefore, in FIG. 1, each part of the light receiving element 2 is drawn by a hidden line (broken line).

The light receiving element 2 includes a main waveguide 4, a sub waveguide 12, a first electrode 6, and a second electrode 7. The main waveguide 4 is a low concentration n-type semiconductor containing impurities such as phosphorus. The impurity concentration is, for example, 1×10 ¹⁴ to 1×10 ¹⁷ [cm ⁻³ ]. Since silicon oxide and silicon have a large difference in refractive index, light is confined in the main waveguide 4, which is a low-concentration n-type semiconductor. The width (length in the Y-axis direction in the coordinate system in the figure) and height (length in the Z-axis direction) of the main waveguide are approximately 0.05 [μm]. A waveguide having such a width and height is called a thin line waveguide and is well known, so a detailed description thereof will be omitted.

The length L1 of the main waveguide 4 is set to (nL/2) with respect to the wavelength L of the light to be received. n is a natural number, and in the case of the main waveguide 4 of the embodiment, n=1. That is, the length L1 of the main waveguide 4 is L1=L/2. The reason for determining the length L1 of the main waveguide 4 as described above will be described later. Note that the “light receiving target light” may be referred to as “detection target light” or “measurement target light”.

The sub-waveguide 12 is composed of a sub-waveguide main body 12a made of a low-concentration n-type semiconductor like the main waveguide 4 and a sub-waveguide connecting portion 12b made of undoped high-resistance silicon. The width and height of the sub-waveguide 12 are the same as the width and height of the main waveguide 4. The sub-waveguide 12 is also a thin line waveguide. The sub-waveguide connection portion 12b may be pn-separated.

The sub-waveguide connection portion 12b is arranged at the connection portion of the sub-waveguide 12 with the main waveguide 4. The sub-waveguide connection portion 12b has an insulating property, and the main waveguide 4 and the sub-waveguide main body 12a of the sub-waveguide 12 are insulated. One end of the sub-waveguide 12 (the end opposite to the connection end with the main waveguide 4) reaches the edge of the silicon substrate 3, and the edge corresponds to the light receiving port 12c. The sub-waveguide 12 extends in the direction orthogonal to the extending direction of the main waveguide 4 near the light receiving port 12c, but is curved at a right angle at the connecting portion with the main waveguide 4, It is connected to a sharp angle. Therefore, the light entering from the light receiving port 12c is guided to the sub-waveguide 12 and enters the side surface of the main waveguide 4 along the longitudinal direction of the main waveguide 4.

First electrodes 6 made of aluminum (Al) or silver (Ag) are arranged at both ends of the main waveguide 4 in the longitudinal direction (X-axis direction of the coordinate system in the drawing). The first electrode 6 is in ohmic contact with the end of the main waveguide 4. Aluminum (Al) or silver (Ag) reflects light. The first electrode 6 also functions as a reflecting mirror that reflects the light traveling in the main waveguide 4. The joint surface of the first electrode 6 with the main waveguide 4 is orthogonal to the longitudinal direction (Y-axis direction) of the main waveguide 4.

First electrodes 6 that reflect light are arranged at both ends of the main waveguide 4. The light guided to the sub waveguide 12 and incident on the main waveguide 4 is reflected by the first electrodes 6 at both ends. As described above, the length L1 of the main waveguide 4 is L/2 (L is the wavelength of the light to be received), so that the light incident on the main waveguide 4 is It becomes a standing wave with a belly. The antinode of the standing wave is located at a distance L/4 from the end of the main waveguide 4. In addition, in FIG. 2, the curve of the symbol SW schematically represents the standing wave generated in the main waveguide 4.

The second electrode 7 is arranged on the side surface of the main waveguide 4. The second electrode 7 is made of gold (Au), for example, and is Schottky-bonded to the main waveguide 4. Therefore, in the second electrode 7, the photovoltaic effect is generated by the standing wave of light inside the main waveguide 4. The second electrode 7 is arranged at a position having a length L2 from one end of the main waveguide 4. Here, the length L2 is set to {L/4+(mL)/2}. Here, L is the wavelength of the light to be received (center wavelength of the wavelength range of the light to be received), and m is an integer 0≦m<n. Since n=1 in the embodiment, m=0 and the second electrode 7 is arranged at a position of length L/4 from one end of the main waveguide 4. This length L2=L/4 corresponds to the position of the antinode of the standing wave. The photovoltaic effect is generated in the vicinity of the second electrode 7 of the main waveguide 4, but since the position of the second electrode 7 corresponds to the antinode of the standing wave of light, the light intensity is highest inside the main waveguide 4. Therefore, the greatest photovoltaic effect can be expected.

On the other hand, the high-concentration n-type region 5 having the impurity concentration higher than that of the central portion of the main waveguide 4 is formed in the portions in contact with the first electrode 6 at both ends of the main waveguide 4. The high concentration n-type region 5 is in ohmic contact with the first electrode 6. The impurities injected into the high concentration n-type region 5 may be the same as the impurities injected into the central portion of the main waveguide 4 or may be different impurities. One example of different impurities is arsenic (As). The concentration of the high concentration n-type region 5 is, for example, 1×10 ¹⁸ to 1×10 ²² [cm ⁻³ ].

A low-concentration n-type semiconductor main waveguide 4, a high-concentration n-type region 5 provided at an end of the main waveguide 4, a first electrode 6 ohmic-bonded to the high-concentration n-type region 5, and a shot in the middle of the main waveguide 4. The second electrode 7 that is key-joined constitutes a diode 10. In the diode 10, the first electrode 6 corresponds to the cathode electrode and the second electrode 7 corresponds to the anode electrode.

The light measurement principle of the light receiving element 2 will be briefly described. In the light receiving element 2, a voltage is applied with the first electrode 6 as a positive electrode and the second electrode 7 as a negative electrode. When no light is incident on the main waveguide 4, the first electrode 6 corresponds to the cathode electrode and the second electrode 7 corresponds to the anode electrode, so that no current flows. When light is incident on the main waveguide 4, a photovoltaic effect is generated in the second electrode 7 (anode electrode), so that a current can flow from the main waveguide 4 to the second electrode 7. Since a positive potential voltage is applied to the first electrode 6, when the photovoltaic effect occurs, a current flows from the first electrode 6 (cathode electrode) toward the second electrode 7 (anode electrode). .. The magnitude of the current has a positive correlation with the magnitude of the photovoltaic effect. Therefore, light is received by measuring the magnitude of the current flowing from the first electrode 6 to the second electrode 7 (or by measuring the voltage between the first electrode 6 and the second electrode 7). The intensity of light can be specified.

As described above, since the position of the second electrode 7 that produces the photovoltaic effect corresponds to the antinode of the standing wave, the intensity of the received light becomes maximum at the position of the second electrode 7. Therefore, the light receiving element 2 has high light measurement sensitivity. In particular, the light incident on the main waveguide 4 is reflected at both ends of the main waveguide 4 to form a standing wave. The intensity of the standing wave light due to the light reflected at both ends of the optical path is higher than the intensity of the standing wave light due to the light reflected at only one end of the optical path. Therefore, the light receiving element 2 has high light measurement sensitivity.

When gold (Au) is used as the second electrode 7, since the barrier height of silicon (a constituent material of the main waveguide 4) viewed from gold (Au) is 0.7 [eV], the light receiving element 2 is , Light having a wavelength shorter than the wavelength of 1770 [nm] can be detected. That is, since the infrared wavelength range is 700 to 1000 [nm], the light receiving element 2 can detect infrared light and light having a shorter wavelength than infrared light. When the wavelength range of the light to be measured is determined, the length L1 of the main waveguide 4 and the distance L2 from the end of the main waveguide 4 to the second electrode 7 are determined based on the center wavelength L of the wavelength range. ..

(Second Embodiment) FIG. 3 shows a sectional view of a light receiving element 2a of the second embodiment. The light receiving element 2a is different from the light receiving element 2 of the first embodiment in the length L1 of the main waveguide 4a and the number of the second electrodes 7a-7c. The rest of the configuration of the light receiving element 2a is the same as the configuration of the light receiving element 2 of the first embodiment, so description will be omitted.

As described above, the length of the main waveguide 4a may be (nL/2) with respect to the wavelength L of the light to be received (n is a natural number). In the case of the light receiving element 2a of the second embodiment, n=3, and the length L1 of the main waveguide 4a is L1=(3L)/2. At this time, the light guided to the main waveguide 4a becomes a standing wave having three antinodes of vibration. The symbol SW in FIG. 3 schematically represents the standing wave generated in the main waveguide 4a.

The light receiving element 2a includes three second electrodes 7a-7c. The second electrode 7a is arranged at a distance L2=L/4 from the left end of the main waveguide 4a in the figure. The second electrode 7b is arranged at a position of the distance L2=(3L)/4. The second electrode 7c is arranged at a position of distance L2=(5L)/4. All of the three second electrodes 7a-7c are arranged so as to be located at the antinode of the standing wave generated in the main waveguide 4a. Although not shown, the three second electrodes 7a-7c are connected by conductor wires, and electrically form one second electrode.

A main waveguide 4a made of an n-type semiconductor, a high-concentration n-type region 5 provided at both ends of the main waveguide 4a, a first electrode 6 ohmic-bonded to the high-concentration n-type region 5, and a side surface of the main waveguide 4a. The second electrodes 7a-7c that are in Schottky junction form a diode 10a. Although not shown, a voltage is applied with the first electrode 6 as a positive electrode and the second electrodes 7a-7c as a negative electrode.

No current flows from the first electrode 6 to the second electrodes 7a-7c in the state where light is not incident on the main waveguide 4a. When light enters the main waveguide 4a through the sub-waveguide 12 (see FIG. 1), a photovoltaic effect occurs in the second electrodes 7a-7c, and a current flows from the first electrode 6 toward the second electrodes 7a-7c. .. The amount of current depends on the amount of light incident on the main waveguide 4a. The light receiving element 2a identifies the amount of incident light from the value of the current flowing from the first electrode 6 toward the second electrodes 7a-7c or the voltage between the first electrode 6 and the second electrodes 7a-7c. can do.

In the light receiving element 2a of the second embodiment, the length of the main waveguide 4a is determined so that a standing wave having three antinodes is generated, and three second electrodes 7a are provided at the respective antinode positions. -7c is arranged. The position of the antinode of the standing wave is the place where the light of the standing wave becomes the strongest. Since the second electrodes 7a-7c are arranged for each of the three antinodes of the standing wave, the light receiving element 2a has a higher light measurement sensitivity.

In the light receiving element 2a of the second embodiment, the number of second electrodes is preferably three, which is the same as the number of antinodes, but the number of second electrodes may be two. There may be only one.

The length L1 of the main waveguide may be a length that satisfies (nL/2). Here, L is the wavelength of the light to be received, and n is a natural number. The position where the second electrode is arranged (the distance L2 from the end of the main waveguide) may be a length that satisfies L2={L/4+(mL)/2}. Here, m is an integer that satisfies 0≦m<n. The second electrodes are preferably provided at all antinode positions of the standing wave generated in the main waveguide, but the second electrodes may be provided at at least one antinode position.

(Third Embodiment) Fourth, a sectional view of a light receiving element 2b of the third embodiment is shown. In the light receiving element 2b of the third embodiment, the main waveguide 14, the first electrodes 16a and 16b, and the second electrode 17 form a bipolar transistor 10b. The light receiving element 2b is also formed on the silicon substrate 3. The main waveguide 14, the first electrodes 16a and 16b, and the second electrode 17 are surrounded by the upper silicon oxide layer 18 and the lower silicon oxide layer 19.

The main waveguide 14 is made of a low concentration n-type semiconductor. However, a part of the main waveguide 14 is composed of different semiconductor materials. The impurity contained in the n-type low concentration region (low concentration region 14a) of the main waveguide 14 is, for example, phosphorus (P), and the concentration thereof is, for example, 1×10 ¹⁴ to 1×10 ¹⁷ [cm ^{− 3} ]. The n-type low-concentration region 14a corresponds to the base of the transistor. Below, the low-concentration region 14a may be referred to as the base region 14a.

The light-receiving element 2b also includes a sub-waveguide, which is the same as the sub-waveguide 12 of the light-receiving element 2 of the first embodiment and is connected to the main waveguide 14 at an acute angle. The description of the sub-waveguide of the light receiving element 2b is omitted.

First electrodes 16 a and 16 b are arranged at both ends of the main waveguide 14. The first electrodes 16 a and 16 b are made of metal such as gold (Au) and are ohmic-bonded to the main waveguide 14. The first electrodes 16a and 16b also function as reflecting mirrors that reflect the light that has entered the main waveguide 14. An emitter region 15a containing a high concentration of p-type impurities is formed at one end of the main waveguide 14, and a collector region 15b containing a high concentration of p-type impurities is formed at the other end. There is. The pair of first electrodes 16a is in ohmic contact with the emitter region 15a, and the other first electrode 16b is in ohmic contact with the collector region 15b. The impurity contained in the emitter region 15a and the collector region 15b is, for example, boron (B), and the concentration thereof is, for example, 1×10 ¹⁸ to 1×10 ²² [cm ⁻³ ].

A drift region 21 containing a p-type impurity in a concentration lower than that of the collector region 15b is provided between the base region 14a and the collector region 15b of the main waveguide 14. An example of the p-type impurity is, for example, boron (B), and the concentration thereof is, for example, 1×10 ¹⁴ to 1×10 ¹⁸ [cm ⁻³ ]. The presence or absence of the drift region 21 is arbitrary, and the drift region 21 may be omitted when the collector voltage is set low.

The second electrode 17 is Schottky-bonded to the base region 14a of the main waveguide 14.

The length L1 of the main waveguide 14 is determined to satisfy L1=(nL)/2. Here, L is the wavelength of the light to be received, and n is a natural number. The distance L2 from the end of the main waveguide 14 of the second electrode 17 is set to a length that satisfies L2={L/4+(mL)/2}. Here, m is an integer that satisfies 0≦m<n. In the case of the light receiving element 2b, n=1 and m=0. That is, the length L1 of the main waveguide 14 is L1=L/2, and the distance L2 from the end of the main waveguide 14 to the second electrode 17 is L2=L/4. The length L1 of the main waveguide 14 and the position (distance L2) of the second electrode 17 are the same as those of the main waveguide 4 of the light receiving element 2 of the first embodiment. Therefore, when light enters the main waveguide 14, a photovoltaic effect is generated in the vicinity of the second electrode 17.

Below, the 1st electrode 16a joined to the emitter region 15a may be called the emitter electrode 16a, and the 1st electrode 16b joined to the collector region 15b may be called the collector electrode 16b. In the light receiving element 2b, a voltage is applied with the collector electrode 16b as a negative electrode and the emitter electrode 16a as a positive electrode. No current flows between the collector electrode 16b and the emitter electrode 16a when no light is incident on the main waveguide 14. When light enters the main waveguide 14, a photovoltaic effect occurs (electrons are excited) in the second electrode 17, and electrons are injected into the n-type base region 14a. As a result, the emitter region 15a and the collector region 15b are brought into conduction, and a current flows from the emitter electrode 16a to the collector electrode 16b. The amount of current depends on the amount of light incident on the main waveguide 14. The light receiving element 2b can specify the amount of incident light from the value of the current flowing from the emitter electrode 16a to the collector electrode 16b or the voltage between the emitter electrode 16a and the collector electrode 16b.

Also in the light receiving element 2b of the third embodiment, the second electrode 17 is arranged at a position corresponding to the antinode of the standing wave generated in the main waveguide 14. Therefore, since the photovoltaic effect is generated in the portion where the intensity of the incident light is high, the light receiving element 2b has a high light measurement sensitivity. In particular, since it is the antinode of the standing wave formed by the light reflected at both ends of the main waveguide 14, its intensity is higher than the strength of the standing wave formed by one reflection.

(Fourth Embodiment) FIG. 5 shows a plan view of a light receiving element 2c of the fourth embodiment. The main waveguide 4, the first electrode 6, the second electrode 7, and the high-concentration n-type region 5 of the light receiving element 2c are the same as those of the light receiving element 2 of the first embodiment. That is, the main waveguide 4, the first electrode 6, the second electrode 7, and the high-concentration n-type region 5 of the light receiving element 2c form the diode 10. In the light receiving element 2c of the fourth embodiment, the sub waveguide 22 is different from the sub waveguide 12 of the light receiving element 2.

The sub-waveguide 22 is composed of a straight path 22a extending parallel to the main waveguide 4 and a circular path 22b in contact with the straight path 22a. One end of the straight path 22a is a light receiving port 22c, and light enters through this light receiving port 22c. The incident light enters the circular path 22b from the straight path 22a and goes around the circular path 22b. The circular path 22b is close to and separated from the main waveguide 4 at the position indicated by the arrow A. An upper silicon oxide layer 8 separates the circular path 22b and the main waveguide 4. The distance between the circular path 22b and the main waveguide 4 is approximately one wavelength of light. The sub-waveguide 22 and the upper silicon oxide layer 8 have greatly different refractive indices, and the main waveguide 4 and the upper silicon oxide layer 8 also have significantly different refractive indices. The circular path 22b (sub-waveguide 22), the main waveguide 4, and the upper silicon oxide layer 8 that separates them from each other are located at a position where the circular path 22b is close to and apart from the main waveguide 4 (a position indicated by an arrow A in the figure). , Meets the conditions that give rise to an evanescent field. Therefore, when light passes through the circular path 22b, evanescent light is generated from the circular path 22b to the main waveguide 4 through the upper silicon oxide layer 8. That is, the light incident on the sub-waveguide 22 is guided to the main waveguide 4.

Points to be noted regarding the technique described in the embodiment will be described. In each of the light receiving elements of the examples, the main waveguide is formed of an n-type semiconductor. In the technique disclosed in this specification, the main waveguide may be formed of a p-type semiconductor. In that case, p-type regions having a higher concentration than the central portion of the main waveguide are formed at both ends of the main waveguide. At this time, the first electrode, the second electrode, and the main waveguide form a diode. Further, an n-type region (collector region and emitter region) is provided at both ends of the p-type main waveguide, and an n-type drift region is provided between the p-type region (base region) and the collector region at the center of the main waveguide. To provide. With this configuration, the first electrode (the emitter electrode and the collector electrode at both ends of the main waveguide), the second electrode, and the main waveguide (including the base region, the collector region, and the emitter region) form a transistor.

Specific examples of the present invention have been described above in detail, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. The technical elements described in the present specification or the drawings exert technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technique illustrated in the present specification or the drawings can achieve a plurality of purposes at the same time, and achieving one of the purposes has technical utility.

2, 2a, 2b, 2c: light receiving element 3: silicon substrate 4: main waveguide 4a: main waveguide 5: high concentration n-type region 6: first electrodes 7, 7a, 7b, 7c: second electrode 8: upper silicon oxide Layer 9: Lower silicon oxide layer 10, 10a: Diode 10b: Bipolar transistor 12: Sub-waveguide 12a: Sub-waveguide body 12b: Sub-waveguide connection 12c: Light receiving port 14: Main waveguide 14a: Base region (low concentration) region)
15a: emitter region 15b: collector region 16a: emitter electrode (first electrode)
16b: collector electrode (first electrode)
17: second electrode 18: upper silicon oxide layer 19: lower silicon oxide layer 21: drift region 22: sub-waveguide 22a: straight path 22b: circular path 22c: light receiving port

Claims (5)

A light receiving element,
A first conductivity type main waveguide formed on the semiconductor substrate;
A sub-waveguide that guides light to the main waveguide from the outside,
A first electrode made of a metal that reflects light, and ohmic-bonded to both ends of the main waveguide in the longitudinal direction;
A second electrode that is a Schottky junction in the middle of the main waveguide,
Is equipped with
The main waveguide, the first electrode and the second electrode constitute a semiconductor element,
The length of the main waveguide has a length of (nL)/2 (where L is the wavelength of the light to be received and n is a natural number),
The said 2nd electrode is a light receiving element arrange|positioned in the position of the distance of {L/4+(mL)/2} (however, m is an integer of 0<=m<n) from one end of the said main waveguide. A high-concentration first conductivity type semiconductor region having an impurity concentration higher than that of a central portion of the main waveguide is formed in a portion of each of the both ends of the main waveguide, which is joined to each of the first electrodes.
The light receiving element according to claim 1, wherein the first electrode, the second electrode, and the main waveguide form a diode. A second conductivity type semiconductor region is formed at a portion joined to each of the first electrodes at both ends of the main waveguide, and one of the second conductivity type semiconductor region and the first conductivity type region is formed. And a low-concentration second-conductivity-type semiconductor region having an impurity concentration lower than that of the second-conductivity-type semiconductor region,
The light receiving element according to claim 1 , wherein the first electrode, the second electrode, and the main waveguide at both ends of the main waveguide form a transistor. The light receiving element according to claim 1, wherein the sub-waveguide is connected to the main waveguide at an acute angle. The light receiving element according to claim 1, wherein the sub-waveguide is close to and apart from the main waveguide, and causes light to be incident on the main waveguide by evanescent light.

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