JP5223201B2 - Field effect transistor - Google Patents

Description

  The present invention relates to a field effect transistor (FET), and more particularly to a FET structure having a diamond semiconductor as a main material.

  Diamond semiconductors have a high output with a breakdown electric field strength 3 times that of gallium nitride (GaN), 20 times that of gallium arsenide (GaAs), 4 times that of silicon carbide (SiC), or 40 times that of GaAs. Excellent physical properties for device applications. Nevertheless, since the n-type conductive layer cannot be formed by impurity doping due to the hardness and denseness of the diamond crystal, it has been difficult to put an electronic device using a diamond semiconductor into practical use.

  Almost all FETs using a diamond semiconductor as a channel material that have been reported in the past have been hole-conducting. This is because the hydrogen-terminated surface of diamond that is spontaneously formed is used as a carrier supply source when a diamond crystal is grown using a chemical vapor deposition method, and the hole mobility is low. As a result, there are problems that high frequency operation and high current density are difficult. Further, since the threshold voltage depends on the interface state of the hydrogen termination surface, there is a problem that it is difficult to control the threshold voltage.

  In order to solve such a problem, an FET using a heterojunction of diamond and a different kind of semiconductor has been reported. FIG. 15 is a diagram showing a cross-sectional structure of an FET using a heterojunction according to the prior art. Such an FET is reported in Patent Document 1 by Kaji and Kobayashi.

In FIG. 15, 200 is a substrate, 201 is a diamond layer forming a channel, 202 is an electron made of aluminum nitride (AlN) to which an n-type impurity (Si) is added or boron nitride (BN) to which an n-type impurity is added. It is a supply layer. As the impurities in the electron supply layer 202 are activated, a two-dimensional electron gas (e) is formed in the vicinity of the interface with the electron supply layer 202 in the diamond layer 201. A source electrode 205 and a drain electrode 206 are formed on the electron supply layer 202 and are in ohmic contact. A gate electrode 207 is formed in a portion sandwiched between the source electrode 205 and the drain electrode 206 on the electron supply layer 202 and is in Schottky contact. Such an FET is an electron conduction type, and can operate at a high frequency and have a high current density due to the high mobility of electrons. Further, since the threshold voltage is determined by the impurity concentration and thickness of the electron supply layer 202, the threshold voltage can also be controlled. FIG. 16 shows an energy band diagram of an FET using Si-added AlN.
JP 2002-324812 A

In the conventional FET shown in FIG. 15, AlN or BN to which an n-type impurity is added is used as the electron supply layer. However, it is known that due to the large band gap of AlN and BN, impurities in AlN and BN tend to form deep levels, and the activation rate of added impurities is low. For this reason, in order to obtain a practical two-dimensional electron concentration as an FET, impurities must be doped at an extremely high concentration. For example, if AlN is doped with silicon (Si) to supply carrier electrons of 1 × 10 ¹⁸ cm ⁻³ , high concentration of Si of 10 ²⁰ cm ⁻³ must be doped. Such a low activation rate of impurity atoms affects the uniformity and reproducibility of the effective impurity concentration, which causes variations in device characteristics within the wafer surface and between lots.

  The object of the present invention is to eliminate the above-mentioned problems in FETs mainly composed of diamond semiconductors, enable high-frequency operation, high current density, easy control of threshold voltage, and variations in device characteristics within the wafer surface. An object of the present invention is to provide an FET in which the problem of variation between lots is solved.

  In order to achieve the above object, according to the present invention, in a field effect transistor in which a substrate, a diamond semiconductor layer, and a compound semiconductor layer are formed in this order, the diamond semiconductor layer is composed of (111) plane diamond, There is provided a field effect transistor comprising a (0001) plane hexagonal compound semiconductor or a (111) plane cubic compound semiconductor.

  According to the present invention, at least the spontaneous polarization effect, i.e., the spontaneous polarization effect or the positive polarization effect due to the spontaneous polarization effect and the piezopolarization effect, is present at the interface between the compound semiconductor layer as the electron supply layer and the diamond semiconductor layer. A two-dimensional electron gas is generated near the interface between the electron supply layer and the diamond semiconductor layer as well as having a fixed charge. Due to the high mobility of two-dimensional electrons, high frequency operation and high current density are possible. Since the threshold voltage is determined by the thickness of the electron supply layer, the threshold voltage can be easily controlled. Furthermore, since the polarization charge density is determined by the piezoelectric coefficient specific to the material, the reproducibility and uniformity are excellent, and problems such as in-plane variation in element characteristics and variation between lots are solved. This greatly contributes to the high-frequency performance and power performance of the FET.

  The present invention is characterized in that, when forming a heterojunction between a diamond semiconductor layer and a compound semiconductor layer, the plane orientation and the crystal form of the compound semiconductor are defined. In the above-mentioned Patent Document 1, there is no description or suggestion that the definition of the plane orientation or the crystal form of the compound semiconductor is an important requirement. When AlN and BN are selected as the compound semiconductor layer, the present invention It is a selective invention. AlN and BN are materials having a larger band gap than the diamond layer. However, even in other compound semiconductors, particularly materials having a smaller band gap than the diamond layer, the plane orientation and the crystal shape of the compound semiconductor should be controlled. Thus, a good heterojunction with the diamond layer can be formed. When a compound semiconductor layer with a band gap smaller than that of the diamond layer is stacked, a channel formed by a two-dimensional electron gas is formed near the interface with the diamond layer in the compound semiconductor layer, but some of the carriers travel through the diamond layer. As in the case where a channel is formed in the diamond layer, high-frequency operation and high current density are possible due to the high mobility of the two-dimensional electron gas. In particular, in the present invention, the polarization charge is formed by forming a spontaneous polarization effect or a positive fixed charge resulting from the spontaneous polarization effect and the piezoelectric polarization effect at the interface between the compound semiconductor layer, which is an electron supply layer, and the diamond layer. The density is determined by the piezoelectric coefficient specific to the material, and an FET having excellent reproducibility can be obtained.

  The III-V hexagonal semiconductor has a structure in which group III atomic planes and group V atomic planes are alternately arranged in the (0001) crystal axis direction. Therefore, when a (0001) plane thin film is formed, its upper interface and lower Polarization charges are generated at the interface. Similarly, since the group III-V cubic semiconductor has a structure in which group III atomic planes and group V atomic planes are alternately arranged in the (111) crystal axis direction, when a (111) plane thin film is formed, Polarization charges are generated at the interface and the lower interface.

  In the present invention, the two-dimensional electron gas which is a carrier is generated by utilizing the polarization charge formed in the (0001) plane hexagonal semiconductor thin film or the (111) plane cubic thin film. In order to stably form a hexagonal thin film or a (111) plane cubic thin film, it is known that a (111) plane cubic semiconductor may be used as a growth substrate. That is, by performing crystal growth on (111) plane diamond, a (0001) plane hexagonal thin film or a (111) plane cubic thin film can be spontaneously oriented and formed. This is because the (111) direction of the cubic crystal and the (0001) direction of the hexagonal crystal are similar in crystal structure. That is, both the (111) direction of the cubic crystal and the (0001) direction of the hexagonal crystal are parallel to one of the covalent bonds composed of sp3 hybrid orbits extending from the constituent atoms in the apex direction of the tetrahedron. Since the bonding directions are the same, a (0001) plane hexagonal thin film or a (111) plane cubic thin film can be stably grown on a cubic (111) plane diamond.

  The stable crystal structure of GaAs, AlAs, InAs, GaP, AlP, and InP is only a cubic crystal, and there is no stable hexagonal crystal structure. Therefore, when these compound semiconductors are selected as the electron supply layer, these compound semiconductors may be crystal-grown on the (111) plane diamond by an ordinary method. Thereby, a (111) plane cubic crystal can be formed automatically. Here, the normal method is, for example, a metal-organic chemical vapor deposition (MOCVD) method or a molecular beam epitaxy (MBE) method.

  On the other hand, BN, GaN, AlN, InN, and SiC have not only a hexagonal crystal structure but also a cubic crystal structure as a stable crystal structure. Therefore, when these compound semiconductors are selected as the electron supply layer, care must be taken in crystal growth.

For example, since hexagonal GaN is more stable than cubic GaN, an MOCVD method for growing in a thermal equilibrium state is used. For example, trimethyl gallium (TMG) is used as the group III material, and ammonia (NH ₃ ) is used as the group V material. By using (111) plane diamond as a growth substrate and setting the substrate temperature between about 900 ° C. and about 1100 ° C. at which NH ₃ decomposes, a more stable hexagonal GaN thin film can be formed.

On the other hand, since cubic GaN is unstable compared with hexagonal GaN, crystal growth needs to be performed in a lower temperature non-equilibrium state. For this reason, in order to form cubic GaN, the MBE method that allows growth in a non-equilibrium state is used. For example, metal gallium (Ga) is used as the group III material, and nitrogen (N ₂ ) that is plasmatized by applying high frequency is used as the group V material. By using (111) plane diamond as the growth substrate and setting the substrate temperature between about 400 ° C. and about 600 ° C., a cubic GaN thin film can be formed.

For example, a plasma-enhanced chemical vapor deposition (PECVD) method is used to form the (111) plane diamond crystal. For example, hydrocarbon (CH ₄ ) that has been plasmatized by applying a high frequency is used as the source gas. For example, a (111) plane diamond crystal can be formed by using a (111) plane diamond substrate and setting the substrate temperature between about 1000 ° C. and about 1200 ° C. Other substrates include (111) plane Si substrate, (0001) plane hexagonal SiC substrate, (111) plane cubic SiC substrate, (0001) plane hexagonal GaN substrate, and (111) plane cubic GaN substrate. A (0001) plane sapphire substrate or the like can be used.

  Hereinafter, the present invention will be specifically described with reference to examples.

(First embodiment)
FIG. 1 is a diagram showing a cross-sectional structure of a first embodiment of an FET according to the present invention.

  In FIG. 1, 10 is a substrate, 11 is a (111) plane diamond layer, 12 is a carrier supply layer made of (0001) plane undoped hexagonal boron nitride (h-BN), 13 is a two-dimensional electron gas, and 15 is a source electrode. , 16 are drain electrodes, and 17 is a gate electrode.

  Such an FET is manufactured as follows. On the substrate 10, a (111) plane undoped diamond layer 11 and a (0001) plane undoped h-BN layer 12 are sequentially grown by, for example, PECVD and MOCVD. Here, the layer thickness of the diamond layer 11 is between 10 nm and 10 μm. In this embodiment, for example, it is set to 2 μm. The layer thickness of the h-BN layer 12 is between 1 nm and 100 nm. In this embodiment, for example, it is set to 20 nm.

  On the h-BN layer 12, for example, a metal such as titanium (Ti) / gold (Au) is vapor-deposited and alloyed to form the source electrode 15 and the drain electrode 16, respectively, and make ohmic contact. Next, a gate electrode 17 is formed by evaporating a metal such as aluminum (Al), for example, on a portion sandwiched between the source electrode 15 and the drain electrode 16 on the surface of the h-BN layer. Take Schottky contact. Thus, the semiconductor device as shown in FIG. 1 was produced.

Here, the lattice constant of diamond is a = 0.356 nm, but the interval between (111) plane carbon (C) atoms is a / 2 ^1/2 = 0.252 nm. On the other hand, the a-axis length of h-BN is 0.25 nm. Since the difference between the two is as small as 0.8%, when a (0001) plane h-BN layer is grown on a (111) plane diamond, if it is below the critical film thickness of dislocation generation, it becomes a strained layer in which tensile strain is generated. A good heterojunction can be formed. The film thickness 20 nm of the h-BN layer 12 of this example is not more than the critical film thickness for generating dislocations.

The III-V hexagonal semiconductor has a structure in which group III atomic planes and group V atomic planes are alternately arranged in the (0001) crystal axis direction. Therefore, when a (0001) plane thin film is formed, its upper interface and lower It is known that polarization charges are generated at the interface. Here, the polarization effect is caused by spontaneous polarization generated when the bond between the group III atom and the group V atom deviates from the ideal tetrahedral ideal structure, and the difference in the a-axis length between the thin film and the substrate. There is piezo polarization. spontaneous polarization density of h-BN was calculated to 0.029C / ^{m 2,} piezo polarization density of the h-BN on diamond is calculated to 0.013C / ^{m 2.} Therefore, a positive charge of 0.042 C / m ² is generated at the heterointerface between the diamond layer 11 and the h-BN layer 12 based on spontaneous polarization and piezoelectric polarization. This charge corresponds to a two-dimensional electron having an areal density of 2.6 × 10 ¹³ cm ⁻² .

  FIG. 2 is an energy band diagram under the gate in this embodiment.

As described above, positive charges having a surface density of 0.042 C / m ² are generated at the heterointerface between the diamond layer 11 and the h-BN layer 12 due to spontaneous polarization and piezoelectric polarization. The band gap of h-BN is 5.9 eV, which is larger than that of diamond. Therefore, a two-dimensional electron gas 13 is generated in the vicinity of the heterointerface with the h-BN layer 12 in the diamond layer 11 to form an n-type channel. Since the h-BN layer 12 has a larger band gap than diamond, the h-BN layer 12 acts as a good gate insulating layer, and the gate leakage current is reduced.

  In such an FET, due to the high mobility of the two-dimensional electron gas 13, high frequency operation and high current density are possible. Due to the large band gap of h-BN, a high concentration two-dimensional electron gas can be accumulated and the gate leakage current is reduced. Further, since the threshold voltage is determined by the thickness of the h-BN layer 12 that is the electron supply layer, the threshold voltage can be easily controlled. Furthermore, since the polarization charge density is determined by the piezoelectric coefficient specific to the material, the reproducibility and uniformity are excellent, and problems such as in-plane variation of element characteristics and lot-to-lot variation are solved.

(Second embodiment)
FIG. 3 is a diagram showing a cross-sectional structure of a second embodiment of the FET according to the present invention.

The feature of this embodiment is that the (0001) plane undoped h-BN layer in the first embodiment is replaced with a (0001) plane n-type h-BN layer 22. Here, as the n-type impurity added to the h-BN layer, for example, silicon (Si), germanium (Ge), tin (Sn), lead (Pb), or the like is used, and the impurity concentration is 1 × 10 ¹⁸ cm ^{−. Between 3} and 1 × 10 ²² cm ⁻³ .

  FIG. 4 is an energy band diagram under the gate in this embodiment.

As described above, positive fixed charges having a surface density of 0.042 C / m ² are generated at the heterointerface between the diamond layer 11 and the h-BN layer 22 due to spontaneous polarization and piezoelectric polarization. The band gap of h-BN is 5.9 eV, which is larger than that of diamond. For this reason, the two-dimensional electron gas 13 is generated in the vicinity of the heterointerface with the h-BN layer 22 in the diamond layer 11 to form an n-type channel. Since the electron supply effect by impurity doping is added to the electron supply effect by spontaneous polarization and piezo polarization, the current density increases as compared with the first embodiment. Due to the positive fixed charge inside the h-BN layer 22, the conduction band energy has a downwardly convex shape. For this reason, the energy barrier with respect to an electron becomes low, and the contact resistance in the source electrode 15 and the drain electrode 16 is reduced.

  In such an FET, due to the high mobility of the two-dimensional electron gas 13, high frequency operation and high current density are possible. Since the threshold voltage is determined by the thickness of the h-BN layer 22 that is an electron supply layer and the impurity concentration, the threshold voltage can be easily controlled. Furthermore, the contact resistance is reduced by the fixed charge resulting from the ionization of the n-type impurity inside the h-BN layer 22.

(Third embodiment)
FIG. 5 is a diagram showing a cross-sectional structure of a third embodiment of the FET according to the present invention.

  The feature of this embodiment is that the (0001) plane undoped h-BN layer 12 in the first embodiment is replaced with a (0001) plane undoped hexagonal aluminum nitride (h-AlN) layer 32. Here, the layer thickness of the h-AlN layer 32 is between 1 nm and 100 nm. In this embodiment, for example, it is set to 10 nm.

As described above, the C atom interval of the (111) plane diamond is a / 2 ^1/2 = 0.252 nm, whereas the a-axis length of h-AlN is 0.31 nm. The difference between the two is large, and the layer thickness 10 nm of the h-AlN layer 32 of the present embodiment is equal to or greater than the critical film thickness of dislocation generation. For this reason, the h-AlN layer 32 is lattice-relaxed.

The spontaneous polarization density of h-AlN is calculated to be 0.081 C / m ² . On the other hand, since the h-AlN layer 32 of this embodiment is lattice-relaxed, piezoelectric polarization does not occur. Therefore, a positive charge of 0.081 C / m ² is generated at the heterointerface between the diamond layer 11 and the h-BN layer 12 based on spontaneous polarization. This charge corresponds to a two-dimensional electron having a surface density of 5.1 × 10 ¹³ cm ⁻² .

  The energy band diagram under the gate of this embodiment is the same as that of the first embodiment (FIG. 2).

As described above, a positive charge of 0.081 C / m ² is generated at the heterointerface between the diamond layer 11 and the h-AlN layer 32 due to spontaneous polarization. The band gap of h-AlN is 6.2 eV, which is larger than 5.5 eV of diamond. For this reason, the two-dimensional electron gas 13 is generated near the hetero interface with the h-AlN layer 32 in the diamond layer 11 to form an n-type channel. Since the h-AlN layer 32 has a larger band gap than diamond, the h-AlN layer 32 acts as a good gate insulating layer, and the gate leakage current is reduced.

  Such an FET enables high-frequency operation and high current density due to the high mobility of the two-dimensional electron gas 13. The gate leakage current is reduced due to the large band gap of h-AlN. Further, since the threshold voltage is determined by the thickness of the h-AlN layer 32 that is an electron supply layer, the threshold voltage can be easily controlled. Furthermore, since the polarization charge density is determined by the piezoelectric coefficient specific to the material, the reproducibility and uniformity are excellent, and problems such as in-plane variation of element characteristics and lot-to-lot variation are solved.

(Fourth embodiment)
FIG. 6 is a diagram showing a cross-sectional structure of a fourth embodiment of the FET according to the present invention.

  The feature of this embodiment is that the (0001) plane undoped h-BN layer 12 in the first embodiment is replaced with a (0001) plane undoped hexagonal gallium nitride (h-GaN) layer 42. Here, the layer thickness of the h-GaN layer 42 is between 1 nm and 100 nm. In this embodiment, for example, it is set to 10 nm.

As described above, the C atom spacing of the (111) plane diamond is a / 2 ^1/2 = 0.252 nm, whereas the a-axis length of h-GaN is 0.32 nm. The difference between the two is large, and the layer thickness 10 nm of the h-GaN layer 42 of the present embodiment is equal to or greater than the critical film thickness at which dislocation occurs. For this reason, the h-GaN layer 42 is lattice-relaxed.

The spontaneous polarization density of h-GaN is calculated as 0.029 C / m ² . On the other hand, since the h-GaN layer 42 of this embodiment is lattice-relaxed, piezoelectric polarization does not occur. Therefore, a positive charge of 0.029 C / m ² is generated at the heterointerface between the diamond layer 11 and the h-GaN layer 42 based on spontaneous polarization. This charge corresponds to a two-dimensional electron having a surface density of 1.8 × 10 ¹³ cm ⁻² .

  FIG. 7 is an energy band diagram under the gate in this embodiment.

As described above, positive charges having a surface density of 0.029 C / m ² are generated at the heterointerface between the diamond layer 11 and the h-GaN layer 42 due to spontaneous polarization. Further, the band gap of h-GaN is 3.4 eV, which is smaller than 5.5 eV of diamond. Therefore, the two-dimensional electron gas 13 is generated near the hetero interface with the diamond layer 11 in the h-GaN layer 42 to form an n-type channel.

  In such an FET, due to the high mobility of the two-dimensional electron gas 13, high frequency operation and high current density are possible. Since the threshold voltage is determined by the thickness of the h-GaN layer 42 which is an electron supply layer, the threshold voltage can be easily controlled. Furthermore, since the polarization charge density is determined by the piezoelectric coefficient specific to the material, the reproducibility and uniformity are excellent, and problems such as in-plane variation of element characteristics and lot-to-lot variation are solved.

(Fifth embodiment)
FIG. 8 is a diagram showing a cross-sectional structure of a fifth embodiment of the FET according to the present invention.

The feature of this embodiment is that the (0001) plane undoped h-GaN layer 42 in the fourth embodiment is replaced with a (0001) plane n-type h-GaN layer 52. Here, for example, Si, Ge, Sn, Pb, or the like is used as the n-type impurity added to GaN, and the impurity concentration is between 1 × 10 ¹⁸ cm ⁻³ and 1 × 10 ²² cm ⁻³ .

  FIG. 9 is an energy band diagram under the gate in this embodiment.

As described above, positive fixed charges having a surface density of 0.029 C / m ² are generated at the heterointerface between the diamond layer 11 and the h-GaN layer 52 due to spontaneous polarization. Moreover, the band gap of h-GaN is smaller than 3.4 eV and 5.5 eV of diamond. Therefore, the two-dimensional electron gas 13 is generated near the hetero interface with the diamond layer 11 in the h-GaN layer 52 to form an n-type channel. Since the electron supply effect by impurity doping is added to the electron supply effect by spontaneous polarization, the current density is increased as compared with the fourth embodiment. Due to the positive ionization charge in the h-GaN layer 52, the conduction band energy has a downwardly convex shape. For this reason, the energy barrier with respect to an electron becomes low, and the contact resistance in the source electrode 15 and the drain electrode 16 is reduced.

  In such an FET, due to the high mobility of the two-dimensional electron gas 13, high frequency operation and high current density are possible. Since the threshold voltage is determined by the thickness of the h-GaN layer 52 as an electron supply layer and the impurity concentration, the threshold voltage can be easily controlled. Furthermore, the contact resistance is reduced due to the fixed charges resulting from the ionization of the n-type impurities in the h-GaN layer 52.

(Sixth embodiment)
FIG. 10 is a diagram showing a cross-sectional structure of a sixth embodiment of the FET according to the present invention.

  The feature of this embodiment is that the (0001) plane undoped h-BN layer 12 in the first embodiment is replaced with a (0001) plane undoped hexagonal indium nitride (h-InN) layer 62. Here, the layer thickness of the h-InN layer 62 is between 1 nm and 100 nm. In this embodiment, for example, it is set to 5 nm.

As described above, the C atom spacing of the (111) plane diamond is a / 2 ^1/2 = 0.252 nm, whereas the a-axis length of h-InN is 0.35 nm. The difference between the two is large, and the thickness 5 nm of the h-InN layer 62 of the present embodiment is equal to or greater than the critical thickness of dislocation generation. For this reason, the h-InN layer 62 is lattice-relaxed.

spontaneous polarization density of h-InN is calculated to 0.032C / ^{m 2.} On the other hand, since the h-InN layer 62 of this embodiment is lattice-relaxed, no piezoelectric polarization occurs. Therefore, a positive charge of 0.032 C / m ² is generated at the heterointerface between the diamond layer 11 and the h-InN layer 62 based on spontaneous polarization. This charge corresponds to a two-dimensional electron having an areal density of 2.0 × 10 ¹³ cm ⁻² .

  The energy band diagram under the gate of this embodiment is the same as that of the fourth embodiment (FIG. 7).

As described above, a positive charge of 0.032 C / m ² is generated at the heterointerface between the diamond layer 11 and the h-InN layer 62 due to spontaneous polarization. The band gap of h-InN is 0.9 eV, which is smaller than 5.5 eV of diamond. For this reason, the two-dimensional electron gas 13 is generated near the heterointerface with the diamond layer 11 in the h-InN layer 62 to form an n-type channel.

  Such an FET enables high-frequency operation and high current density due to the high mobility of the two-dimensional electron gas 13. Since the threshold voltage is determined by the thickness of the h-InN layer 62 that is an electron supply layer, the threshold voltage can be easily controlled. Furthermore, since the polarization charge density is determined by the piezoelectric coefficient specific to the material, the reproducibility and uniformity are excellent, and problems such as in-plane variation of element characteristics and lot-to-lot variation are solved.

(Seventh embodiment)
FIG. 11 is a diagram showing a cross-sectional structure of a seventh embodiment of the FET according to the present invention.

  The feature of this embodiment is that the (0001) plane undoped h-BN layer 12 in the first embodiment is replaced with a (111) plane undoped cubic boron nitride (c-BN) layer 72. Here, the layer thickness of the c-BN layer 72 is between 1 nm and 100 nm. In this embodiment, for example, it is set to 20 nm. The c-BN layer 72 of this example has a thickness of 20 nm or less which is less than the critical thickness for generating dislocations.

Since the group III-V cubic semiconductor has a structure in which group III atom planes and group V atom planes are alternately arranged in the (111) crystal axis direction, when the (111) plane thin film is formed, the upper interface and the lower layer are formed. It is known that polarization charges are generated at the interface. Here, the polarization effect is caused by spontaneous polarization generated when the bond between the group III atom and the group V atom deviates from the ideal tetrahedral ideal structure, and the difference in the a-axis length between the thin film and the substrate. Although there are a piezoelectric polarization, spontaneous polarization density of c-BN it was calculated to 0.029C / ^{m 2,} piezo polarization density of c-BN on diamond is calculated to 0.013C / ^{m 2.} Therefore, a positive charge of 0.042 C / m ² is generated at the heterointerface between the diamond layer 11 and the c-BN layer 72 based on spontaneous polarization and piezo polarization. This charge corresponds to a two-dimensional electron having an areal density of 2.6 × 10 ¹³ cm ⁻² .

  The energy band diagram under the gate of this embodiment is the same as that of the fourth embodiment (FIG. 7).

As described above, positive charges having a surface density of 0.042 C / m ² are generated at the heterointerface between the diamond layer 11 and the c-BN layer 72 due to spontaneous polarization. The band gap of c-BN is about 5 eV, which is smaller than that of diamond. For this reason, the two-dimensional electron gas 13 is generated near the heterointerface with the diamond layer 11 in the c-BN layer 72 to form an n-type channel.

  In such an FET, due to the high mobility of the two-dimensional electron gas 13, high frequency operation and high current density are possible. Since the threshold voltage is determined by the thickness of the c-BN layer 72 that is an electron supply layer, the threshold voltage can be easily controlled. Furthermore, since the polarization charge density is determined by the piezoelectric coefficient specific to the material, the reproducibility and uniformity are excellent, and problems such as in-plane variation of element characteristics and lot-to-lot variation are solved.

(Eighth embodiment)
FIG. 12 is a diagram showing a cross-sectional structure of an eighth embodiment of the FET according to the present invention.

  The feature of this embodiment is that the (0001) plane undoped h-AlN layer 32 in the third embodiment is replaced with a (111) plane undoped cubic aluminum nitride (c-AlN) layer 82. Here, the layer thickness of the c-AlN layer 82 is between 1 nm and 100 nm. In this embodiment, for example, it is set to 10 nm. The c-AlN layer 82 of this example has a thickness of 10 nm which is equal to or greater than the critical film thickness for generating dislocations. For this reason, the c-AlN layer 82 is lattice-relaxed.

The spontaneous polarization density of c-AlN is calculated to be 0.081 C / m ² . On the other hand, since the c-AlN layer 82 of this embodiment is lattice-relaxed, piezoelectric polarization does not occur. Therefore, a positive charge of 0.081 C / m ² is generated at the heterointerface between the diamond layer 11 and the c-AlN layer 82 based on spontaneous polarization. This charge corresponds to a two-dimensional electron having a surface density of 5.1 × 10 ¹³ cm ⁻² .

  The energy band diagram under the gate of this embodiment is the same as that of the fourth embodiment (FIG. 7).

As described above, positive charges having a surface density of 0.081 C / m ² are generated at the heterointerface between the diamond layer 11 and the c-AlN layer 82 due to spontaneous polarization. The band gap of c-AlN is 5.1 eV, which is smaller than that of diamond. For this reason, the two-dimensional electron gas 13 is generated near the hetero interface with the diamond layer 11 in the c-AlN layer 82 to form an n-type channel.

  In such an FET, due to the high mobility of the two-dimensional electron gas 13, high frequency operation and high current density are possible. Since the threshold voltage is determined by the thickness of the c-AlN layer 82 as an electron supply layer, the threshold voltage can be easily controlled. Furthermore, since the polarization charge density is determined by the piezoelectric coefficient specific to the material, the reproducibility and uniformity are excellent, and problems such as in-plane variation of element characteristics and lot-to-lot variation are solved.

(Ninth embodiment)
FIG. 13 is a diagram showing a cross-sectional structure of a ninth embodiment of the FET according to the present invention.

  The feature of this embodiment is that the (0001) plane undoped h-GaN 42 in the fourth embodiment is replaced with a (111) plane undoped cubic gallium nitride (c-GaN) layer 92. Here, the layer thickness of the c-GaN layer 92 is between 1 nm and 100 nm. In this embodiment, for example, it is set to 10 nm. The c-GaN layer 92 of this example has a thickness of 10 nm that is equal to or greater than the critical thickness for the occurrence of dislocations. For this reason, the c-GaN layer 92 is lattice-relaxed.

The spontaneous polarization density of c-GaN is calculated as 0.029 C / m ² . On the other hand, since the c-GaN layer 92 of this example is lattice-relaxed, piezoelectric polarization does not occur. Therefore, a positive charge of 0.029 C / m ² is generated at the heterointerface between the diamond layer 11 and the c-GaN layer 92 based on spontaneous polarization. This charge corresponds to a two-dimensional electron having a surface density of 1.8 × 10 ¹³ cm ⁻² .

  The energy band diagram under the gate of this embodiment is the same as that of the fourth embodiment (FIG. 7).

As described above, positive charges having a surface density of 0.029 C / m ² are generated at the heterointerface between the diamond layer 11 and the c-GaN layer 92 due to spontaneous polarization. In addition, the band gap of c-GaN is 3.3 eV, which is smaller than 5.5 eV of diamond. For this reason, the two-dimensional electron gas 13 is generated near the hetero interface between the c-GaN layer 92 and the diamond layer 11 to form an n-type channel.

  In such an FET, due to the high mobility of the two-dimensional electron gas 13, high frequency operation and high current density are possible. Since the threshold voltage is determined by the thickness of the c-GaN layer 92 which is an electron supply layer, the threshold voltage can be easily controlled. Furthermore, since the polarization charge density is determined by the piezoelectric coefficient specific to the material, the reproducibility and uniformity are excellent, and problems such as in-plane variation of element characteristics and lot-to-lot variation are solved.

(Tenth embodiment)
FIG. 14 is a diagram showing a cross-sectional structure of a tenth embodiment of the FET according to the present invention.

  The feature of this embodiment is that the (0001) plane undoped h-InN 62 in the sixth embodiment is replaced with a (111) plane undoped cubic indium nitride (c-InN) layer 102. Here, the thickness of the c-InN layer 102 is between 1 nm and 100 nm. In this embodiment, for example, it is set to 5 nm. The thickness of the c-InN layer 102 in this example is 5 nm or more than the critical thickness for the occurrence of dislocations. For this reason, the c-InN layer 102 is lattice-relaxed.

The spontaneous polarization density of c-InN is calculated to be 0.032 C / m ² . On the other hand, since the c-InN layer 102 of this embodiment is lattice-relaxed, piezoelectric polarization does not occur. Therefore, a positive charge of 0.032 C / m ² is generated at the heterointerface between the diamond layer 11 and the c-InN layer 102 based on spontaneous polarization. This charge corresponds to a two-dimensional electron having an areal density of 2.0 × 10 ¹³ cm ⁻² .

  The energy band diagram under the gate of this embodiment is the same as that of the fourth embodiment (FIG. 7).

As described above, positive charges having a surface density of 0.032 C / m ² are generated at the heterointerface between the diamond layer 11 and the c-InN layer 102 due to spontaneous polarization. The band gap of c-InN is about 0.9 eV, which is smaller than 5.5 eV of diamond. For this reason, the two-dimensional electron gas 13 is generated near the hetero interface with the diamond layer 11 in the c-InN layer 102 to form an n-type channel.

  In such an FET, due to the high mobility of the two-dimensional electron gas 13, high frequency operation and high current density are possible. Since the threshold voltage is determined by the thickness of the c-InN layer 102 which is an electron supply layer, the threshold voltage can be easily controlled. Furthermore, since the polarization charge density is determined by the piezoelectric coefficient specific to the material, the reproducibility and uniformity are excellent, and problems such as in-plane variation of element characteristics and lot-to-lot variation are solved.

  As mentioned above, although this invention was demonstrated according to the said Example, this invention is not limited only to the said aspect, Of course, various aspects according to the principle of this invention are included.

  For example, in the above embodiment, the case where a hexagonal compound semiconductor (h-BN, h-AlN, h-GaN, h-InN) is used as the electron (carrier) supply layer material is shown. A semiconductor such as hexagonal silicon carbide (h-SiC) may be used. Alternatively, a mixed crystal semiconductor including at least two different semiconductor materials among h-BN, h-AlN, h-GaN, and h-InN may be used.

  In addition, although a case where a cubic compound semiconductor (c-BN, c-AlN, c-GaN, c-InN) is used as an electron (carrier) supply layer material, other cubic compound semiconductors, for example, cubic Using crystalline silicon carbide (c-SiC), gallium arsenide (GaAs), aluminum arsenide (AlAs), indium arsenide (InAs), gallium phosphide (GaP), aluminum phosphide (AlP), indium phosphide (InP) Also good. Alternatively, a mixed crystal semiconductor made of at least two different semiconductor materials among c-BN, c-AlN, c-GaN, c-InN, GaAs, AlAs, InAs, GaP, AlP, and InP may be used.

  As a substrate material, a diamond substrate, a Si substrate, a SiC substrate, a GaN substrate, a sapphire substrate, or the like can be used.

It is a figure which shows the cross-section of FET which is the 1st Example of this invention. It is an energy band figure of FET which is a 1st Example of this invention. It is a figure which shows the cross-section of FET which is the 2nd Example of this invention. It is an energy band figure of FET which is a 2nd Example of this invention. It is a figure which shows the cross-section of FET which is the 3rd Example of this invention. It is a figure which shows the cross-section of FET which is the 4th Example of this invention. It is an energy band figure of FET which is the 4th example of the present invention. It is a figure which shows the cross-section of FET which is the 5th Example of this invention. It is an energy band figure of FET which is the 5th Example of this invention. It is a figure which shows the cross-section of FET which is the 6th Example of this invention. It is a figure which shows the cross-section of FET which is the 7th Example of this invention. It is a figure which shows the cross-section of FET which is the 8th Example of this invention. It is a figure which shows the cross-section of FET which is the 9th Example of this invention. It is a figure which shows the cross-section of FET which is the 10th Example of this invention. It is a figure which shows the cross-section of FET by a prior art. It is an energy band figure of FET by a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10, 200 ... Substrate 11 ... (111) Diamond layer 12 ... (0001) plane undoped h-BN layer 13, 203 ... Two-dimensional electron gas 15, 205 ... Source electrode 16, 206 ... Drain electrode 17, 207 ... Gate electrode 22 ... (0001) plane n-type h-BN layer 32 ... (0001) plane undoped h-AlN layer 42 ... (0001) plane undoped h-GaN layer 52 ... (0001) plane n-type h-GaN layer 62 ... (0001) Plane undoped h-InN layer 72 (111) Plane undoped c-BN layer 82 (111) Plane undoped c-AlN layer 92 (111) Plane undoped c-GaN layer 102 (111) Plane undoped c-InN layer 201 ... Diamond layer 202 ... n-type AlN layer or n-type BN layer

Claims (6)

A field effect transistor formed by contacting a substrate, a diamond semiconductor layer, and a compound semiconductor layer in this order, wherein the diamond semiconductor layer is formed of a diamond semiconductor formed in parallel with a (111) crystal plane, and the compound semiconductor Any one of BN, GaN, InN, and SiC having a layer formed in parallel with the (0001) crystal plane , or two or more mixed crystals of BN, GaN, AlN, InN, and SiC (however, A field effect transistor comprising a hexagonal compound semiconductor ( except AlGaN) . A substrate, a diamond semiconductor layer, and a compound semiconductor layer are formed in contact with each other in this order, and the diamond semiconductor layer is made of an undoped diamond semiconductor formed in parallel with a (111) crystal plane, and A field effect transistor comprising a compound semiconductor layer made of an undoped AlN hexagonal compound semiconductor formed in parallel with a (0001) crystal plane . A field effect transistor formed by contacting a substrate, a diamond semiconductor layer, and a compound semiconductor layer in this order, wherein the diamond semiconductor layer is formed of a diamond semiconductor formed in parallel with a (111) crystal plane, and the compound semiconductor A field effect transistor, wherein the layer is made of a cubic compound semiconductor formed in parallel with a (111) crystal plane.   4. The field effect transistor according to claim 3, wherein the cubic compound semiconductor is one or more of BN, GaN, AlN, InN, SiC, GaAs, AlAs, InAs, GaP, AlP, and InP. A field effect transistor characterized by being a mixed crystal. In the field effect transistor according to any one of claims 1, 3, 4, field effect transistors, characterized in that n-type impurity into the compound semiconductor layer is added. In the field effect transistor according to any one of claims 1 to 5, at the interface between the diamond semiconductor layer of said compound semiconductor layer, which has a positive fixed charge resulting from the least spontaneous polarization, said compound semiconductor layer And a field effect transistor characterized in that a two-dimensional electron gas is generated in the vicinity of the interface of the diamond semiconductor layer.

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