heterojunction bipolar transistor

Abstract

This HBT comprises: a subcollector layer (102) formed on a substrate (101); and a collector layer (103) formed on the group-V polarity surface side of the subcollector layer (102). The HBT further comprises: a base layer (104) formed in contact with the group-V polarity surface of the collector layer (103); an emitter layer (105) formed in contact with the group-V polarity surface of the base layer (104); and an emitter contact layer (106) formed on the group-V polarity surface side of the emitter layer (105). The base layer (104) is composed of InGaN having less In composition as compared to the collector layer (103), and the emitter layer (105) is composed of GaN or InGaN having less In composition as compared to the base layer (104).

Description

[Technical Field]

[0001] The present invention relates to a heterojunction bipolar transistor. [Background technology]

[0002] Nitride semiconductors have a wide band gap, making them promising materials for high-speed, high-voltage electronic devices. Many research institutes are actively researching high-electron mobility transistors that utilize the high density of sheet carriers generated by polarization in AlGaN / GaN, and these transistors have already been put to practical use as amplifying transistors for communication amplifiers and as highly efficient power devices.

[0003] Heterojunction bipolar transistors (HBTs) are device structures that can achieve high voltage resistance by using high-voltage materials in the collector layer, and can achieve both high speed and high voltage resistance. There have been many reports of HBT structures using III-V compound semiconductors with InP or GaAs substrate materials, and IV materials with SiGe base layers, that have achieved cutoff frequencies of several hundred GHz, maximum oscillation frequencies, and high voltage resistance at the same time.

[0004] Even with GaN-based wide-gap materials, it is expected that by creating HBTs using this material, it will be possible to realize transistors with even higher breakdown voltages and higher speeds than those made from conventional III-V compound semiconductors.

[0005] However, it is difficult to make nitride semiconductors such as GaN p-type at high concentrations for the following reasons: First, in nitride semiconductors, the ionization energy of the impurities that function as acceptors is very large. Also, while nitride semiconductors are grown using common growth techniques such as MOCVD, there is an essential issue in that the dopants (Mg, Zn, etc.) are inactivated by H (hydrogen) contained in the carrier gas and raw materials during p-type doping, making it impossible to increase the hole concentration.

[0006] To increase the speed of an HBT, it is necessary to increase the concentration, reduce the resistance, and lower the contact resistance of both the n-type and p-type. However, as mentioned above, it is extremely difficult to achieve high speeds in HBTs made of nitride semiconductors, which are difficult to increase the concentration of the p-type.

[0007] One technique proposed for achieving high hole concentrations in semiconductor devices using nitride semiconductors such as GaN is to fabricate devices with an N-polar plane as the principal plane orientation. Nitride semiconductors are materials with polarization in the c-axis direction, and devices are generally fabricated by growing crystals in a plane orientation known as a group III polarity plane (+c-axis direction). In the case of group III polarity, when AlGaN is grown on GaN, the band bending occurs due to the electric field caused by the difference in spontaneous polarization between the materials and the polarization electric field generated by strain in the AlGaN layer, generating two-dimensional electron gas at the interface between the AlGaN and GaN. Utilizing this, a GaN channel HEMT structure has been realized, and high-frequency devices using this structure have already been put to practical use.

[0008] On the other hand, a configuration in which the primary surface is an N-polar (group V polar) plane is a reverse of a group III polar plane. In this case, the direction of the electric field generated by polarization is reversed compared to the case of a group III polar plane. For example, when AlGaN is formed on GaN with an N-polar principal plane orientation, a two-dimensional hole gas is generated at the AlGaN / GaN interface due to the polarization electric field (see Non-Patent Document 1). In this way, in an HBT using GaN with an N-polar principal plane orientation, the above-mentioned two-dimensional hole gas can be used to overcome the issues related to p-type doping control.

[0009] However, in HBTs using a two-dimensional hole gas formed using an N-polar surface, there are issues to be overcome regarding the ohmic contact between the base layer and the base electrode. In HBT structures with an N-polar surface as the principal surface orientation, a technique is used to increase the concentration of p-base layer 304a by creating two-dimensional hole gas 321 at the interface between emitter layer 305 made of AlGaN and p-type GaN p-base layer 304a, as shown in Figure 8A.

[0010] This HBT includes a buffer layer 307 formed on a substrate 301, a sub-collector layer 302 made of an n-type nitride semiconductor formed on the buffer layer 307, a collector layer 303 made of n-type GaN formed on the sub-collector layer 302, a p-base layer 304a made of p-type GaN formed on the collector layer 303, a base layer 304b made of undoped GaN formed on the p-base layer 304a, an emitter layer 305 formed on the base layer 304b, and an emitter cap layer 306 made of an n-type nitride semiconductor formed on the emitter layer 305.

[0011] This HBT also includes an emitter electrode 311 formed on the emitter cap layer 306, a base electrode 312 formed on the base layer on one side of the emitter layer 305, and a collector electrode 313 connected to the sub-collector layer 302. In the emitter-top structure shown in Fig. 8A, ohmic contacts between each metal electrode and the emitter cap layer 306, base layer 304b, and sub-collector layer 302 must be formed from the top surface (front surface) of the device.

[0012] In this HBT, the base layer is highly concentrated by the two-dimensional hole gas 321, so it is important that the emitter layer 305 also exists directly below the base electrode 312. However, the emitter layer 305 made of AlGaN has high resistance. Therefore, forming the base electrode 312 directly on the emitter layer 305, as shown in Figure 8A, increases the ohmic contact resistance. To obtain good ohmic contact between the base layer 304b and the base electrode 312, it is necessary to take measures such as thinning the emitter layer 305b directly below the base electrode 312 by partially removing it by etching, as shown in Figure 8B.

[0013] However, as shown in FIG. 8C, if the emitter layer 305c is completely removed, the concentration of the two-dimensional hole gas 321 will be drastically reduced (disappeared), and therefore, etching of the emitter layer for forming the base electrode requires a very high degree of controllability. [Prior art documents] [Non-patent literature]

[0014] [Non-Patent Document 1] Takeru Kumebe et al., "Emitter-top GaN HBTs with two-dimensional hole gases fabricated by epitaxial lift-off," Proceedings of the 80th Autumn Meeting of the Japan Society of Applied Physics, 21a-E301-5, July 13, 2019. Summary of the Invention [Problem to be solved by the invention]

[0015] As mentioned above, in GaN-based HBT structures using nitride semiconductors such as GaN, it is not easy to achieve a high concentration of p-type, and it is difficult to obtain good ohmic contact between the base layer and the base electrode.

[0016] The present invention has been made to solve the above problems, and has as its object to obtain good ohmic contact between the base layer and the base electrode in a GaN-based bipolar transistor structure. [Means for solving the problem]

[0017] A heterojunction bipolar transistor according to the present invention comprises: a subcollector layer made of an n-type nitride semiconductor formed on a substrate; a collector layer made of InGaN and made to be n-type, formed on the V group polarity face side of the subcollector layer; a base layer made of InGaN formed in contact with the V group polarity face of the collector layer and having a lower In composition than the collector layer; an emitter layer made of InGaN or GaN formed in contact with the V group polarity face of the base layer and having a lower In composition than the base layer; an emitter contact layer made of an n-type nitride semiconductor formed on the V group polarity face side of the emitter layer; an emitter electrode connected to the emitter contact layer; a base electrode connected to the base layer; a collector electrode connected to the subcollector layer; and the base layer. Emitterand a two-dimensional hole gas formed in each of the base layer near the interface with the collector layer and the collector layer near the interface with the base layer. [Effects of the Invention]

[0018] As described above, according to the present invention, the collector layer is made of InGaN, the base layer is made of InGaN with a smaller In composition than the collector layer, and the emitter layer is made of InGaN. , Be Since the base layer is made of InGaN or GaN with a smaller In composition than the base layer, a good ohmic contact can be obtained between the base layer and the base electrode in a GaN-based bipolar transistor structure. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a cross-sectional view showing the configuration of a heterojunction bipolar transistor according to a first embodiment of the present invention. [Figure 2A] FIG. 2A is a band diagram showing the band state of the heterojunction bipolar transistor according to the first embodiment of the present invention. [Figure 2B] FIG. 2B is a characteristic diagram showing the results of calculation of the sheet carrier density of the heterojunction bipolar transistor according to the first embodiment of the present invention. [Figure 3A] FIG. 3A is a band diagram showing the band state of the heterojunction bipolar transistor according to the first embodiment of the present invention. [Figure 3B] FIG. 3B is a characteristic diagram showing the results of calculation of the sheet carrier density of the heterojunction bipolar transistor according to the first embodiment of the present invention. [Figure 4A] FIG. 4A is a cross-sectional view showing the state of the heterojunction bipolar transistor in the middle of a process, for illustrating the method for manufacturing the heterojunction bipolar transistor according to the first embodiment of the present invention. [Figure 4B] FIG. 4B is a cross-sectional view showing the state of the heterojunction bipolar transistor in the middle of a process, for illustrating the method for manufacturing the heterojunction bipolar transistor according to the first embodiment of the present invention. [Figure 4C] FIG. 4C is a cross-sectional view showing the state of the heterojunction bipolar transistor in the middle of a process, for illustrating the method for manufacturing the heterojunction bipolar transistor according to the first embodiment of the present invention. [Figure 4D] FIG. 4D is a cross-sectional view showing the state of the heterojunction bipolar transistor in the middle of a process, for illustrating the method for manufacturing the heterojunction bipolar transistor according to the first embodiment of the present invention. [Figure 5A] FIG. 5A is a characteristic diagram showing the relationship between the In composition and the hole concentration in a collector layer made of InGaN when the base layer is made of GaN (the In composition is set to 0). [Figure 5B] FIG. 5B is a characteristic diagram showing the relationship between the In composition of the collector layer made of InGaN and the hole concentration when the In composition of the base layer 104 made of InGaN is set to 0.05. [Figure 5C] FIG. 5C is a characteristic diagram showing the relationship between the In composition and the hole concentration in the collector layer made of InGaN when the In composition of the base layer 104 made of InGaN is set to 0.10. [Figure 6] FIG. 6 is a cross-sectional view showing a configuration of a heterojunction bipolar transistor according to a second embodiment of the present invention. [Figure 7] FIG. 7 is a cross-sectional view showing a configuration of a heterojunction bipolar transistor according to a third embodiment of the present invention. [Figure 8A] FIG. 8A is a cross-sectional view showing a conventional GaN-based HBT structure having an N-polarity principal plane orientation. [Figure 8B] FIG. 8B is a cross-sectional view showing a conventional GaN-based HBT structure having an N-polarity principal plane orientation. [Figure 8C] FIG. 8C is a cross-sectional view showing a conventional GaN-based HBT structure having an N-polarity principal plane orientation. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, a heterojunction bipolar transistor according to an embodiment of the present invention will be described.

[0021] [Embodiment 1] First, the structure of the heterojunction bipolar transistor according to Embodiment 1 of the present invention will be described with reference to FIG. 1. This heterojunction bipolar transistor (HBT) first includes a subcollector layer 102 formed on a substrate 101 and a collector layer 103 formed on the V-group polar surface side of the subcollector layer 102. In this example, when viewed from the side of the substrate 101, the subcollector layer 102 is formed on the buffer layer 107, and the collector layer 103 is formed on the subcollector layer 102.

[0022] The subcollector layer 102 can be composed of a nitride semiconductor (GaN or InGaN) doped with n-type at a high concentration. For example, the subcollector layer 102 can be composed of GaN doped with n-type at a high concentration. Since the subcollector layer 102 also functions as a contact layer for realizing an ohmic contact with the collector electrode 113 described later, the doping concentration is set to a relatively high concentration (for example, 5×10 18 cm -3 or more). Also, the subcollector layer 102 is grown relatively thick within a range that does not affect the device characteristics. For example, it is desirable to set the thickness of the subcollector layer 102 to at least 1 μm or more in order to also function as a buffer layer for improving crystal quality.

[0023] The collector layer 103 is composed of In x Ga 1-x N (0 < x < 1). The doping concentration for making the InGaN constituting the collector layer 103 n-type is set, for example, to be smaller than that of the subcollector layer 102. For example, the collector layer 103 can be composed of n-type InGaN having an n-type impurity concentration of about 10 17 cm -3 .

[0024] This HBT also includes a base layer 104 formed in contact with the group V polarity surface of the collector layer 103, an emitter layer 105 formed in contact with the group V polarity surface of the base layer 104, and an emitter contact layer 106 formed on the group V polarity surface side of the emitter layer 105. The base layer 104 is made of InGaN with a lower In composition than the collector layer 103. This configuration allows a two-dimensional hole gas 121 to be generated in the collector layer 103 near the interface between the collector layer 103 and the base layer 104 due to a polarization electric field. The base layer 104 can also be made of p-type InGaN.

[0025] For example, the collector layer 103 can be made of InGaN having an In composition 0.05 or more higher than the InGaN constituting the base layer 104. The base layer 104 can be made of InGaN having an In composition 0.1 or more. The base layer 104 can have a thickness of, for example, about 4 nm.

[0026] The emitter layer 105 is made of InGaN or GaN, which has a smaller In composition than the base layer 104. This difference in In composition allows a two-dimensional hole gas 121 to be generated in the base layer 104 near the interface between the base layer 104 and the emitter layer 105 due to the polarization effect. Furthermore, the emitter layer 105 has a larger band gap than the base layer 104, which suppresses the reverse absorption of holes toward the emitter side, thereby increasing the current gain.

[0027] The emitter contact layer 106 is made of an n-type nitride semiconductor. The emitter contact layer 106 is a layer for forming an ohmic contact with low contact resistance, and the n-type impurity concentration is set to a high concentration. For example, the emitter contact layer 106 has an n-type impurity concentration of 5×10 18 cm -3In this layer, it is effective to increase the impurity concentration and narrow the band gap in order to achieve ohmic contact with the metal. Therefore, the emitter contact layer 106 is not limited to GaN, but can be made of InGaN or other materials. The emitter contact layer 106 can have a thickness of approximately 100 nm.

[0028] The HBT also includes an emitter electrode 111 formed on and connected to the emitter contact layer 106, a base electrode 112 connected to the base layer 104, and a collector electrode 113 connected to the sub-collector layer. In the first embodiment, the base electrode 112 is formed on and in contact with the emitter layer 105.

[0029] In the first embodiment, the sub-collector layer 102, the collector layer 103, the base layer 104, the emitter layer 105, and the emitter contact layer 106 are stacked in this order on the substrate 101 with their main surfaces being V-polar planes. The emitter contact layer 106 is formed in a mesa shape, and the base electrode 112 is formed on the emitter layer 105 on the side of the emitter contact layer 106.

[0030] As described above, the structure in which each layer is stacked on the substrate 101 with the main surface being a group V polarity plane can be realized as follows. First, the substrate 101 is made of a material used to form a nitride semiconductor device, and the material of the substrate 101 is selected so that the main surface has an N polarity plane as its orientation (the main surface is a group V polarity plane). For example, the substrate 101 can be made of sapphire, a C-face SiC substrate, an N polarity GaN substrate, an N polarity AlN substrate, or the like.

[0031] When the substrate 101 is a sapphire substrate, the buffer layer 107 can be a nitride layer formed on the surface of the substrate by subjecting the surface of the substrate 101 to high-temperature heat treatment in an atmosphere of a source gas such as ammonia. A nitride semiconductor having an N-polar plane as its primary plane orientation can be grown on the buffer layer 107 formed by nitriding. On the other hand, when the substrate 101 is a GaN single crystal substrate or an AlN single crystal substrate having an N-polar plane as its primary plane orientation, a nitride semiconductor can be grown on the substrate 101 without using a special buffer layer.

[0032] As described above, the HBT according to the first embodiment has two-dimensional hole gas 121 formed in collector layer 103 near the interface between base layer 104 and collector layer 103, and in base layer 104 near the interface between base layer 104 and emitter layer 105.

[0033] The HBT according to the first embodiment, in which the two-dimensional hole gas 121 is formed, will be described in more detail below. The results of calculations of band changes and accompanying changes in sheet carrier density when the In composition of the base layer 104 is changed in the layer structure of the HBT according to the first embodiment described above will be described with reference to FIGS. 2A and 2B. The following conditions were used in the calculations:

[0034] The sub-collector layer 102 has an impurity concentration of 10 19 cm -3 The base layer 104 was made of n-type GaN with an In composition of about 0.15, and the collector layer 103 was made of InGaN with an In composition of 0.15 and had a thickness of 100 nm. The base layer 104 was made of 5 nm and the emitter layer 105 was made of 20 nm. In Figures 2A and 2B, the numbers (0.05, 0.07, 0.10, 0.12) indicate the In composition of the InGaN that constitutes the base layer 104.

[0035] The amount of two-dimensional hole gas (2DHG) generated by polarization depends on the magnitude of the difference in composition at the heterointerface. In this example, the emitter layer 105 is GaN (i.e., In composition = 0), and the collector layer 103 has an In composition of 0.15. Therefore, when the In composition of the base layer 104 is changed within a range of 0.05 to 0.12, the difference in In composition between the emitter layer 105 and the base layer 104 increases as the In composition increases. On the other hand, when the In composition of the base layer 104 is changed within a range of 0.05 to 0.12, the difference in In composition between the base layer 104 and the collector layer 103 decreases. Therefore, as the In composition of the base layer 104 increases, the hole concentration of the two-dimensional hole gas 121 formed in the base layer 104 increases, and the hole concentration of the two-dimensional hole gas 121 formed in the collector layer 103 decreases.

[0036] Focusing on the two-dimensional hole gas 121 formed in the base layer 104, as shown in FIG. 2B, it can be seen that when the In composition is 0.07 or less, the hole concentration peak suggesting the formation of the two-dimensional hole gas 121 is hardly formed. From this, it can be seen that when the emitter layer 105 is made of GaN, it is desirable that the In composition of the InGaN applied to the base layer 104 be at least greater than 0.10. In other words, a more effective effect is achieved when the In composition of the InGaN constituting the base layer 104 is 0.10 or greater.

[0037] Next, the results of calculations of the band change and the accompanying change in sheet carrier density when the In composition of collector layer 103 is changed in the layer structure of the HBT according to the first embodiment will be described with reference to Figures 3A and 3B. Figures 3A and 3B show the effect of the In composition of collector layer 103. The following conditions were used in the calculations:

[0038] The sub-collector layer 102 has an impurity concentration of 10 19 cm -3The base layer 104 was made of InGaN with an In composition of 0.10 and had a thickness of 5 nm. The collector layer 103 was made of 100 nm thick, and the emitter layer 105 was made of 20 nm thick. In Figures 3A and 3B, the numbers (0.10, 0.12, 0.15, 0.17) indicate the In composition of the InGaN that constitutes the collector layer 103.

[0039] When the In composition of the collector layer 103 is the same as that of the base layer 104 (0.10), two-dimensional hole gas is not formed in the collector layer 103 near the interface between the collector layer 103 and the base layer 104. Only when the In composition of the collector layer 103 becomes larger than that of the base layer 104 does two-dimensional hole gas 121 form in the collector layer 103 near the interface between the collector layer 103 and the base layer 104 due to the influence of polarization at the heterointerface caused by the difference in In composition. It can be seen that the concentration of two-dimensional hole gas 121 formed in the collector layer 103 increases as the In composition of the collector layer 103 increases within the range of 0.1 to 0.17.

[0040] For this reason, it is necessary to set the In composition of the InGaN constituting the collector layer 103 to be larger than the In composition of the base layer 104. In other words, "In composition of the InGaN constituting the collector layer 103>In composition of the InGaN constituting the base layer 104>0".

[0041] 3A, when the In composition of the base layer 104 is 0.10, by setting the In composition of the collector layer 103 to 0.15 or more, high concentrations can be obtained in both the two-dimensional hole gas 121 formed in the base layer 104 and the two-dimensional hole gas 121 formed in the collector layer 103. Therefore, it is effective that the In composition of the InGaN constituting the collector layer 103 is 0.05 or more higher than the In composition of the InGaN constituting the base layer 104. In other words, it can be said that a sufficient hole concentration can be obtained under the condition "In composition of the InGaN constituting the collector layer 103 - In composition of the InGaN constituting the base layer 104 ≧ 0.05."

[0042] Next, the manufacturing method of the HBT according to the first embodiment will be described with reference to FIGS. 4A to 4D. .Ma 4A, buffer layer 107, subcollector layer 102, collector-forming layer 203 made of InGaN, base-forming layer 204 made of InGaN, emitter-forming layer 205 made of GaN, and emitter-contact-forming layer 206 made of an n-type nitride semiconductor (e.g., GaN) are grown by crystal growth in this order on substrate 101, with the main surfaces being group V polar planes. For example, the above-mentioned crystal growth can be performed by well-known methods such as molecular beam epitaxy (MBE) or metalorganic vapor phase epitaxy (MOVPE).

[0043] Next, the emitter electrode 111 is formed on the emitter contact formation layer 206. The emitter electrode 111 is formed using a material and under conditions that allow for an ohmic connection with the emitter contact formation layer 206 (emitter contact layer 106). For example, the emitter electrode 111 may have a layered structure of Ti / Al / Ni / Au or the like. An ohmic connection can be formed between the emitter electrode 111 and the emitter contact formation layer 206 by performing a predetermined heat treatment. This heat treatment may deteriorate the morphology and shape of the electrode surface and end portion, so a protective film can be formed to protect the emitter electrode 111 from these problems.

[0044] Next, the emitter contact formation layer 206 is patterned to form a mesa-shaped emitter contact layer 106 as shown in FIG. 4B. For example, the emitter contact formation layer 206 is etched by a self-aligned dry etching process using the emitter electrode 111 as a mask, thereby forming the emitter contact layer 106. In this process, the etching is stopped at the surface of the emitter formation layer 205. The etching process for the emitter contact formation layer 206 made of GaN is performed by from It is not easy to stop the etching on the surface of the emitter-contact-forming layer 205. For this reason, the emitter-contact-forming layer 206 is made of a material other than GaN, which makes it easier to obtain an etching selectivity with respect to GaN.

[0045] It should be noted that if it is anticipated that the electrode shape will become non-uniform due to the heat treatment for forming an ohmic connection, or that the emitter electrode 111 will be damaged by dry etching, the above-described self-alignment treatment does not necessarily have to be employed.

[0046] Next, as shown in Fig. 4C, a base electrode 112 is formed on the emitter-forming layer 205 around the mesa-shaped emitter contact layer 106. The base electrode 112 is formed on and in contact with the emitter-forming layer 205. Furthermore, by performing a heat treatment, an ohmic contact is formed between the base-forming layer 204 (base layer 104) and the base electrode 112 via the emitter-forming layer 205 made of GaN. The base electrode 112 can be made of a material system that allows such an ohmic contact to be formed.

[0047] Because the emitter-forming layer 205 (emitter layer 105) is made of GaN, electrical connection with the base layer 104 is possible without increasing resistance even when the base electrode 112 is formed on the emitter layer 105. Furthermore, because the base electrode 112 is formed with the emitter layer 105 remaining, the two-dimensional hole gas 121 formed in the base layer 104 directly below the emitter layer 105 is not lost, and a high hole concentration and low contact resistance can be achieved even in the region directly below the base electrode 112.

[0048] Next, the emitter-forming layer 205, the base-forming layer 204, and the collector-forming layer 203 are patterned to form a mesa consisting of the emitter layer 105, the base layer 104, and the collector layer 103, as shown in Fig. 4D. Thereafter, the collector electrode 113 is formed, thereby obtaining an HBT as shown in Fig. 1.

[0049] Next, the In composition of the InGaN of the collector layer 103 and the In composition of the InGaN of the base layer 104 will be described with reference to FIGS. 5A, 5B, and 5C.

[0050] 5A, when the base layer is made of GaN (In composition is 0), there is no difference in composition with the emitter layer 105, and therefore, as shown by the black circles, no two-dimensional hole gas is generated in the base layer due to the polarization effect. When the In composition of the collector layer 103 exceeds about 0.10, the hole concentration of the two-dimensional hole gas 121 in the collector layer 103, shown by the black squares, and the overall hole concentration, shown by the black triangles, increase dramatically.

[0051] 5B and 5C, in the base layer 104 made of InGaAs, the overall hole concentration indicated by the black triangles begins to increase when the In composition of the collector layer 103 exceeds about 0.1. For this reason, by setting the I composition of the collector layer 103 to 0.1 or more, the effects of the present invention can be more effectively obtained.

[0052] 5B, by setting the In composition of the base layer 104 to 0.05, the rise in the hole concentration of the two-dimensional hole gas 121 formed in the collector layer 103, indicated by the black squares, shifts toward a higher In composition. Similarly, as shown in FIG. 5C, by setting the In composition of the base layer 104 to 0.1, the rise in the hole concentration of the two-dimensional hole gas 121 formed in the collector layer 103 shifts to a position where the In composition increases to about 0.15, corresponding to the increase in the In composition. Therefore, in order to appropriately set both the hole concentration of the two-dimensional hole gas 121 formed in the collector layer 103 and the hole concentration of the two-dimensional hole gas 121 formed in the base layer 104, it is preferable that the In composition difference between the base layer 104 and the collector layer 103 be 0.05 or more.

[0053] [Embodiment 2] Next, the configuration of a heterojunction bipolar transistor according to a second embodiment of the present invention will be described with reference to FIG. 6. In the second embodiment, the base layer 104 includes a p-type base layer 104a made of InGaN in the center in the thickness direction. The upper base layer 104c above the p-type base layer 104a and the lower base layer 104b below the p-type base layer 104a are undoped or p-type with a lower impurity concentration than the p-type base layer 104a. As described above, the In composition of these three layers is set to be smaller than that of the collector layer 103. The other configurations are the same as those of the first embodiment, and therefore will not be described again.

[0054] With this configuration, the minimum doping concentration of the entire base layer 104 made of InGaN is fixed by the doping of the p-base layer 104a. As a result, even if the base layer 104 is made thick, it is possible to prevent the resistance from increasing and the hole concentration from decreasing.

[0055] [Embodiment 3] Next, the configuration of a heterojunction bipolar transistor according to a second embodiment of the present invention will be described with reference to Fig. 7. In this embodiment, first, a collector contact layer 132 is formed on a substrate 131, and a collector layer 133 is formed on the V polarity surface side of collector contact layer 132. The HBT also includes a base layer 134 formed in contact with the V polarity surface of collector layer 133, an emitter layer 135 formed in contact with the V polarity surface of base layer 134, and an emitter contact layer 136 formed on the V polarity surface side of emitter layer 135.

[0056] The HBT also includes an emitter electrode 141 connected to the emitter contact layer 136, a base electrode 142 formed on the base layer 134, and a collector electrode 143 connected to the collector contact layer.

[0057] In Embodiment 3, when viewed from the side of the substrate 131, an emitter contact layer 136, an emitter layer 135, a base layer 134, a collector layer 133, and a collector contact layer 132 are stacked on the buffer layer 137 with the main surface being a group III polar plane. Further, the collector layer 133 and the collector contact layer 132 are formed in a mesa shape, and the base electrode 142 is formed in contact with the base layer 134 on the side of the collector layer 133.

[0058] Also in Embodiment 3, by adopting the configuration shown below, a two-dimensional hole gas 121 can be generated in the same manner as in Embodiments 1 and 2 described above, and a good ohmic contact between the base layer 134 and the base electrode 142 can be obtained.

[0059] First, the base layer 134 is composed of InGaN having an In composition smaller than that of the collector layer 133. By adopting this configuration, a two-dimensional hole gas 121 due to a polarization electric field can be generated in the collector layer 133 near the interface between the collector layer 133 and the base layer 134.

[0060] The collector contact layer 132 can be composed of a nitride semiconductor (GaN or InGaN) doped with n-type at a high concentration. For example, the collector contact layer 132 can be composed of GaN doped with n-type at a high concentration. The doping concentration of the collector contact layer 132 is set to a relatively high concentration (for example, 5×10 18 cm -3 or higher) in order to achieve an ohmic contact with the collector electrode 143. Note that the collector contact layer 132 corresponds to the sub-collector layer 102 of the HBT according to Embodiment 1.

[0061] The collector layer 133 is composed of In x Ga 1-x N (0 < x < 1). The doping concentration for making the InGaN constituting the collector layer 133 n-type is set to be smaller than that of the collector contact layer 132, for example. For example, the collector layer 133 has an n-type impurity concentration of 10 17 cm-3 The semiconductor layer can be made of n-type InGaN.

[0062] For example, the collector layer 133 can be made of InGaN having an In composition that is 0.05 or more higher than the InGaN that makes up the base layer 134. The base layer 134 can be made of InGaN with an In composition of 0.1 or more. The base layer 134 can have a thickness of, for example, about 4 nm.

[0063] Moreover, the emitter layer 135 is made of InGaN or GaN, which has a smaller In composition than the base layer 134. This difference in In composition allows two-dimensional hole gas 121 to be generated by the polarization effect in the base layer 134 near the interface between the base layer 134 and the emitter layer 135. Furthermore, the emitter layer 135 has a larger band gap than the base layer 134, which suppresses the reverse absorption of holes toward the emitter side, thereby increasing the current gain.

[0064] The emitter contact layer 136 is made of an n-type nitride semiconductor. The emitter contact layer 136 is a layer for forming an ohmic contact with low contact resistance, and the n-type impurity concentration is set to a high concentration. For example, the emitter contact layer 136 has an n-type impurity concentration of 5×10 18 cm -3 In this layer, it is effective to increase the impurity concentration and narrow the band gap in order to achieve ohmic contact with the metal. Therefore, the emitter contact layer 136 is not limited to GaN, but can be made of InGaN, for example.

[0065] The third embodiment has a collector-top (collector-up) structure. Collector electrode 143 is formed on collector contact layer 132, and emitter electrode 141 is formed on emitter contact layer 136 around emitter layer 135. Because collector layer 133, which is used to obtain a breakdown voltage, is disposed on the upper layer of the device, it is desirable to form a passivation or the like to cover the periphery of the device as necessary to prevent a decrease in breakdown voltage.

[0066] As described above, according to the present invention, the collector layer is made of InGaN, the base layer is made of InGaN with a lower In composition than the collector layer, and the emitter layer is made of the base layer and either InGaN or GaN with a lower In composition than the base layer, so that a GaN-based bipolar transistor structure can obtain good ohmic contact between the base layer and the base electrode.

[0067] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention. [Explanation of symbols]

[0068] 101...substrate, 102...subcollector layer, 103...collector layer, 104...base layer, 105...emitter layer, 106...emitter contact layer, 107...buffer layer, 111...emitter electrode, 112...base electrode, 113...collector electrode, 121...two-dimensional hole gas.

Claims

1. a sub-collector layer made of an n-type nitride semiconductor formed on the substrate; a collector layer made of InGaN and made n-type, formed on the V-group polarity surface side of the sub-collector layer; a base layer made of InGaN, the base layer being formed in contact with a group V polar surface of the collector layer and having a smaller In composition than the collector layer; an emitter layer made of InGaN or GaN, the emitter layer being formed in contact with a group V polar surface of the base layer and having a lower In composition than the base layer; an emitter contact layer made of an n-type nitride semiconductor formed on the V-group polar surface side of the emitter layer; an emitter electrode connected to the emitter contact layer; a base electrode connected to the base layer; a collector electrode connected to the sub-collector layer; a two-dimensional hole gas formed in the base layer near the interface between the base layer and the emitter layer and in the collector layer near the interface between the collector layer and the base layer; A heterojunction bipolar transistor comprising:

2. 2. The heterojunction bipolar transistor according to claim 1, A heterojunction bipolar transistor, wherein the collector layer is made of InGaN having an In composition that is 0.05 or more higher than that of the InGaN constituting the base layer.

3. 3. The heterojunction bipolar transistor according to claim 1, The heterojunction bipolar transistor is characterized in that the base layer is made of InGaN having an In composition of 0.1 or more.

4. The heterojunction bipolar transistor according to any one of claims 1 to 3, A heterojunction bipolar transistor, wherein the base layer is made of p-type InGaN.

5. The heterojunction bipolar transistor according to any one of claims 1 to 3, The heterojunction bipolar transistor is characterized in that the base layer includes a p-type base layer made of InGaN at a central portion in a thickness direction.

6. The heterojunction bipolar transistor according to any one of claims 1 to 5, the subcollector layer, the collector layer, the base layer, the emitter layer, and the emitter contact layer are stacked in this order on the substrate with their main surfaces being group V polar surfaces; the emitter contact layer is formed in a mesa shape, The base electrode is formed on the emitter layer on the side of the emitter contact layer. A heterojunction bipolar transistor comprising:

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