. Namely, the end surface 12c and the side surface 11b are located on the same plane. The side surface 11b is a surface extending to intersect the Y direction, and is a flat surface perpendicular to the Y direction in this example. In this example, the side surface 11b and the end surface 12c are cleavage surfaces formed by cutting the wafer.

The first electrode 15 is formed in a planar shape on the major surface 11a of the semiconductor substrate 11. The first electrode 15 is made of, for example, gold and is formed in a square shape by patterning. The first electrode 15 includes a connection portion 15a located on one side in the X direction with respect to the mesa portion 12. In this example, the connection portion 15a is the entirety of the first electrode 15. The first electrode 15 is electrically connected to the first contact layer 13 through the air bridge wiring 16. The first electrode 15 is provided to extract a current output from the active layer 21, to the outside. An area of the first electrode 15 is, for example, 10000 μm²or more. In this case, the first electrode 15 and the first wiring 52 to be described later can be suitably brought into surface contact with each other. In this example, dimensions of the first electrode 15 are 400 μm×400 μm.

The air bridge wiring 16 is a connection wiring that is electrically connected to the first contact layer 13 and the first electrode 15. The air bridge wiring 16 is led out from the first contact layer 13 to the one side in the X direction, and is bridged between the first contact layer 13 and the first electrode 15. In this example, the air bridge wiring 16 is led out from the first contact layer 13 in a direction parallel to the X direction. In a plan view, the air bridge wiring 16 is led out from an end portion of the first contact layer 13 (mesa portion 12) in the X direction. The air bridge wiring 16 is an aerial wiring (three-dimensional wiring) including a bridge portion 16a extending in air. The bridge portion 16a is electrically connected to the first electrode 15 (connection portion 15a) through a plating layer 151 to be described later. The air bridge wiring 16 further includes a portion 16b having a planar shape that is formed on the first contact layer 13. The bridge portion 16a is integrally formed with the portion 16b.

The air bridge wiring 16 is formed, for example, by the following step. First, a resist is formed in a region between the mesa portion 12 and the first electrode 15 on the major surface 11a of the semiconductor substrate 11 by patterning. Subsequently, a gold thin film having a thickness of approximately 5 μm is formed on the resist by plating, and thereafter, the resist is removed. Accordingly, the air bridge wiring 16 having a sheet shape is formed.

The bridge portion 16a is formed in a wide sheet shape (layer shape). A length (width) L3 of the bridge portion 16a in the Y direction (optical waveguide direction A) is longer than a length L4 of the bridge portion 16a in the X direction. In this example, the bridge portion 16a extends curvedly when viewed in the Y direction. In this case, as shown in FIG. 4, the length L4 of the bridge portion 16a in the X direction is a length (actual length) of the bridge portion 16a along an extending direction. The length L3 is, for example, 50 μm or more. The length L3 is, for example, approximately 50 μm to 3000 μm, and is 80 μm in this example. The length L4 is, for example, approximately 5 μm to 200 μm, and is 30 μm in this example. An aspect ratio (ratio of the length L3 to the length L4) of the bridge portion 16a may be 0.25 to 100, preferably 1 to 50. The aspect ratio of the bridge portion 16a may be more preferably 1 to 20, further preferably 2 to 10. A thickness of the bridge portion 16a (air bridge wiring 16) in the Z direction is equal to or more than 1 μm and equal to or less than 10 μn.

The second electrode 17 is formed in a planar shape on the second portion 14b of the second contact layer 14. In other words, the second electrode 17 is formed in a planar shape on the major surface 11a of the semiconductor substrate 11 with the second contact layer 14 interposed therebetween. The second electrode 17 is made of, for example, gold and is formed in a square shape by patterning. The second electrode 17 is disposed on the other side in the X direction (side opposite the first electrode 15) with respect to the mesa portion 12. The second electrode 17 is electrically connected to the second contact layer 14. The second electrode 17 is provided to extract a current output from the active layer 21, to the outside. The second electrode 17 is electrically isolated from the first electrode 15. An area of the second electrode 17 is, for example, 10000 μm²or more. In this case, the second electrode 17 and the second wiring 53 to be described later can be suitably brought into surface contact with each other. In this example, dimensions of the second electrode 17 are 400 μm×400 μn, and are the same as those of the first electrode 15.

When plating is performed in the step of forming the air bridge wiring 16 described above, plating layers may be formed on the first electrode 15 and on the second electrode 17. In FIGS. 2 to 4, these plating layers are indicated by reference signs 151 and 171. The plating layers 151 and 171 can be regarded as forming the first electrode 15 and the second electrode 17, respectively. In this example, the plating layer 151 is formed with a size smaller than the first electrode 15 (formed on only a part of the first electrode 15), but may be formed on the entire surface of the first electrode 15. Similarly, the plating layer 171 is formed with a size smaller than the second electrode 17, but may be formed on the entire surface of the second electrode 17. The plating layer 151 can be regarded as forming the air bridge wiring 16. The plating layers 151 and 171 may not be formed.

[Fixing Member]

As shown in FIGS. 1 and 6, the fixing member 50 includes an insulating substrate 51, the first wiring 52, and the second wiring 53. In FIG. 6, the first wiring 52 and the second wiring 53 are hatched for ease of understanding. The insulating substrate 51 is formed in, for example, a rectangular flat plate shape, and has a major surface 51a and a 51b opposite the major surface 51a. In this example, the major surfaces 51a and 51b are flat surfaces parallel to each other. A base material of the insulating substrate 51 is, for example, alumina (aluminum oxide). In this case, dielectric loss in a high frequency band can be reduced. The base material of the insulating substrate 51 may be SiC or ceramic that is a highly insulating material.

A recessed portion 54 is formed in the major surface 51a of the insulating substrate 51. In this example, the recessed portion 54 is a groove extending along the Y direction (optical waveguide direction A), and is open on a side surface 51c of the insulating substrate 51. Namely, the recessed portion 54 extends to reach the side surface 51c. The side surface 51c is a surface extending to intersect the Y direction, and is a flat surface perpendicular to the Y direction in this example. As one example, the recessed portion 54 has a rectangular shape in a plan view, and has a substantially semicircular shape in a cross section perpendicular to the Y direction.

The first wiring 52 is formed in a planar shape on the major surface 51a of the insulating substrate 51. The first wiring 52 is made of, for example, gold and is formed in a predetermined shape by patterning. The first wiring 52 is disposed on the one side in the X direction with respect to the recessed portion 54. The first wiring 52 is formed in a rectangular shape in which long sides are parallel to the Y direction, and extends across the side surfaces 51c and 51d of the insulating substrate 51. The side surface 51d is a surface of the insulating substrate 51 on a side opposite to the side surface 51c.

The second wiring 53 is formed in a planar shape on the major surface 51a of the insulating substrate 51. The second wiring 53 is electrically isolated from the first wiring 52. The second wiring 53 is made of, for example, gold and is formed in a predetermined shape by patterning. The second wiring 53 is formed to reach (wrap around) from the major surface 51a of the insulating substrate 51 to the major surface 51b through a side surface 51e. Namely, the second wiring 53 includes a first portion 53a disposed on the major surface 51a, and a second portion 53b disposed over the major surface 51b and the side surface 51e. The first portion 53a is disposed on the other side in the X direction (side opposite the first wiring 52) with respect to the recessed portion 54. The first portion 53a includes a portion 53a1 formed in a rectangular shape in which long sides are parallel to the X direction, and a portion 53a2 formed in a rectangular shape in which long sides are parallel to the Y direction. The portion 53a2 extends from the portion 53a1 to reach the side surface 51d of the insulating substrate 51. The second portion 53b is formed on the entireties of the major surface 51b and of the side surface 51e.

A solder layer 55 is formed on the first wiring 52, and a solder layer 56 is formed on the second wiring 53. The solder layer 56 is formed on the portion 53a1 of the second wiring 53. Each of the solder layers 55 and 56 is formed in, for example, a square shape from a metal material, and has a thickness of approximately equal to or more than 1 μm and equal to or less than 10 μm.

As shown in FIG. 1, the photodetector 10 is fixed to the fixing member 50 in a state where the major surface 11a of the semiconductor substrate 11 faces the major surface 51a of the insulating substrate 51. In a fixed state where the photodetector 10 is fixed to the fixing member 50, at least a part of the mesa portion 12 is disposed inside the recessed portion 54. In this example, a tip portion (end portion opposite the semiconductor substrate 11 in the Y direction) of the mesa portion 12 is disposed inside the recessed portion 54. The mesa portion 12 is separated from an inner surface 54a of the recessed portion 54. A longitudinal direction of the mesa portion 12 is parallel to a longitudinal direction of the recessed portion 54. The end surface 12c of the mesa portion 12 is exposed from the recessed portion 54 when viewed in the Y direction. In the fixed state, a part of the air bridge wiring 16 (bridge portion 16a) is also disposed inside the recessed portion 54 to be separated from the inner surface 54a. Accordingly, a connection portion between the air bridge wiring 16 and the first contact layer 13 is disposed inside the recessed portion 54.

The photodetector 10 is fixed to the fixing member 50 by connecting the first electrode 15 to the first wiring 52 and by connecting the second electrode 17 to the second wiring 53. The first electrode 15 is fusion-bonded to the first wiring 52 using the solder layer 55, and is electrically connected to the first wiring 52 in a state where the first electrode 15 is in surface contact with the first wiring 52 with the solder layer 55 interposed therebetween. The second electrode 17 is fusion-bonded to the second wiring 53 using the solder layer 56, and is electrically connected to the second wiring 53 in a state where the second electrode 17 is in surface contact with the second wiring 53 with the solder layer 56 interposed therebetween. Each of a contact area between the first electrode 15 and the first wiring 52 and a contact area between the second electrode 17 and the second wiring 53 is 10000 μm²or more. In the fixed state, a part of the first wiring 52 is exposed such that a terminal 81 of a connector 80 to be described later can come into contact therewith (FIGS. 7 and 8).

[Mounting State of Photodetection Module]

As shown in FIGS. 7, 8, and 9, for example, the photodetection module 1 is used by being connected to the connector 80. The connector 80 is, for example, a SMA connector and includes the terminal 81 that is electrically connected to a signal wire of a SMA cable. The terminal 81 is mechanically connected to the first wiring 52 of the fixing member 50, and is electrically connected to the first wiring 52. Alternatively, the terminal 81 may be connected to the first wiring 52 by soldering. In such a manner, in this example, the first wiring 52 is a signal side wiring that is electrically connected to the signal wire.

The connector 80 further includes a body member 82 and a support member 83 that is integrally formed with the body member 82. The body member 82 and the support member 83 are electrically connected to a ground wire of the SMA cable. The photodetection module 1 is fixed to the connector 80 by connecting the fixing member 50 to the support member 83. For example, the second portion 53b of the second wiring 53 of the fixing member 50 is connected to a gold layer formed on a surface of the support member 83 by soldering. For example, the gold layer is formed to cover the entire surface of the support member 83. Alternatively, the body member 82 may be configured separately from the support member 83. In this case, the support member 83 to which the fixing member 50 is connected may be fixed to the body member 82 by screwing or by soldering. The second wiring 53 is electrically connected to the ground wire by the above-described connection. Namely, in this example, the second wiring 53 is a ground side wiring that is electrically connected to the ground wire. In this example, electrical connection between the photodetector 10 and the connector 80 is realized without using bonding wires (wireless connection). Accordingly, the generation of inductance caused by the bonding wires can be avoided. In addition, when bonding wires are used, the wires may be damaged by impact, being caught, or the like, and thus care must be taken in handling the device. In contrast, in this example, since bonding wires are not used, the handling of the device can be facilitated.

A shape (wiring pattern) of the first wiring 52 is set in consideration of impedance matching. In this example, the first wiring 52 is configured as a microstrip line. As one example, when a thickness of the insulating substrate 51 is 0.5 mm, a dielectric constant of the insulating substrate 51 is 9.8, a thickness of the first wiring 52 is 0.8 μm, and a matching resistance value at 20 GHz is 50Ω, a width W of the first wiring 52 is set to 0.5 mm or less, and a length L5 of the first wiring 52 is set to 1.45 mm or less (FIG. 6). Accordingly, the length L5 of the first wiring 52 is ¼ or less of a wavelength (electrical length) of an electrical signal propagating through the first wiring 52. In such a manner, impedance matching in a desired frequency band can be realized by setting the length L5 of the first wiring 52 to ¼ or less of an electrical length calculated based on design parameters of the first wiring 52. Incidentally, the first wiring 52 may be configured as a coplanar line instead of a microstrip line.

As shown in FIG. 8, the photodetection module 1 (photodetector 10) further includes a lens 40. The lens 40 is disposed to face the end surface 12c of the mesa portion 12 that is a light incident surface, and converges the detection light DL toward the end surface 12c to condense the detection light DL on the end surface 12c. A numerical aperture (NA) of the lens 40 is, for example, 0.4 or more, and a condensed light diameter is several μn to several tens of μm. A low reflection layer including a dielectric multilayer film may be formed on both surfaces of the lens 40 and the end surface 12c. In this case, a transmittance of these surfaces for light in a sensitivity wavelength range of the photodetector 10 can be 95% or more.

Functions and Effects

In the photodetector 10, the first contact layer 13 and the first electrode 15 are connected to each other by the air bridge wiring 16. Accordingly, for example, compared to when the first contact layer 13 and the first electrode 15 are connected to each other by wire bonding, a wiring length can be shortened, and an area of the mesa portion 12 in a plan view can be reduced. In the case of wire bonding, an area required for a wiring is determined from an area required for crimping a tip of a wire, and for example, an area of approximately 100 μm×100 μm is required. Inductance can be reduced by shortening the wiring length, and response speed can be increased. In addition, parasitic capacitance can be reduced by reducing the area of the mesa portion 12, and thus the response speed can also be increased. On the other hand, when the area of the mesa portion 12 is simply reduced, it is concerned that output signal intensity decreases. In this respect, in the photodetector 10, when viewed in the Z direction (direction perpendicular to the major surface 11a of the semiconductor substrate 11), the length L1 of the mesa portion 12 in the optical waveguide direction A (Y direction) is longer than the length L2 of the mesa portion 12 in the X direction (direction perpendicular to the optical waveguide direction A). Accordingly, a path along which the detection light DL travels through the mesa portion 12 can be lengthened, and light can be effectively absorbed in the mesa portion 12. As a result, even when the area of the mesa portion 12 is reduced, output signal intensity can be ensured. Further, in the photodetector 10, the air bridge wiring 16 is led out from the first contact layer 13 to the one side in the X direction, and is bridged between the first contact layer 13 and the first electrode 15. Accordingly, a width (length in the optical waveguide direction A) of the air bridge wiring 16 can be ensured. When the wiring width is wide, a cross-sectional area increases, so that inductance can be further reduced and strength of the air bridge wiring 16 can be ensured to ensure reliability. Therefore, according to the photodetector 10, increasing response speed, ensuring output signal intensity, and ensuring reliability can be realized.

FIG. 10 is a graph showing a response characteristic of the photodetector 10, and FIG. 11 is a graph showing an example of an output signal in a high frequency region. As shown in FIG. 10, in the photodetector 10, a cutoff frequency that is a frequency when signal intensity decreases to −3 dB is 20 GHz or higher. For example, in an MCT (HgCdTe) sensor used as a mid-infrared photodetector, only a cutoff frequency of approximately several hundreds of MHz (approximately 1 GHz at its maximum) can be realized. In contrast, in the photodetector 10, the cutoff frequency can be increased, and the response speed can be increased. In addition, as shown in FIG. 11, a high signal intensity higher than 30 dB is obtained in a frequency band higher than 26 GHz.

The first electrode 15 includes the connection portion 15a located on the one side in the X direction with respect to the mesa portion 12, and the air bridge wiring 16 is led out from the first contact layer 13 to the one side in the X direction and is connected to the connection portion 15a of the first electrode 15. Accordingly, the air bridge wiring 16 can be led out from the first contact layer 13 to the one side in the X direction, and the width of the air bridge wiring 16 can be certainly ensured.

The air bridge wiring 16 includes the bridge portion 16a extending in air, and the length L3 of the bridge portion 16a in the optical waveguide direction A is longer than the length L4 of the bridge portion 16a in the X direction. Accordingly, the width of the bridge portion 16a (air bridge wiring 16) can be certainly ensured.

Each of the length L1 of the mesa portion 12 in the optical waveguide direction A and the length L3 of the bridge portion 16a in the optical waveguide direction A is 50 μm or more. Accordingly, the width of the bridge portion 16a can be certainly ensured. In addition, the length of the mesa portion 12 in the optical waveguide direction A can be ensured, and light can be more effectively absorbed in the mesa portion 12.

The ratio of the length L1 to the length L2 of the mesa portion 12 is greater than 1 and less than 100. Accordingly, the length L1 of the mesa portion 12 in the optical waveguide direction A can be ensured, and light can be more effectively absorbed in the mesa portion 12.

The ratio of the length L3 to the length L4 of the bridge portion 16a is greater than 1 and less than 50. Accordingly, the width of the bridge portion 16a (air bridge wiring 16) can be certainly ensured.

The thickness of the bridge portion 16a is equal to or more than 1 μm and equal to or less than 10 μm. Accordingly, reliability can be certainly ensured. Namely, a shape of the bridge portion 16a and mechanical strength for maintaining the shape can be ensured by setting the thickness of the bridge portion 16a to 1 μm or more. In addition, the shape of the bridge portion 16a can be maintained against its own weight by setting the thickness of the bridge portion 16a to 10 μm or less.

The one end surface 12c of the mesa portion 12 in the optical waveguide direction A is an incident surface of the detection light DL. Accordingly, light can be more effectively absorbed in the mesa portion 12, and output signal intensity can be certainly ensured.

The one end surface 12c of the mesa portion 12 in the optical waveguide direction A is flush with the side surface 11b of the semiconductor substrate 11. Accordingly, the end surface 12c of the mesa portion 12 can be easily used as an incident surface of the detection light DL.

The photodetector 10 includes the lens 40 that is disposed to face the end surface 12c of the mesa portion 12, and that converges the detection light DL toward the end surface 12c of the mesa portion 12. Accordingly, a width (length in the X direction) of the end surface 12c of the mesa portion 12 can be narrowed, and the area of the mesa portion 12 in a plan view can be reduced. As a result, parasitic capacitance can be reduced, and the response speed can be further increased. In addition, when the area of the mesa portion 12 is assumed to be constant, the length of the mesa portion 12 in the optical waveguide direction A can be lengthened by narrowing the width of the mesa portion 12. As a result, light can be more effectively absorbed in the mesa portion 12, and output signal intensity can be certainly ensured. Even when the active layer 21 is thickened, absorption efficiency can be increased by increasing the number of cycles of the unit stacked bodies 24 in the active layer 21. An increase in the thickness of the active layer 21 contributes to reducing parasitic capacitance and also to reducing noise through an increase in element resistance. On the other hand, when the number of cycles is too large, quantum efficiency decreases, so that it is preferable that the number of cycles is approximately 10 to 150.

The side surfaces 12a of the mesa portion 12 in the X direction are exposed. Accordingly, the generation of parasitic capacitance can be suppressed, and the response speed can be further increased. Hereinafter, this point will be described with reference to FIGS. 12A and 12B.

FIG. 12A is a front view of the photodetector 10, and FIG. 12B is a front view of a photodetector 10A of a first modification example. As shown in FIG. 12A, in the photodetector 10, both the side surfaces 12a of the mesa portion 12 in the X direction are exposed. In contrast, as shown in FIG. 12B, in the photodetector 10A, both the side surfaces 12a of the mesa portion 12 are covered with an insulating layer 90. The insulating layer 90 is, for example, a silicon nitride (SiN) film. The insulating layer 90 enters between the first contact layer 13 and a metal layer (the portion 16b of the air bridge wiring 16) on the first contact layer 13. Since the portion has a structure where an insulator is sandwiched between metals, for example, when misalignment occurs during formation of the insulating layer 90, there is a possibility that a small parasitic capacitance is generated in the portion. In this respect, in the photodetector 10, since both the side surfaces 12a of the mesa portion 12 are exposed, the generation of such parasitic capacitance can be suppressed.

In a plan view, the second contact layer 14 includes the first portion 14a located between the major surface 11a of the semiconductor substrate 11 and the mesa portion 12, and the second portion 14b located outside the mesa portion 12, and the second electrode 17 is formed on the second portion 14b. Accordingly, a large area of the second electrode 17 can be ensured, and a connection member can be well connected to the second electrode 17. For example, in the above-described example, the second wiring 53 is electrically connected to the second electrode 17 in a surface contact state. Accordingly, for example, compared to when the second electrode 17 and the second wiring 53 are connected to each other by wire bonding, inductance can be reduced, and the response speed can be further increased. In addition, since the second electrode 17 and the second wiring 53 are connected to each other in a surface contact state, the photodetector 10 and the fixing member 50 can be firmly fixed, and reliability can be certainly ensured. In addition, since the area of the second electrode 17 is large, a large contact area between the second electrode 17 and the second wiring 53 can be ensured. Incidentally, the connection member connected to the second electrode 17 may be a ribbon-shaped electrode or a bonding wire. Even in such a case, since the area of the second electrode 17 is large, the connection member can be well connected to the second electrode 17.

In the photodetection module 1, the first electrode 15 on the semiconductor substrate 11 in the photodetector 10 is electrically connected to the first wiring 52 in a state where the first electrode 15 is in surface contact with the first wiring 52 on the insulating substrate 51 in the fixing member 50 (surface-to-surface connected). Accordingly, for example, compared to when the first electrode 15 and the first wiring 52 are connected to each other by wire bonding, the generation of inductance caused by wires can be avoided, and inductance can be reduced. As a result, the response speed can be increased. In addition, since the first electrode 15 and the first wiring 52 are connected to each other in a surface contact state, the photodetector 10 and the fixing member 50 can be firmly fixed, and reliability can be ensured. Further, in the photodetection module 1, at least a part of the mesa portion 12 of the photodetector 10 is disposed inside the recessed portion 54 formed in the insulating substrate 51. Accordingly, the mesa portion 12 that is delicate can be protected, and reliability can be ensured. For example, the mesa portion 12 can be protected from damage caused by external contact, the attachment of floating matter, air pressure, or the like (mechanical damage caused by external factors).

The recessed portion 54 is open on the side surface 51c of the insulating substrate 51. Accordingly, the detection light can be easily incident on the end surface 12c of the mesa portion 12 in the Y direction. At least a part of the air bridge wiring 16 is disposed inside the recessed portion 54. Accordingly, the air bridge wiring 16 that is delicate can be protected, and reliability can be certainly ensured. The first wiring 52 has a length of ¼ or less of a wavelength of an electrical signal propagating through the first wiring 52. Accordingly, impedance matching can be realized. The mesa portion 12 is separated from the inner surface of the recessed portion 54. Accordingly, the mesa portion 12 can be certainly protected. When the photodetector 10 is a detector that detects the detection light DL using intersubband absorption in the quantum well structure, detection can be performed without applying an external voltage, so that heat is unlikely to be generated in the mesa portion 12. For this reason, the mesa portion 12 can be separated from the insulating substrate 51.

The first electrode 15 is in surface contact with the first wiring 52 with the solder layer 55 sandwiched therebetween, and the second electrode 17 is in surface contact with the second wiring 53 with the solder layer 56 sandwiched therebetween. Accordingly, the first electrode 15 and the first wiring 52 can be electrically and mechanically firmly connected to each other, and the second electrode 17 and the second wiring 53 can be electrically and mechanically firmly connected to each other. In addition, when the first electrode 15 and the first wiring 52 are connected to each other and the second electrode 17 and the second wiring 53 are connected to each other, even in a case where there is a height difference between the first electrode 15 and the second electrode 17, the height difference can be absorbed by the solder layers 55 and 56, and good surface contact at each connection location can be realized.

Modification Examples

In the photodetection module 1 of a second modification example shown in FIGS. 13 and 14, the detection light DL travels through the semiconductor substrate 11, and then is incident on the mesa portion 12. In the second modification example, the side surface 11b of the semiconductor substrate 11 is an inclined surface inclined with respect to the Z direction. As one example, an inclination angle θ of the side surface 11b with respect to the Z direction when viewed in the X direction is 45 degrees. The side surface 11b is a polished surface formed, for example, by polishing. The detection light DL is incident on the side surface 11b in a direction perpendicular to the side surface 11b, travels through the semiconductor substrate 11, and then is incident on a surface 12d of the mesa portion 12 through the second contact layer 14. The surface 12d is a surface on a semiconductor substrate 11 side of the mesa portion 12. Namely, in this example, the side surface 11b is an incident surface of the detection light DL. The end surface 12c of the mesa portion 12 is not flush with the side surface 11b of the semiconductor substrate 11. The detection light DL incident on the mesa portion 12 is multiply reflected inside the mesa portion 12, and an electric field vibrational component of the detection light DL that is parallel to the Z direction is absorbed in the active layer 21. The detection light DL may be multiply reflected inside the semiconductor substrate 11, and incident on the surface 12d of the mesa portion 12 a plurality of times. Incidentally, in FIG. 13, the first electrode 15 and the like are omitted and hatching indicating a cross section is omitted.

As shown in FIG. 14, in the second modification example, the recessed portion 54 is configured as a hole formed in the major surface 51a of the insulating substrate 51. The recessed portion 54 is not open on the side surface 51c of the insulating substrate 51 and is separated from an outer edge of the major surface 51a. The recessed portion 54 has a substantially semicircular shape in a cross section perpendicular to the X direction. In the second modification example, at least a part of the mesa portion 12 is also disposed inside the recessed portion 54. The mesa portion 12 is formed in a shape that allows the mesa portion 12 to be disposed inside the recessed portion 54. In the second modification example as described above, similarly to the above embodiment, increasing response speed, ensuring output signal intensity, and ensuring reliability can also be realized. In addition, since the recessed portion 54 is configured as a hole that is not open in the side surface 51c of the insulating substrate 51, the mesa portion 12 can be more certainly protected.

In a third modification example shown in FIG. 15, the photodetector 10 includes a height adjustment layer 19. The height adjustment layer 19 is formed in a planar shape on the major surface 11a of the semiconductor substrate 11. The height adjustment layer 19 is formed on the major surface 11a to be separated from the second contact layer 14, and is electrically isolated from the second contact layer 14. The height adjustment layer 19 is formed simultaneously with the formation of the second contact layer 14, and has the same configuration as that of the second contact layer 14. Namely, the height adjustment layer 19 has the same thickness as that of the second contact layer 14. The first electrode 15 is formed on the height adjustment layer 19. In the third modification example as described above, similarly to the above embodiment, increasing response speed, ensuring output signal intensity, and ensuring reliability can also be realized. In addition, when the first electrode 15 and the first wiring 52 are connected to each other and the second electrode 17 and the second wiring 53 are connected to each other, even in a case where there is a height difference between the first electrode 15 and the second electrode 17, the height difference between the first electrode 15 and the second electrode 17 can be reduced by the height adjustment layer 19, and good surface contact at each connection location can be realized.

The present disclosure is not limited to the above embodiment. For example, the material and the shape of each configuration are not limited to the material and the shape described above, and various materials and shapes can be adopted. The length L3 of the bridge portion 16a in the optical waveguide direction A may not necessarily be longer than the length L