Patent Application
20240413227
The present invention relates to a bipolar transistor.
A nitride semiconductor has a large band gap, and therefore is promising as a high-speed and high-withstand-voltage electronic device material. A high-electron-mobility transistor utilizing a high-density sheet carrier generated by polarization of AlGaN/GaN has been studied actively by many research institutions, and has already been put into practical use as an amplification transistor for a communication amplifier and a high-efficiency power device.
A hetero-junction bipolar transistor (HBT) has a device structure capable of achieving a high withstand voltage, using a high-withstand-voltage material for a collector layer, and achieving both high-speed performance and high-withstand-voltage performance. In Group III-V compound semiconductors using InP or GaAs as a substrate material and a Group IV material using SiGe as a base layer, there are many reports that a cut-off frequency, a maximum oscillation frequency, and a high withstand voltage of several hundred GHz are compatible in the HBT structure.
It is expected that, even with a GaN-based material as a wide gap material, by realizing an HBT using this material, a transistor with an even higher withstand voltage and higher speed than conventional Group III-V compound semiconductors can be realized.
However, it is difficult to make a nitride semiconductor such as GaN into a p-type at a high concentration as will be described below. First, in the nitride semiconductor, ionization energy of the impurity functioning as an acceptor is very great. In addition, the nitride semiconductor is grown by a general growth technique such as a MOCVD, but there is an essential problem that a doped dopant (Mg, Zn, or the like) is inactivated by H (hydrogen) contained in a carrier gas or a raw material at the time of p-type doping, and the hole concentration cannot be increased.
In order to increase the HBT speed, although it is necessary to obtain a high concentration, a low resistance, and a low contact resistance of both the n-type and the p-type, as described above, it is very difficult to achieve high speed in the HBT using the nitride semiconductor with which it is difficult to achieve a high concentration of p-type.
In the semiconductor device using the nitride semiconductor such as GaN, one technique for obtaining a high hole concentration has been devised for manufacturing a device with an N polar plane as a principal plane orientation. The nitride semiconductor is a material having polarization in a c-axis direction, and generally crystal growth is carried out in the (+c-axis direction) plane orientation called a Group III polar plane to manufacture a device. In the case of a Group III polar plane, when AlGaN is grown on GaN, the band is bent by an electric field due to a difference in magnitude of spontaneous polarization between materials and a polarization electric field generated by distortion generated in an AlGaN layer, and a two-dimensional electron gas is generated at an interface between AlGaN and GaN. A GaN channel HEMT structure has been realized by utilizing this structure, and a high-frequency device using this structure has already been put into practical use.
On the other hand, in the configuration in which the principal surface is an N polar (Group V polarity) surface, the Group III polarity surface is reversed. In this case, the direction of the electric field generated by polarization is reversed from the case of the Group III polar plane. For example, in a case where AlGaN is formed on GaN with an N polar plane as a principal plane orientation, a two-dimensional hole gas is generated by a polarization electric field at an AlGaN/GaN interface (see NPL 1). In the HBT using GaN having the N polar plane as the principal plane orientation, the above-mentioned two-dimensional hole gas is utilized to overcome the problem related to the p-type doping control.
However, in the HBT that utilizes a two-dimensional hole gas formed using an N polar plane, there is a problem to be overcome with respect to ohmic contact between the base layer and the base electrode. In the HBT structure having an N polar plane as a principal plane orientation, a technique for increasing the concentration of a p-base layer 304a by obtaining a two-dimensional hole gas 321 at an interface between an emitter layer 305 made of AlGaN and the p-base layer 304a made of p-type GaN is used as shown in
The HBT includes a buffer layer 307 formed on a substrate 301, a sub-collector layer 302 formed on the buffer layer 307 and made of an n-type nitride semiconductor, a collector layer 303 formed on the sub-collector layer 302 and made of n-type GaN, a p-base layer 304a formed on the collector layer 303 and made of p-type GaN, a base layer 304b formed on the p-base layer 304a and made of undoped GaN, an emitter layer 305 formed on the base layer 304b, and an emitter cap layer 306 formed on the emitter layer 305 and made of an n-type nitride semiconductor.
The HBT has an emitter electrode 311 formed on the emitter cap layer 306, a base electrode 312 formed on the base layer lateral to the emitter layer 305, and a collector electrode 313 connected to the sub-collector layer 302. In the structure of the emitter top shown in
In the HBT, since the base layer is highly concentrated by the two-dimensional hole gas 321, it is important that the emitter layer 305 also be immediately under the base electrode 312, but the emitter layer 305 made of AlGaN has a high resistance. Therefore, as shown in
However, as shown in
As described above, in the GaN-based HBT structure having the N polarity as the principal plane orientation, there is a problem that it is difficult to obtain good ohmic contact between the base layer and the base electrode. Although the emitter layer plays an important role for generating a two-dimensional hole gas, since it has a high resistance, the ohmic resistance at the time of electrode formation is increased. However, if the emitter layer immediately below the base electrode is completely removed, the two-dimensional hole gas immediately below the emitter is lost, causing an increase in the base resistance and an increase in the base contact resistance.
Embodiments of the present invention have been made to solve the above-mentioned problems, and an object of embodiments of the present invention is to obtain good ohmic contact between a base layer and a base electrode in a GaN-based bipolar transistor structure having N polarity as a principal plane orientation.
A bipolar transistor according to embodiments of the present invention includes a sub-collector layer which is formed on a substrate and made of an n-type nitride semiconductor; an n-type collector layer which is formed on the sub-collector layer and made of InGaN; a base layer which is formed on the collector layer and made of GaN; a mesa-shaped emitter layer which is made of a nitride semiconductor containing Al formed on the base layer; an emitter cap layer which is formed on the emitter layer and made of an n-type nitride semiconductor; an emitter electrode formed on the emitter cap layer; a base electrode which is formed on the base layer beside the emitter layer and is ohmically connected to the base layer; a collector electrode connected to the sub-collector layer; and a two-dimensional hole gas which is formed in each of the base layer near an interface between the base layer and the collector layer and the collector layer near an interface between the collector layer and the base layer, in which the sub-collector layer, the collector layer, the base layer, the emitter layer, and the emitter cap layer are formed on the substrate with principal surfaces being Group V polar planes.
As described above, according to embodiments of the present invention, since the base layer made of GaN is formed on the collector layer made of InGaN with each principal surface being a Group V polar plane, and an emitter layer made of the nitride semiconductor containing Al is formed on the base layer, good ohmic contact between the base layer and the base electrode can be obtained in a GaN-based bipolar transistor structure having N polarity as a principal plane orientation.
Hereinafter, a bipolar transistor (hetero-junction bipolar transistor: HBT) according to an embodiment of the present invention will be described with reference to
The substrate 101 is used for forming a nitride semiconductor device, and the material of the substrate 101 is selected so that the N polar plane is set to the principal plane orientation (a state in which the principal surface is set to the group V polar plane). For example, as the substrate 101, sapphire, a C-plane SiC substrate, an N polar GaN, an N polar AlN substrate, or the like can be used.
As the buffer layer 107, when the substrate 101 is a sapphire substrate, a nitride layer on the substrate surface formed by subjecting the surface of the substrate 101 to high-temperature heat treatment under a raw material gas atmosphere such as ammonia can be set as the buffer layer 107. A nitride semiconductor having an N polar plane as a principal plane orientation can be crystal-grown on the buffer layer 107 formed by nitriding. On the other hand, when a GaN single crystal substrate or an AlN single crystal substrate having an N polar plane as a principal plane orientation is used as the substrate 101, a nitride semiconductor having an N polar plane as a principal plane orientation can be crystal-grown without using a special buffer layer.
The sub-collector layer 102 can be made of a highly n-type doped nitride semiconductor (GaN or InGaN). For example, the sub-collector layer 102 can be made of n-type doped GaN with a high concentration. Since the sub-collector layer 102 also functions as a contact layer for realizing ohmic contact with a collector electrode 113 to be described later, the doping concentration is set to a relatively high concentration (for example, 5×1018 cm−3 or more). Further, the sub-collector layer 102 grows relatively thick in a range that does not affect the device characteristics. For example, it is desirable that the thickness of the sub-collector layer 102 be set to at least 1 μm or more to function as a buffer layer for improving the crystal quality.
The collector layer 103 is made of InxGa1-xN (0<x<1) in which the In composition is always set to be greater than 0. The doping concentration for making InGaN constituting the collector layer 103 n-type is set to be smaller than that of the sub-collector layer 102. For example, the collector layer 103 can be made of n-type InGaN having an n-type impurity concentration of about 1017 cm−3. As will be described later, the collector layer 103 can be made of InGaN with an In composition of 0.05 or more. The thickness of the collector layer 103 can be about 50 nm.
The HBT includes a base layer 104 formed on the collector layer 103, an emitter layer 105 formed on the base layer 104, and an emitter cap layer 106 formed on the emitter layer 105. The emitter layer 105 and the emitter cap layer 106 have a mesa shape.
The base layer 104 is formed of GaN. In this example, the base layer 104 is provided with a p-base layer 104a made of p-type GaN at a central part in the thickness direction. As is well known, although GaN has a certain limitation in p-type formation, it is desirable to form the p-base layer 104a from GaN into which p-type impurities are introduced at a high concentration within a possible range in consideration of crystal quality or the like. An upper base layer 104c on the upper side of the p-base layer 104a and a lower base layer 104b on the lower side are undoped or become a p-type with an impurity concentration lower than that of the p-base layer 104a. The lower base layer 104b can have a thickness of about 2 nm. The p-base layer 104a can have a thickness of about 5 nm. The upper base layer 104c can have a thickness of about 2 nm.
The emitter layer 105 is made of a nitride semiconductor containing Al. The emitter layer 105 can be made of AlGaN (Al0.25Ga0.75N). The thickness of the emitter layer 105 can be about 20 nm.
An emitter cap layer 106 is made of an n-type nitride semiconductor. The emitter cap layer 106 is a layer for forming an ohmic contact having a low contact resistance, and the n-type impurity concentration is set to a high concentration. For example, the emitter cap layer 106 can have an n-type impurity concentration of 5×1018 cm−3 or more. In this layer, it is also effective to increase the concentration of impurities and to narrow the band gap for ohmic contact with metal. Therefore, the emitter cap layer 106 may be made of InGaN or the like, for example, without being limited to GaN. The thickness of the emitter cap layer 106 can be about 100 nm.
Further, the sub-collector layer 102, the collector layer 103, the base layer 104, the emitter layer 105, and the emitter cap layer 106 can be formed on the substrate 101 in a state in which the principal surface is a group V polar plane.
The HBT includes an emitter electrode 11 formed on the emitter cap layer 106, a base electrode 112 formed on the base layer 104 lateral to the emitter layer 105 and ohmically connected to the base layer 104, and a collector electrode 113 connected to the sub-collector layer 102. The base electrode 112 can be formed in contact with the upper part of the base layer 104 around the emitter layer 105 formed in a mesa shape.
The HBT having the above-described structure according to the embodiment includes two-dimensional hole gases 121 formed in each of the base layer 104 in the vicinity of the interface between the base layer 104 and the emitter layer 105 and in the collector layer 103 in the vicinity of the interface between the collector layer 103 and the base layer 104.
The HBT according to an embodiment in which the two-dimensional hole gas 121 is formed will be described in more detail below.
As shown in the band diagram of
First, the two-dimensional hole gas 121 is generated by the influence of a polarization electric field caused by a hetero structure in each of an interface between the collector layer 103 and the base layer 104 (the lower base layer 104b) and an interface between the emitter layer 105 and the base layer 104 (the upper base layer 104c).
At the interface between the emitter layer 105 and the base layer 104 (the upper base layer 104c), the band is raised upward because the magnitude of polarization of each layer is different. In this state, since the p-base layer 104a exists in the central part (immediately under the upper base layer 104c) of the base layer 104, the energy of the valence band at the interface exceeds Fermi energy (Fermi level) and the two-dimensional hole gas 121 of high concentration is formed.
Next, attention is paid to the p-base layer 104a, the lower base layer 104b, and the collector layer 103. First, InGaN has a larger spontaneous polarization than GaN. Further, since the collector layer 103 made of InGaN exists between the p-base layer 104a and the sub-collector layer 102, spontaneous polarization acts in a direction for promoting an internal electric field of the collector layer 103. Further, the lower base layer 104b made of undoped GaN exists between the p-base layer 104a and the collector layer 103. Therefore, the band is raised upward even at the interface between the lower base layer 104b and the collector layer 103. As a result, the two-dimensional hole gas 121 having a high concentration is formed on the interface.
On the other hand, in the N polar plane GaN hetero-junction bipolar transistor of the related art, as shown in a band diagram of
As compared with a structure of the related art, since an interface is formed by the lower base layer 104b made of GaN and the collector layer 103 made of InGaN in embodiments of the present invention, as described above, a two-dimensional hole gas is generated by the effect of hetero-junction, and as a result, the hole concentration of the two-dimensional hole gas 121 formed in the collector layer 103 in the vicinity of the base layer can be set higher.
Next, the effects of embodiments of the present invention when manufacturing the device will be described in detail. In an HBT structure having an N polar plane as a principal plane orientation, a technique for increasing the concentration of the base layer by forming a two-dimensional hole gas at an interface between the emitter layer and the base layer is used. In such an emitter top structure, ohmic contacts between the metal electrode and the emitter contact layer, the base layer, and the sub-collector layer need to be formed from the upper surface side (front surface side) of the device.
However, since the emitter layer made of AlGaN has a high resistance, when the base electrode is formed from immediately above the emitter, the ohmic contact resistance becomes high (
However, in embodiments of the present invention, even if the emitter layer 105 does not exist at all at the place where the base electrode 112 is formed, the two-dimensional hole gas 121 exists also at the interface between the collector layer 103 and the base layer 104 (the lower base layer 104b). Therefore, even if the emitter layer 105 at the position where the base electrode 112 is formed is completely removed and the base electrode 112 is formed in contact with the upper part of the base layer 104 (the upper base layer 104c), a decrease in the hole concentration of the base layer 104 in this region can be suppressed. As a result, good ohmic contact between the base layer 104 and the base electrode 112 can be realized.
Next, the results obtained by performing band calculation and calculation of sheet carrier density in the HBT layer structure according to the above-described embodiment will be described with reference to
The sub-collector layer 102 is made of n-type GaN having an impurity concentration of about 1019 cm−3, and the collector layer 103 is made of InGaN and has a thickness of 50 nm. In
The lower base layer 104b is made of undoped GaN and has a thickness of 2 nm, the p-base layer 104a is made of p-type GaN having an impurity concentration of about 1019 cm−3 and has a thickness of 5 nm, and the upper base layer 104c is made of undoped GaN and has a thickness of 2 nm. Further, the emitter layer 105 is made of Al0.25Ga0.75N and has a thickness of 20 nm, and the emitter cap layer 106 is made of n-type GaN with an impurity concentration of about 1019 cm−3 and has a thickness of 100 nm.
Under the condition that the In composition of the collector layer 103 is 0, the influence of polarization caused by a hetero structure is not generated on the interface between the lower base layer 104b (base layer 104) and the collector layer 103 made of GaN, and the band of the interface is not raised. Therefore, as indicated by a dotted line in
When the In composition of the collector layer 103 is higher than 0.05, the band at the interface between the lower base layer 104b (base layer 104) and the collector layer 103 rises due to the influence of the polarization electric field caused by the hetero structure, and as shown in
By adjusting the above-mentioned layer structure (thickness, composition, doping concentration, etc.), the polarization effect can be obtained even with a lower In composition, and the high hole concentration can also be obtained. It is important that the collector layer 103 made of InGaN, the lower base layer 104b, and the p-base layer 104a are laminated in this order. In addition, the orientation of the polarity of the layer structure is important, and it is important that they are laminated in this order from the substrate side for N polarity, and they are laminated in the opposite order for group III polarity.
Although the configuration in which the p-base layer 104a is provided at the central part in the thickness direction of the base layer 104 is provided in the above description, this is not the necessary configuration. As shown in
The results obtained by performing the band calculation and the sheet carrier density calculation in the HBT layer structure of the case of this configuration will be described with reference to
The sub-collector layer 102 is made of n-type GaN having an impurity concentration of about 1019 cm−3, the collector layer 103 is made of In0.1Ga0.9N and has a thickness of 50 nm, and the base layer 104 is made of undoped GaN and has a thickness of 4 nm. The emitter layer 105 is made of Al0.25Ga0.75N and has a thickness of 20 nm, and the emitter cap layer 106 is made of n-type GaN with an impurity concentration of about 1019 cm−3 and has a thickness of 100 nm.
In this structure, no p-base layer is introduced. Therefore, the structure is simply formed by laminating the emitter layer 105 made of undoped AlGaN, the base layer 104 made of undoped GaN, and the collector layer 103 made of undoped InGaN. Even with such a structure that does not use the p-type doping layer, a two-dimensional hole gas is generated in the collector layer 103 near the interface due to the influence of a polarization electric field caused by the hetero structure between the collector layer 103 and the base layer 104, and a high hole concentration can be obtained. In the hetero structure of the emitter layer 105 and the base layer 104, a two-dimensional hole gas is generated in the base layer 104 near the interface due to the influence of the polarization electric field caused by the hetero structure of the emitter layer 105 and the base layer 104.
It has been reported that GaN is a material which has a high ionization energy of a dopant and is difficult to increase a hole concentration, even if p-type doping is performed by Mg or the like, and that the dopant is inactivated by the influence of H used in a raw material or a carrier gas during growth. Due to these reasons, introduction of the p-type layer may be a significant restriction from the viewpoint of device process and crystal quality. However, since the present structure can achieve a high hole concentration in the base layer without using any p-type layer, an HBT structure can be achieved in a state of high crystal quality and high mobility, and further improvement of high frequency characteristics can be expected.
As described above, according to embodiments of the present invention, the base layer made of GaN is formed on the collector layer made of InGaN with the principal surface thereof being a group V polar plane, and the emitter layer made of a nitride semiconductor containing Al is formed on the base layer. As a result, two-dimensional hole gas is formed in each of the base layer near the interface between the base layer and the emitter layer and the collector layer near the interface between the collector layer and the base layer, and good ohmic contact between the base layer and the base electrode can be obtained in a GaN-based bipolar transistor structure having N polarity as a principal plane orientation.
Note that it is clear that the embodiments of the present invention are not limited to the embodiments described above and within the technical concept of the present invention and many modifications and combinations can be implemented by those skilled in the art.
This application is a national phase entry of PCT Application No. PCT/JP2021/042008, filed on Nov. 16, 2021, which application is hereby incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/042008 | 11/16/2021 | WO |
