JP7588028B2 - Semiconductor photodetector - Google Patents

Description

The present invention relates to a semiconductor light receiving element.

As a semiconductor light receiving element applied to high-speed optical communication, a PIN type photodiode is used in which an undoped semiconductor layer that is depleted when a bias voltage is applied in use and acts as an absorption layer for optical signals from an optical fiber is sandwiched between p-type and n-type semiconductor layers. Patent Document 1 reports a configuration in which the absorption layer is made of a p-type doped semiconductor layer and an undoped semiconductor layer in order to achieve both light receiving sensitivity and high-speed response. Patent Document 2 also discloses a semiconductor light receiving element as a structure that enables high-speed response to high optical input even at low voltage, in which the absorption layer between the N layer and the P layer is made of a layer containing n-type impurities whose concentration gradually decreases from the junction surface with the N layer toward the inside, and a layer containing p-type impurities whose concentration gradually decreases from the junction surface with the P layer toward the inside.

Patent Publication 2019-160900 Japanese Patent Application Publication No. 3-38887

Reducing the element capacitance is effective in increasing the speed of semiconductor light receiving elements. The element capacitance is composed of the capacitance of the PN junction and the parasitic capacitance, but the capacitance of the PN junction can be reduced by thickening the depletion layer. For example, in the configuration of Patent Document 1, thickening the undoped absorption layer is effective. However, even if the undoped absorption layer is thickened, if the undoped absorption layer cannot be entirely depleted by the bias voltage applied under the operating conditions, the actual depletion layer will be narrow, and the expected capacitance reduction effect will not be obtained. At a constant bias voltage, the lower the carrier concentration, the wider the depleted region. Therefore, if the undoped absorption layer is an intrinsic semiconductor layer that does not contain any carriers, it is easy to completely deplete it even at a low voltage.

However, the undoped absorption layer actually contains a low concentration of carriers. The undoped absorption layer is formed by crystal growth using, for example, MOCVD or MBE. During this crystal growth, the absorption layer is formed by not intentionally doping p-type or n-type carriers. However, in reality, a finite background level of carriers is contained in the absorption layer, and it does not become an intrinsic semiconductor layer in the complete sense. The concentration of these unintentionally contained carriers (carriers contained at the background level) depends on the growth device and growth conditions, but is, for example, about 2 x 10^15/cm^3 for n-type. Therefore, when an actual undoped absorption layer is used, the region that can be depleted under a certain voltage is limited depending on the carrier concentration contained. In other words, simply thickening the undoped absorption layer may not be enough to ensure the thickness of the depletion layer under actual usage conditions.

As mentioned above, even if an undoped absorption layer is used, the region that can be depleted is limited because it actually contains a low concentration of carriers. If the bias voltage applied here is increased, the region that can be depleted will expand even with the same carrier concentration, making it possible to reduce the capacitance. However, the bias voltage that can be applied to a semiconductor light-receiving element is limited from the perspective of power consumption, and there are cases in which a sufficient bias voltage cannot be obtained. Therefore, in practical use, a semiconductor light-receiving element that has a large region that can be depleted at a lower voltage is desirable.

The objective of the present invention is to provide a semiconductor light-receiving element that has a large area that can be depleted, is low-capacity, and has excellent high-speed response.

The semiconductor light receiving element according to the present invention comprises a semiconductor substrate, a first conductive type high concentration layer formed on the semiconductor substrate, a first conductive type low concentration layer formed on and in contact with the first conductive type high concentration layer, a second conductive type low concentration layer forming a PN junction interface with the first conductive type low concentration layer, and a second conductive type high concentration layer formed on and in contact with the second conductive type low concentration layer, wherein the carrier concentrations of the first conductive type low concentration layer and the second conductive type low concentration layer are less than 1×10^16/cm^3, the first conductive type high concentration layer has a higher carrier concentration than the first conductive type low concentration layer, the second conductive type high concentration layer has a higher carrier concentration than the second conductive type low concentration layer, the carrier concentrations of the first conductive type high concentration layer and the second conductive type high concentration layer are 1×10^17/cm^3 or more, and at least one of the first conductive type low concentration layer or the second conductive type low concentration layer includes an absorption layer having a band gap that absorbs incident light.

This invention provides a semiconductor light-receiving element that is low-capacity and has excellent high-speed response.

1 is a cross-sectional view of a semiconductor light receiving element according to a first embodiment of the present invention. 1 is a band diagram of a semiconductor light receiving element according to a first embodiment of the present invention. 4 is a graph showing the electric field intensity of the semiconductor light receiving element according to the first embodiment of the present invention. FIG. 11 is a cross-sectional view of a semiconductor light receiving element according to a comparative example. 1 is a band diagram of a semiconductor light receiving element according to a comparative example. 11 is a graph showing the electric field intensity of a semiconductor light receiving element according to a comparative example. FIG. 4 is a cross-sectional view of a semiconductor light receiving element according to a second embodiment of the present invention. 4 is a band diagram of a semiconductor light receiving element according to a second embodiment of the present invention. 6 is a graph showing the electric field intensity of the semiconductor light receiving element according to the second embodiment of the present invention. FIG. 11 is a cross-sectional view of a semiconductor light receiving element according to a third embodiment of the present invention. 11 is a band diagram of a semiconductor light receiving element according to a third embodiment of the present invention. 10 is a graph showing the electric field intensity of a semiconductor light receiving element according to a third embodiment of the present invention. FIG. 11 is a cross-sectional view of a semiconductor light receiving element according to a fourth embodiment of the present invention. 11 is a band diagram of a semiconductor light receiving element according to a fourth embodiment of the present invention. 10 is a graph showing the electric field intensity of a semiconductor light receiving element according to a fourth embodiment of the present invention. FIG. 13 is a cross-sectional view of a semiconductor light receiving element according to a modified example of the fourth embodiment of the present invention. 13 is a band diagram of a semiconductor light receiving element according to a modified example of the fourth embodiment of the present invention. 13 is a graph showing an electric field intensity of a semiconductor light receiving element according to a modified example of the fourth embodiment of the present invention.

The following describes an embodiment of the present invention in detail with reference to the drawings. In all drawings used to explain the embodiment, components having the same function are given the same reference numerals, and repeated explanations will be omitted. Note that the drawings shown below are merely for explaining examples of the embodiment, and the size of the drawings does not necessarily match the scale described in this example.

[First embodiment]
1A is a cross-sectional view of a semiconductor light receiving element 10 according to a first embodiment of the present invention. The semiconductor light receiving element 10 is a surface-illuminated type semiconductor light receiving element capable of receiving an optical signal having a wavelength of 840 nm to 950 nm transmitted, for example, through a multimode fiber. The semiconductor light receiving element 10 has a semiconductor multilayer structure in which an n-type InP contact layer 12 having a thickness of 1 μm and doped with Si at a concentration of 5×10^18/cm^3, an n-type low-concentration InGaAs absorption layer 13 having a thickness of Wn, a p-type low-concentration InGaAs absorption layer 14 having a thickness of Wp and doped with Be at a concentration of 2×10^15/cm^3, and a p-type InGaAs contact layer 16 having a thickness of 0.1 μm and doped with Be at a concentration of 5×10^19/cm^3 are sequentially stacked on an Fe-doped semi-insulating InP substrate 11. Here, the n-type low-concentration InGaAs absorption layer 13 is formed as an undoped layer in the multi-layer growth, but as a result, it is an n-type semiconductor layer with a carrier density of 2×10^15/cm^3. Note that it is acceptable to intentionally dope Si or the like in the multi-layer growth to create a desired carrier concentration. The n-type low-concentration InGaAs absorption layer 13, the p-type low-concentration InGaAs absorption layer 14, and the p-type InGaAs contact layer 16 are layers having a band gap capable of absorbing incident light. In this embodiment, Wn=0.9 μm, Wp=0.9 μm. Here, the n-type InP contact layer 12 is not limited to InP. Similarly, the p-type InGaAs contact layer 16 may be a semiconductor layer formed of another material.

The n-type low-concentration InGaAs absorption layer 13, the p-type low-concentration InGaAs absorption layer 14, and the p-type InGaAs contact layer 16 are etched into a cylindrical shape to form a light-receiving mesa. A ring-shaped p-type electrode 17 is electrically connected to the p-type InGaAs contact layer 16 on the top of the light-receiving mesa. Furthermore, an n-type electrode 18 is electrically connected to the n-type InP contact layer 12. The surface except for the p-type electrode 17 and the n-type electrode 18 is coated with a silicon nitride film 19. The n-type InP contact layer 12, the n-type low-concentration InGaAs absorption layer 13, the p-type low-concentration InGaAs absorption layer 14, and the p-type InGaAs contact layer 16 are lattice-matched with the InP substrate 11. The silicon nitride film 19 functions as a reflective film with a reflectance of 1% or less for the wavelength of incident light inside the ring-shaped p-type electrode 17.

When a reverse bias voltage is applied between the p-type electrode 17 and the n-type electrode 18 of the semiconductor light receiving element 10, the n-type low-concentration InGaAs absorption layer 13 and the p-type low-concentration InGaAs absorption layer 14 become depleted and act as an absorption layer for signal light with a wavelength of 840 nm to 950 nm that is incident inside the ring-shaped p-type electrode 17. The p-type InGaAs contact layer 16 also has a band gap capable of absorbing incident light, but since it is highly doped, it does not become depleted at the voltage level of practical use and does not actually function as an absorption layer. The relationship between carrier concentration and depletion will be described later.

Figure 1 (B) shows a band diagram of the semiconductor light receiving element 10 when a reverse bias voltage is applied between the p-type electrode 17 and the n-type electrode 18.

2 is a graph showing the electric field strength when a reverse bias voltage of 2V is applied between the p-type electrode 17 and the n-type electrode 18. The horizontal axis of FIG. 2 indicates the thickness of each layer when the PN junction interface is set to 0. In FIG. 2, the + (plus) direction indicates the n-type electrode 18 side from the PN junction interface, and the - (minus) direction indicates the p-type electrode 17 side. The vertical axis indicates the electric field strength. The interface between the n-type low-concentration InGaAs absorption layer 13 and the p-type low-concentration InGaAs absorption layer 14 is the PN junction, where the electric field strength is maximum, and the electric field strength decreases as it moves away from the PN junction interface. If the electric field strength at the PN junction interface is E0 (V/m), the electric field strength En (V/m) in the n-type low-concentration InGaAs absorption layer 13 is expressed by Equation 4.

Here, q is the elementary charge (=1.6×10^19 (C)), and εn is the dielectric constant of the n-type low-concentration InGaAs absorption layer 13. Here, the dielectric constant of InGaAs is (=1.23×10^-10 (F/m)). Nn is the carrier density of the n-type low-concentration InGaAs absorption layer 13, which is 2×10^15/cm^3. Dn is the distance from the PN junction interface. If the distance from the PN junction interface at which the electric field intensity En becomes zero is Dn0, E0 is expressed by Equation 5.

Similarly, the electric field strength Ep (V/m) in the p-type low-concentration InGaAs absorption layer 14 is expressed by Equation 6.

εp is the dielectric constant of the p-type low-concentration InGaAs absorption layer 14. Here, it is the dielectric constant of InGaAs (=1.23×10^-10 (F/m)). Np is the carrier density of the p-type low-concentration InGaAs absorption layer 14, which is 2×10^15/cm^3, and Dp is the distance from the PN junction interface. If the distance from the PN junction interface at which the electric field intensity Ep becomes zero is Dp0, E0 is expressed by Equation 7.

If the reverse bias voltage is Vr and the built-in potential at the interface between the n-type low-concentration InGaAs absorption layer 13 and the p-type low-concentration InGaAs absorption layer 14 is Vb, the sum of the value obtained by integrating the electric field intensity En from the PN junction interface to Dn0 and the value obtained by integrating the electric field intensity Ep from the PN junction interface to Dp0 is (Vr+Vb) (V), and therefore the following formula 8 is established.

From the formulas 5, 7, and 8, the formulas 1, 2, and 3 are derived.

Optical modules (optical transceivers) used in optical communications are often driven by a single 3.3V power supply. At this time, the voltage applied to the semiconductor light receiving element 10 built into the optical module is a minimum of about 2V due to the effect of the voltage drop in the internal circuit (e.g., the transimpedance amplifier circuit) of the optical module. The built-in potential is determined by the semiconductor material, and in this case, it is 0.54V because it is a PN junction between InGaAs layers. Here, if Vr = 2V and Vb = 0.54V are substituted into formula 1, Dp0 = 0.988μm. Furthermore, from formulas 2 and 3, Dn0 = 0.988μm. In other words, if the carrier concentration of the n-type low-concentration InGaAs absorption layer 13 and the p-type low-concentration InGaAs absorption layer 14 is 2×10^15/cm^3, it can be seen that it is possible to deplete to a width of 0.988μm when a voltage of 2V is applied. In this embodiment, Wn = 0.9 μm and Wp = 0.9 μm, so the entire region of the low-concentration n-type InGaAs absorption layer 13 and the low-concentration p-type InGaAs absorption layer 14 is depleted. In other words, the width (thickness) of the semiconductor light receiving element 10 that is depleted is 1.8 μm.

The present invention is characterized in that the absorption layer is composed of a low-concentration p-type semiconductor layer and a low-concentration n-type semiconductor layer. Here, low concentration refers to a concentration that can be sufficiently depleted when a voltage of several volts is applied, which is actually used. Specifically, it is preferable that the carrier concentration is less than 1 x 10^16/cm^3. The Be of the p-type InGaAs contact layer 16 is 5 x 10^19/cm^3, and it is not depleted at several volts and does not actually function as an absorption layer. By forming the absorption layer from a low-concentration p-type semiconductor layer and an n-type semiconductor layer, the depletion layer region can be enlarged when a certain bias voltage is applied. This makes it possible to provide a semiconductor light receiving element that achieves low capacitance and has excellent high-speed response.

Here, the effect of the present invention will be described using a comparative example. FIG. 3A is a cross-sectional view of a semiconductor light receiving element 30 according to the comparative example. The difference from the semiconductor light receiving element 10 according to the present embodiment is the configuration of the absorption layer, and the other configurations are the same. The absorption layer of the semiconductor light receiving element 30 is composed of an n-type low-concentration InGaAs absorption layer 33 with a thickness Wnc. As with the n-type low-concentration InGaAs absorption layer 13 of the present embodiment, it is an n-type semiconductor layer with an n-type carrier concentration of 2×10^15/cm^3 contained at the background level. Wnc is set to 1.8 μm. That is, the absorption layer has the same concentration and thickness as the semiconductor light receiving element 10. The only difference is that all the absorption layers are n-type. FIG. 3B is a band diagram of the semiconductor light receiving element 30 when a reverse bias is applied.

Figure 4 shows the electric field strength in a comparative example when a reverse bias voltage of 2 V is applied between the p-type electrode 17 and the n-type electrode 18 of the semiconductor light receiving element 30. As in Figure 2, the horizontal axis is set to 0 at the PN junction interface. The interface between the n-type low-concentration InGaAs absorption layer 33 and the p-type InGaAs contact layer 16 is the PN junction. The electric field strength in the n-type low-concentration InGaAs absorption layer 33 is maximum at the PN junction interface and decreases with distance from the PN junction interface. If the electric field strength at the PN junction interface is E0 (V/m), the electric field strength En (V/m) in the n-type low-concentration InGaAs absorption layer 33 is expressed by Equation 4. Similarly, if the distance from the PN junction interface at which the electric field strength En becomes zero is Dn0, E0 is expressed by Equation 5.

If the reverse bias voltage is Vr and the built-in potential at the interface between the n-type InGaAs absorption layer 33 and the p-type InGaAs contact layer 16 is Vb, the integration of the electric field intensity E from the PN junction interface to Dn0 is (Vr+Vb) (V), and therefore Dn0 is expressed by Equation 9.

When Vr = 2 V and Vb = 0.54 (V) are substituted into Equation 9, Dn0 = 1.397 μm. In other words, the n-type low-concentration InGaAs absorption layer 33 has a thickness of 1.8 μm, but when 2 V is actually applied, only 1.397 μm is depleted.

The semiconductor light receiving element 10 according to this embodiment has an absorption layer composed of an n-type low-concentration InGaAs absorption layer 13 and a p-type low-concentration InGaAs absorption layer 14, and the thickness of the absorption layer is 1.8 μm. When 2 V is applied, all of the absorption layers are depleted. On the other hand, the semiconductor light receiving element 30 according to the comparative example has an absorption layer composed only of n-type low-concentration InGaAs, and the thickness of the absorption layer is 1.8 μm. However, when 2 V is applied, only 1.397 μm is depleted. Therefore, the semiconductor light receiving element 10 according to this embodiment has a smaller capacity than the semiconductor light receiving element 30, and is a semiconductor light receiving element with excellent high-speed response. In addition, it can be said that the semiconductor light receiving element 10 according to this embodiment can reduce the voltage required to obtain the same depleted region compared to the semiconductor light receiving element 30 of the comparative example. Therefore, the voltage required to obtain the same capacity, that is, the same level of high-speed response, can be smaller in absolute value, and it can be said to be a semiconductor light receiving element with excellent low power consumption. This characteristic is achieved by combining a p-type low-concentration absorption layer and an n-type low-concentration absorption layer, each of which has a carrier concentration capable of depleting the absorption layer of less than 1 x 10^16/cm^3.

Patent Document 2 shows a semiconductor light receiving element consisting of a layer containing n-type impurities whose concentration gradually decreases from the junction surface with the N layer toward the inside, and a layer containing p-type impurities whose concentration gradually decreases from the junction surface with the P layer toward the inside. At the interface between the n-type impurity layer and the p-type impurity layer, the concentration of both is 0. In this structure, the junction surface with the P-type layer or N-type layer in contact with both the p-type InGaAs absorption layer and the n-type InGaAs absorption layer has a high concentration of 1×10^16/cm^3 or more, and the concentration gradually decreases from the junction surface toward the inside. Therefore, the entire n-type impurity layer and the p-type impurity layer are not depleted, and only the regions with a carrier concentration of less than 1×10^16/cm^3 are depleted. Patent Document 2 aims to generate an internal electric field with a concentration gradient in each impurity layer, and not to reduce the carrier movement speed even at low voltage. Therefore, it is not assumed that the carrier concentration of the entire impurity layer will be less than 1×10^16/cm^3. In addition, in the n-type impurity layer, the carrier concentration gradually changes even at a concentration of about 1×10^16/cm^3. Therefore, the electric field is not applied exactly to the region that is actually depleted, and the interface between the depleted region and the non-depleted region becomes blurred. As a result, the carrier movement speed is not obtained sufficiently, which is a disadvantage in terms of high-speed response. In the semiconductor light receiving element 10 according to this embodiment, the n-type low-concentration InGaAs absorption layer 13 is in contact with the n-type InP contact layer 12 that is doped at a concentration of 5×10^18/cm^3, which is 10 times higher than its own concentration. Therefore, the electric field is applied exactly to the n-type low-concentration InGaAs absorption layer 13, and the semiconductor light receiving element is capable of high-speed response with excellent carrier discharge properties. In order to apply a sufficient electric field to the depletion layer region, the carrier concentration of the layer in contact with the low-concentration layer is preferably 1×10^17/cm^3 or more.

Second Embodiment
FIG. 5A is a cross-sectional view of a semiconductor light receiving element 50 according to a second embodiment of the present invention. FIG. 5B shows a band diagram in a state where a reverse bias voltage is applied to the semiconductor light receiving element 50. The difference from the semiconductor light receiving element 10 shown in the first embodiment is the difference in concentration and thickness of the n-type low concentration InGaAs absorption layers 13, 53 and the p-type low concentration InGaAs absorption layers 14, 54. In this embodiment, the n-type low concentration InGaAs absorption layer 53 has a concentration of 3×10^15/cm^3 and a thickness of 0.6 μm. On the other hand, the p-type low concentration InGaAs absorption layer 54 has a concentration of 2×10^15/cm^3 and a thickness of 0.9 μm. That is, the n-type low concentration InGaAs absorption layer 53 is formed thinner than the p-type low concentration InGaAs absorption layer 54, and the thickness of the entire absorption layer is 1.5 μm. The other configurations are the same as those of the semiconductor light receiving element 10.

Figure 6 is a graph showing the electric field strength when a reverse bias voltage of 2V is applied between the p-type electrode 17 and the n-type electrode 18 of the semiconductor light receiving element 50. As in Figure 2, the horizontal axis is set to 0 at the PN junction interface. The depleted region, that is, the distances Dn0 and Dp0 from the PN junction interface where the electric field strength is 0, can be calculated from Formulas 1, 2, and 3. Substituting Vr = 2V and Vb = 0.54 (V) into Formulas 1, 2, and 3, respectively, results in Dp0 = 1.082 μm and Dn0 = 0.721 μm. In other words, it shows that both the p-type low-concentration InGaAs absorption layer 54 with a thickness of 0.9 μm and the n-type low-concentration InGaAs absorption layer 53 with a thickness of 0.6 μm can be depleted. Furthermore, when the carrier concentrations of the n-type low-concentration InGaAs absorption layer 53 and the p-type low-concentration InGaAs absorption layer 54 are the above values, the maximum depletion layer thickness is Dp0 + Dn0 = 1.803 μm. This shows that although the absorption layer thickness is 1.5 μm in this embodiment, it is possible to completely deplete the absorption layer even if it is 1.803 μm.

On the other hand, for example, in the comparative example, if the concentration of the n-type low-concentration InGaAs absorption layer 33 is set to 3×10^15/cm^3, the same as that of the n-type low-concentration InGaAs absorption layer 53 of this embodiment, the width of the maximum depletion region is 1.14 μm. Therefore, even with the same carrier concentration, it is possible to widen the width of the maximum depletion region by combining a p-type low-concentration absorption layer and an n-type low-concentration absorption layer as shown in this embodiment. Also, as is clear from Equation 1, Equation 2, and Equation 3, in order to obtain the maximum depletion region at a certain bias voltage, the carrier concentrations of the p-type low-concentration absorption layer and the n-type low-concentration absorption layer must be the same.

Light absorption generates electron-hole pairs. The holes then move to the p-type InGaAs contact layer 16 side, and the electrons move to the n-type InP contact layer 12 side. Light absorption occurs throughout the absorption layer, and holes generated near the n-type InP contact layer 12 must move a longer distance than holes generated near the p-type InGaAs contact layer 16 side. Holes are heavier than electrons, so they take longer to move. In high-frequency driving, it is possible to improve high-speed response by quickly moving holes from the depletion layer region. In this embodiment, the PN junction interface where the electric field strength is maximum is closer to the n-side when viewed from the entire absorption layer. Therefore, a strong electric field is applied to the holes generated in the n-type low-concentration absorption layer InGaAs 53, which must move a longer distance. Therefore, these holes are accelerated by the strong electric field, and can move quickly to the p-type InGaAs contact layer 16 side. As a result, it is possible to provide a semiconductor light-receiving element with superior high-speed response.

[Third embodiment]
7A is a cross-sectional view of a semiconductor light receiving element 60 according to a third embodiment of the present invention. FIG. 7B shows a band diagram in a state where a reverse bias voltage is applied to the semiconductor light receiving element 60. The difference from the semiconductor light receiving element 10 shown in the first embodiment is the concentration and thickness of the n-type low concentration InGaAs absorption layers 13, 63 and the p-type low concentration InGaAs absorption layers 14, 64. In this embodiment, the n-type low concentration InGaAs absorption layer 63 has a concentration of 2×10^15/cm^3 and a thickness of 0.9 μm. On the other hand, the p-type low concentration InGaAs absorption layer 64 has a concentration of 3×10^15/cm^3 and a thickness of 0.6 μm. That is, of the n-type low-concentration InGaAs absorption layer 63 and the p-type low-concentration InGaAs absorption layer 64, the one closer to the light incident surface (the p-type low-concentration InGaAs absorption layer 64) is formed thinner than the other (the n-type low-concentration InGaAs absorption layer 63), and the thickness of the entire absorption layer is 1.5 μm. The other configurations are the same as those of the semiconductor light receiving element 10.

Figure 8 is a graph showing the electric field strength when a reverse bias voltage of 2V is applied between the p-type electrode 17 and the n-type electrode 18 of the semiconductor light receiving element 60. As in Figure 2, the horizontal axis is set to 0 at the PN junction interface. The depleted region, that is, the distances Dn0 and Dp0 from the PN junction interface where the electric field strength is 0, can be calculated from Formulas 1, 2, and 3. Substituting Vr = 2V and Vb = 0.54 (V) into Formulas 1, 2, and 3, respectively, results in Dp0 = 0.721 μm and Dn0 = 1.082 μm. In other words, it shows that both the p-type low-concentration InGaAs absorption layer 64 with a thickness of 0.6 μm and the n-type low-concentration InGaAs absorption layer 63 with a thickness of 0.9 μm can be depleted. Furthermore, when the carrier concentrations of the n-type low-concentration InGaAs absorption layer 63 and the p-type low-concentration InGaAs absorption layer 64 are the above values, the maximum depletion layer thickness is Dp0 + Dn0 = 1.803 μm. This shows that although the absorption layer thickness is 1.5 μm in this embodiment, it is possible to completely deplete the absorption layer even if it is 1.803 μm.

On the other hand, for example, in the comparative example, when the concentration of the low-concentration n-type InGaAs absorption layer 33 is set to 2 ×10^15/cm^3, which is the same as that of the low-concentration n-type InGaAs absorption layer 63 of this embodiment, the width of the maximum depletion region is 1.14 μm. Therefore, even with the same carrier concentration, it is possible to increase the width of the maximum depletion region by combining a low-concentration p-type absorption layer and a low-concentration n-type absorption layer as shown in this embodiment.

The semiconductor light receiving element 60 is a surface-incident type optical semiconductor light receiving element, and the received optical signal passes through the p-type InGaAs contact layer 16 and enters the p-type low-concentration InGaAs absorption layer 64. The entered optical signal is absorbed by the p-type low-concentration InGaAs absorption layer 64 and reaches the n-type low-concentration InGaAs absorption layer 63, where it is also absorbed. Therefore, the amount of light absorbed by the p-type low-concentration InGaAs absorption layer 64 is greater than the amount of light absorbed by the n-type low-concentration InGaAs absorption layer 63. Therefore, the number of hole-electron pairs generated by light absorption is also greater in the p-type low-concentration InGaAs absorption layer 64. It is preferable to apply a strong electric field in order to move the large number of carriers generated at high speed. In this embodiment, it is possible to apply a higher electric field strength to the p-type low-concentration InGaAs absorption layer 64, which has a large amount of light absorption, and it is possible to move the carriers efficiently.

As explained in the second embodiment, there are cases where it is better to strengthen the electric field strength of the n-type low-concentration InGaAs absorption layer 63. For example, in the case of a front-illuminated type, the semiconductor light receiving element 60 of the third embodiment is preferable in order to move carriers generated in large quantities on the p-side quickly. On the other hand, in the case of a back-illuminated type semiconductor light receiving element, the n-type low-concentration InGaAs absorption layer 63 absorbs a large amount of light, and the semiconductor light receiving element 50 of the second embodiment has a superior high-speed response. Even in the case of a front-illuminated type semiconductor light receiving element, whether the ability to extract carriers generated in large quantities on the p-side or the ability to extract holes on the n-side affects the overall high-speed response depends on the incident light intensity, the applied voltage, and the thickness and concentration of the absorption layer. Therefore, it is preferable to determine the thickness and concentration of the p-type low-concentration absorption layer and the n-type low-concentration absorption layer as an overall design.

[Fourth embodiment]
FIG. 9A is a cross-sectional view of a semiconductor light receiving element 70 according to the fourth embodiment of the present invention. FIG. 7B shows a band diagram in a state where a reverse bias voltage is applied to the semiconductor light receiving element 70. The difference from the semiconductor light receiving element 10 shown in the first embodiment is that the n-type low concentration InGaAs absorption layer 13 is an n-type low concentration InP wide band gap layer 73 with a carrier concentration of 1×10^15/cm^3, and the thickness of the p-type low concentration InGaAs absorption layer 74 is 0.8 μm. The carrier concentration of the p-type low concentration InGaAs absorption layer 74 is 2×10^15/cm^3, which is the same as that of the first embodiment. The carriers of the n-type low concentration InP wide band gap layer 73 may be either intentionally doped or included in the background. The thickness of the n-type low concentration InP wide band gap layer 73 is 1.5 μm. In other words, the absorption layer is only the p-type low concentration InGaAs absorption layer 74 , and its thickness is 0.9 μm. The other configurations are the same as those of the semiconductor light receiving element 10.

The n-type low-concentration InP wide band gap layer 73 does not act as an absorption layer because the band gap wavelength of InP is shorter than the wavelength of the incident light, 840 nm to 950 nm. However, since the carrier concentration is less than 1×10^16/cm^3, it is a depleted layer. Only electrons among the carriers generated in the p-type low-concentration InGaAs absorption layer 74 can drift at high speed due to the internal electric field. In this way, the structure reduces the element capacitance while suppressing the increase in carrier movement time. In order to improve the high-speed response of the semiconductor light receiving element, it is important not only to reduce the capacitance but also how quickly the carriers escape from the absorption layer. This structure is excellent in the latter respect. Note that the layer is not limited to InP as long as it has a wide band gap for the wavelength of the incident light, and may be, for example, an InAlAs layer or an InAlGaAs layer.

Figure 10 is a graph showing the electric field strength when a reverse bias voltage of 2V is applied between the p-type electrode 17 and the n-type electrode 18 of the semiconductor light receiving element 70. As in Figure 2, the horizontal axis is set to 0 at the PN junction interface. The depleted region, that is, the distances Dn0 and Dp0 from the PN junction interface where the electric field strength is 0, can be calculated from Formulas 1, 2, and 3. In this embodiment, the PN junction interface is the interface between the InGaAs layer and the InP layer, so Vb is 0.59V. The dielectric constant of InP is 1.11 x 10^-10 (F/m). Substituting Vr = 2V and Vb = 0.59 (V) into Formulas 1, 2, and 3, respectively, gives Dp0 = 0.844μm and Dn0 = 1.517μm. In other words, both the p-type low-concentration InGaAs absorption layer 74 with a thickness of 0.8 μm and the n-type low-concentration InP wide band gap layer 73 with a thickness of 1.5 μm can be depleted. In other words, the semiconductor light receiving element 70 has a small capacity because the depletion layer thickness is large at 2.3 μm when 2 V is applied, and furthermore, the n-type low-concentration InP wide band gap layer 73 has excellent carrier discharge properties and is an element capable of high-speed response. Note that, although the n-type low-concentration layer is a wide band gap layer in this embodiment, a wide depletion layer can be obtained similarly to this embodiment even if the p-type low-concentration layer is an InP wide band gap layer and the n-type low-concentration layer is an n-type InGaAs absorption layer.

As shown in the fourth embodiment, the low concentration layer does not necessarily have to be made of only materials with a band gap capable of absorbing incident light, and it is possible to enlarge the depletion layer by forming a PN interface with a p-type low concentration layer and an n-type low concentration layer with a carrier concentration of less than 1 x 10^16/cm^3.

[Modification]
A modified example of the semiconductor light receiving element 70 according to the fourth embodiment of the present invention will be described. FIG. 11A is a cross-sectional view of the semiconductor light receiving element 90 according to the modified example. FIG. 11B shows a band diagram in a state where a reverse bias voltage is applied to the semiconductor light receiving element 90. FIG. 12 is a graph showing an electric field intensity in a state where a reverse bias voltage of 2 V is applied between the p-type electrode 17 and the n-type electrode 18 of the semiconductor light receiving element 90. The difference from the semiconductor light receiving element 70 shown in the fourth embodiment is that a p-type high-concentration InGaAs absorption layer 99 is sandwiched between the p-type low-concentration InGaAs absorption layer 74 and the p-type InGaAs contact layer 16. The p-type high-concentration InGaAs absorption layer 99 is a layer with a thickness of 0.6 μm doped with Be at a concentration of 1×10^18/cm^3. The p-type high-concentration InGaAs absorption layer 99 is not depleted by a voltage of several volts because the carrier concentration is 1×10^16/cm^3 or more. Therefore, the effect of reducing the capacitance due to the presence of this layer cannot be obtained. However, since it has a band gap that absorbs incident light, it is effective in improving the light receiving sensitivity. The depletion region of the semiconductor light receiving element 90 is the same as that of the semiconductor light receiving element 70 shown in the fourth embodiment. The p-type high-concentration InGaAs absorption layer 99 may have a concentration gradient. However, even the lowest concentration region has a concentration that does not cause depletion when a bias voltage is applied under the usage conditions. Specifically, a concentration of 1×10^17/cm^3 or more is preferable. Furthermore, an n-type low-concentration InGaAs absorption layer may be sandwiched between the p-type low-concentration InGaAs absorption layer 74 and the n-type low-concentration InP wide band gap layer 73 as in the first embodiment.

In addition, the layer to be depleted is created to have a thickness derived from Formula 1, Formula 2, and Formula 3, which is the most excellent in terms of low capacity and high-speed response, but the low-concentration layer may be configured as a layer up to about 10% thicker than the depletable layer thickness derived from Formula 1, Formula 2, and Formula 3. In this case, there are layers that are not depleted in the p-type low-concentration layer near the interface with the p-type high-concentration layer, and in the n-type low-concentration layer near the interface with the n-type high-concentration layer. However, since the layers are not thick, they do not have a large effect on the deterioration of the carrier discharge property, and are at a practical level.

Needless to say, the effects of the present invention can be obtained by combining the structures described in the above embodiments with each other. That is, if at least one of the n-type low concentration layer and the p-type low concentration layer includes an absorption layer that absorbs incident light, the n-type low concentration layer and the p-type low concentration layer may be composed of two layers, an absorption layer and a wide band gap layer, or only one of them. Furthermore, the n-type low concentration layer and the p-type low concentration layer may be the same thickness, the n-type low concentration layer may be thicker than the p-type low concentration layer, or the n-type low concentration layer may be thinner than the p-type low concentration layer. In addition, although the above description is of a front-illuminated semiconductor light receiving element capable of receiving optical signals of wavelengths of 840 nm to 950 nm, the front-illuminated semiconductor light receiving element capable of receiving optical signals of other wavelength bands, for example, the 1.3 μm band or the 1.55 μm band, or the back-illuminated semiconductor light receiving element may be used. Furthermore, the fiber to which the optical signal is input may be a single mode fiber. In the above embodiment, the structure was described in which the side closer to the InP substrate was n-type, but it goes without saying that the same effect can be obtained even if the polarity is reversed, that is, the side closer to the InP substrate is p-type. Furthermore, a conductive semiconductor substrate may be used instead of a semi-insulating InP substrate.

10 semiconductor light receiving element, 11 InP substrate, 12 n-type InP contact layer, 13 n-type low-concentration InGaAs absorption layer, 14 p-type low-concentration InGaAs absorption layer, 16 p-type InGaAs contact layer, 17 p-type electrode, 18 n-type electrode, 19 silicon nitride film, 30 semiconductor light receiving element, 33 n-type low-concentration InGaAs absorption layer, 50 semiconductor light receiving element, 53 n-type low-concentration InGaAs absorption layer, 54 p-type low-concentration InGaAs absorption layer, 60 semiconductor light receiving element, 63 n-type low-concentration InGaAs absorption layer, 64 p-type low-concentration InGaAs absorption layer, 70 semiconductor light receiving element, 73 n-type low-concentration InP wide band gap layer, 74 p-type low-concentration InGaAs absorption layer, 90 semiconductor light receiving element, 99 p-type high concentration InGaAs absorption layer.

Claims (18)

A semiconductor substrate;
a first conductivity type high concentration layer formed on the semiconductor substrate;
a first conductivity type low concentration layer formed on and in contact with the first conductivity type high concentration layer;
a second conductivity type low concentration layer forming a PN junction interface with the first conductivity type low concentration layer;
a second conductivity type high concentration layer formed on and in contact with the second conductivity type low concentration layer,
the first conductivity type high concentration layer has a higher carrier concentration than the first conductivity type low concentration layer,
the second conductivity type high concentration layer has a higher carrier concentration than the second conductivity type low concentration layer,
one or both of the first conductivity type low concentration layer and the second conductivity type low concentration layer include an absorption layer having a band gap for absorbing incident light;
The thickness of the first conductivity type low concentration layer is 1.1 times or less a thickness Dn0 defined by Equation 1, Equation 2, and Equation 3,
The thickness of the second conductive type low concentration layer is 1.1 times or less the thickness Dp0 defined by the formula 1, the formula 2, and the formula 3.
Semiconductor photodetector.

Vr: bias voltage applied from the outside Vb: built-in potential q: elementary charge εp: dielectric constant of the second conductivity type low concentration layer εn: dielectric constant of the first conductivity type low concentration layer 2. The semiconductor light receiving element according to claim 1,
the thickness of the first conductivity type low concentration layer is equal to or less than Dn0,
The thickness of the second conductivity type low concentration layer is equal to or less than Dp0.
Semiconductor photodetector. A semiconductor substrate;
a first conductivity type high concentration layer formed on the semiconductor substrate;
a first conductivity type low concentration layer formed on and in contact with the first conductivity type high concentration layer;
a second conductivity type low concentration layer forming a PN junction interface with the first conductivity type low concentration layer;
a second conductivity type high concentration layer formed on and in contact with the second conductivity type low concentration layer,
the first conductivity type high concentration layer has a higher carrier concentration than the first conductivity type low concentration layer,
the second conductivity type high concentration layer has a higher carrier concentration than the second conductivity type low concentration layer,
one or both of the first conductivity type low concentration layer and the second conductivity type low concentration layer include an absorption layer having a band gap for absorbing incident light;
the first conductivity type high concentration layer is a first conductivity type contact layer,
the second conductive type high concentration layer is a second conductive type contact layer,
each of the first conductivity type low concentration layer and the second conductivity type low concentration layer includes an absorption layer having a band gap for absorbing incident light;
the absorption layer in each of the first conductivity type low concentration layer and the second conductivity type low concentration layer is a low concentration InGaAs absorption layer. A semiconductor substrate;
a first conductivity type high concentration layer formed on the semiconductor substrate;
a first conductivity type low concentration layer formed on and in contact with the first conductivity type high concentration layer;
a second conductivity type low concentration layer forming a PN junction interface with the first conductivity type low concentration layer;
a second conductivity type high concentration layer formed on and in contact with the second conductivity type low concentration layer,
the first conductivity type high concentration layer has a higher carrier concentration than the first conductivity type low concentration layer,
the second conductivity type high concentration layer has a higher carrier concentration than the second conductivity type low concentration layer ,
the first conductivity type high concentration layer is a first conductivity type contact layer,
the first conductivity type low concentration layer is a first conductivity type low concentration wide band gap layer having a band gap that does not absorb incident light,
the second conductive type high concentration layer is a second conductive type contact layer,
one of the first conductivity type low concentration layer and the second conductivity type low concentration layer includes an absorption layer having a band gap for absorbing incident light;
the one of the first and second conductive types being a low concentration layer of the second conductive type. 5. The semiconductor light receiving element according to claim 4,
The semiconductor light receiving element, wherein the absorption layer is a low-concentration InGaAs absorption layer. 5. The semiconductor light receiving element according to claim 4,
The first conductivity type low concentration wide band gap layer is made of any one of InP, InAlAs, and InAlGaAs. A semiconductor substrate;
a first conductivity type high concentration layer formed on the semiconductor substrate;
a first conductivity type low concentration layer formed on and in contact with the first conductivity type high concentration layer;
a second conductivity type low concentration layer forming a PN junction interface with the first conductivity type low concentration layer;
a second conductivity type high concentration layer formed on the second conductivity type low concentration layer,
the first conductivity type high concentration layer has a higher carrier concentration than the first conductivity type low concentration layer,
the second conductivity type high concentration layer has a higher carrier concentration than the second conductivity type low concentration layer ,
the first conductivity type high concentration layer is a first conductivity type contact layer,
the first conductivity type low concentration layer is a first conductivity type low concentration wide band gap layer having a band gap that does not absorb incident light,
the second conductivity type high concentration layer is a second conductivity type contact layer and a first second conductivity type high concentration layer;
one of the first conductivity type low concentration layer and the second conductivity type low concentration layer includes an absorption layer having a band gap for absorbing incident light;
the one is a second conductivity type low concentration layer,
the semiconductor light receiving element further including a second second conductivity type high concentration layer formed between the first second conductivity type high concentration layer and the second conductivity type low concentration layer in contact with the first second conductivity type high concentration layer. 8. The semiconductor light receiving element according to claim 7,
The first conductivity type low concentration wide band gap layer is made of any one of InP, InAlAs, and InAlGaAs. 2. The semiconductor light receiving element according to claim 1,
The first conductivity type is n-type, and the second conductivity type is p-type. A semiconductor substrate;
a first conductivity type high concentration layer formed on the semiconductor substrate;
a first conductivity type low concentration layer formed on and in contact with the first conductivity type high concentration layer;
a second conductivity type low concentration layer forming a PN junction interface with the first conductivity type low concentration layer;
a second conductivity type high concentration layer formed on and in contact with the second conductivity type low concentration layer,
the first conductivity type high concentration layer has a higher carrier concentration than the first conductivity type low concentration layer,
the second conductivity type high concentration layer has a higher carrier concentration than the second conductivity type low concentration layer,
one or both of the first conductivity type low concentration layer and the second conductivity type low concentration layer include an absorption layer having a band gap for absorbing incident light;
the first conductivity type is n-type and the second conductivity type is p-type;
The first conductivity type low concentration layer is thinner than the second conductivity type low concentration layer. 11. The semiconductor light receiving element according to claim 10,
The first conductivity type low concentration layer has a higher carrier concentration than the second conductivity type low concentration layer. 10. The semiconductor light receiving element according to claim 9,
The first conductivity type low concentration layer is thicker than the second conductivity type low concentration layer. 13. The semiconductor light receiving element according to claim 12,
The first conductivity type low concentration layer has a carrier concentration lower than that of the second conductivity type low concentration layer. 2. The semiconductor light receiving element according to claim 1,
The first conductivity type low concentration layer has a thickness different from that of the second conductivity type low concentration layer. 2. The semiconductor light receiving element according to claim 1,
The first conductivity type low concentration layer has a carrier concentration different from that of the second conductivity type low concentration layer. 2. The semiconductor light receiving element according to claim 1,
a carrier concentration of the first conductivity type low concentration layer and the second conductivity type low concentration layer is less than 1×10 ¹⁶ /cm ³ ; 2. The semiconductor light receiving element according to claim 1,
the first conductivity type high concentration layer and the second conductivity type high concentration layer each have a carrier concentration of 1×10 ¹⁶ /cm ³ or more. 18. The semiconductor light receiving element according to claim 17,
a carrier concentration of the first conductivity type low concentration layer and the second conductivity type low concentration layer is less than 1×10 ¹⁶ /cm ³ ;

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