TWI572036B - Nitride crystal structure - Google Patents

Description

Nitride crystal structure

This invention relates to a crystal structure, and more particularly to a nitride crystal structure.

With the advancement of technology, the demand for high-energy electronic components is increasing day by day. Traditional Si-based and gallium arsenide (GaAs) components are not suitable for high-power component applications due to their small energy gap, and wide energy gaps must be used. Semiconductor materials, such as tantalum carbide (SiC) or gallium nitride (GaN); compared to tantalum carbide, gallium nitride series materials can achieve higher channel electron mobility and loading by material energy gap modulation and combination Sub-concentration, therefore more suitable as a high-energy electronic component demand application, generally known nitride crystal structure, especially High Electron Mobility Transistor (HEMT), the epitaxial layer structure from top to bottom is the first a layer of aluminum gallium nitride (AlGaN) layer, a second layer of gallium nitride (GaN) layer, a third layer of nucleation layer and a fourth layer of substrate, wherein the first layer of aluminum gallium nitride layer is also The second layer of gallium nitride layer may also be referred to as a channel layer, and the second layer of gallium nitride layer may be a combination of an undoped gallium nitride layer and a semi-insulating gallium nitride layer; Due to the unique polarization effect of the nitride material, the package Spontaneous polarization (Spontaneous Polarization) and piezoelectric polarization (Piezoelectric Polarization), without doping any impurities In the case of the polarization effect, the aluminum gallium nitride/gallium nitride heterostructure can be automatically induced to form two-dimensional electron gas (2DEG) at the interface near the gallium nitride layer, and the electrons in the 2DEG have The higher the moving speed and the magnitude of its electron concentration are related to the polarization. If the epitaxial structure has a component operation function, a source, a drain, and a gate metal electrode must be fabricated thereon, wherein the gate electrode is interposed between the source and the drain electrode. And the ohmic contact between the source and the drain electrode and the semiconductor, the Schottky contact between the gate electrode and the semiconductor; the operation of the device is to provide a current from the source, The current is output from the drain electrode via the 2DEG channel, and a voltage is supplied from the gate electrode. By adjusting the voltage, the channel current switching characteristics and current are controlled.

The component fabricated by the above nitride crystal structure is generally a depletion mode component, or a normally-on component, that is, when the gate electrode is applied with a voltage of 0 V, there is still a gap between the 2DEG channels. Current is generated to bring the source and drain together; however, in high-energy applications, especially for high-power switching applications, the Enchancement Mode component, or Normally Off component, can be used. Improve the safety of components and circuits. When the applied voltage is 0V at the gate electrode, there is no current between the 2DEG channels. The forward bias must be applied to the gate electrode to make the source and drain. Inductive; ideally, there is no 2DEG formation in the channel region directly below the gate electrode, but between the gate and the source, and the gate and the gate There is still 2DEG formed in the channel region between the poles. In order to make the components turn on, a forward bias is applied to the gate electrodes to induce 2DEG formation to conduct the components. There are many ways to achieve this enhanced nitride transistor component, for example, refer to U.S. Patent Nos. 7,728,356 B2 and US 7,851,825 B2 are achieved by varying the structure and component process of the nitride transistor epitaxial layer.

The nitride crystal epitaxial structure described above has a crystal orientation such as Gallium Polar (Ga-polar), or Gallium Face (Ga-face), which is oriented upward from the substrate, and is opposite to the gallium surface. The crystal orientation is a Nitrogen Polar (N-polar), or Nitrogen Face (N-face), and the crystal orientation is a nitrogen-oriented nitride crystal structure, and the epitaxial layer structure is up to The first layer is a first layer of gallium nitride (GaN) layer, a second layer of aluminum gallium nitride (AlGaN) layer, a third layer of gallium nitride (GaN) layer, a fourth layer of nucleation layer and a fifth layer substrate (Substrate), wherein the first layer of gallium nitride layer may also be referred to as a channel layer, and the second layer of aluminum gallium nitride layer may also be referred to as a barrier layer; the nitrogen layer also has a polarization effect on the nitride crystal structure. The aluminum gallium nitride/gallium nitride heterostructure is automatically induced to form a two-dimensional electron gas (2DEG) near the interface of the gallium nitride layer; there are many ways to achieve this nitrogen-oriented nitride crystal structure, for example, refer to the US Patent No. US 8,878,249 B2 is achieved by using an off-angle substrate.

The nitride crystal structure described above, especially the high electron mobility transistor, has a large portion of the characteristics of the transistor component depending on the electron moving speed and electron concentration in the 2DEG channel, so if the 2DEG channel can be improved Properties, such as increased 2DEG electron mobility or electron concentration, all contribute to improved transistor component performance. For example, reference is made to US Pat. No. 6,849,882 B2, which discloses the disclosure of a layer of aluminum nitride (AlN) between an aluminum gallium nitride barrier layer and a gallium nitride channel layer, thereby increasing the electron mobility of the 2DEG channel; For example, reference is made to US Pat. No. 7,795,642 B2, which discloses the use of a gallium aluminum nitride/gallium nitride/aluminum gallium nitride combination layer in place of an aluminum gallium nitride barrier layer and adjusting the aluminum content of aluminum gallium nitride. Increasing the electron concentration of the 2DEG channel is disclosed in US Patent No. US 2006/0163594 A1, which discloses the use of an indium gallium nitride layer in place of an aluminum gallium nitride barrier layer, thereby increasing the electron concentration of the 2DEG channel.

For the sake of his position, the applicant conducted experiments and research, and proposed a nitride crystal structure and fabrication method that can improve the electron mobility and electron concentration of the 2DEG channel, using one or more layers of lower five or three ratio nitride intermediate layers. Below the 2DEG channel, the position is between the 2DEG channel and the substrate, which can improve the electron mobility and electron concentration of the 2DEG channel, thereby improving the performance of the nitride transistor component.

In view of the above disadvantages of the prior art, the main object of the present invention is to provide a nitride transistor structure, which integrates a semiconductor structure such as a nucleation layer, a channel layer, an intermediate layer, a barrier layer, and a buffer layer. Equivalent to increase the electron mobility and carrier concentration of the two-dimensional electron gas channel The efficacy of nitride transistor performance.

The invention proposes to use one or more layers of nitride intermediate layer which is epitaxially grown at a lower five-three ratio to be placed under the 2DEG channel, and the position is between the 2DEG channel and the substrate, which can improve the electron mobility and electron concentration of the 2DEG channel, and further Achieve the performance of improving the performance of nitride transistor components. The fifth to third ratio is one of the main adjustment parameters of the nitride material during the epitaxial growth process, and the ratio thereof is the ratio of the five-element element Moire flow rate and the three-element element Mohr flow rate which are introduced during the epitaxial growth, and the present invention The lower five-three-nitride intermediate layer is placed under the 2DEG channel, and the position is between the 2DEG channel and the substrate, that is, the lower five-three-nitride intermediate layer is in the channel layer, and therefore, the nitrogen The effect of increasing the electron mobility and electron concentration of the 2DEG channel proposed by the present invention can be achieved by the fact that the intermediate layer of the intermediate layer must be less than the ratio of the five to three ratios when the channel layer is grown.

In order to achieve the above object, according to one aspect of the present invention, a gallium-facing nitride transistor structure is provided, including: a substrate; a nucleation layer formed on the substrate; a channel layer, Provided on the nucleation layer, the channel layer has at least one intermediate layer; a barrier layer, the barrier layer is above the channel layer; a source, a drain and a gate three metals An electrode is disposed on the barrier layer, the gate is located between the source and the drain, the source is in ohmic contact with the metal electrode of the drain, and the gate metal electrode is Schott The base contact; wherein the growth of the intermediate layer is compared with the growth of the channel layer to form a growth ratio of five to three.

The gallium according to the above is oriented to a nitride crystal structure, wherein the substrate is selected from the group consisting of sapphire, tantalum carbide, niobium, gallium nitride, aluminum nitride or other substrate material suitable for epitaxial growth of nitride; The gallium is oriented to a nitride crystal structure, wherein the nucleation layer may be gallium nitride, aluminum nitride, aluminum gallium nitride or other nitride material which can effectively reduce the stress and defects of the channel induced channel layer; The gallium is oriented to the nitride transistor structure, wherein the channel layer may be a gallium nitride or other nitride material having a band gap smaller than that of the barrier layer material; and the gallium facing the nitride transistor structure as described above, wherein the barrier layer It may be aluminum gallium nitride, indium aluminum nitride or other nitride material having a larger energy gap than the material of the channel layer; as described above, the gallium faces the nitride crystal structure, wherein the interface between the barrier layer and the channel layer must be formed. a two-dimensional electron gas; wherein the gallium is oriented to a nitride transistor structure, wherein the nitride intermediate layer may be gallium nitride, aluminum gallium nitride or other nitride material; and the gallium facing nitride crystal structure as described above Wherein the nitride intermediate layer fifty-three its growth must be less than the ratio of the channel layer is grown fifty-three ratio; as described in the gallium nitride for transistor structure, wherein the transistor structure may be a depletion mode or enhancement mode device.

The gallium as described above faces the nitride crystal structure, wherein the epitaxial growth is selected from the group consisting of chemical vapor deposition (CVD) and organometallic chemical vapor deposition (Metal Organic Chemical). Vapor Deposition, MOCVD), Molecular Beam Epitaxy (MBE), Hydride Vapor Phase Epitaxy (HVPE), Plasma Enhanced Chemical Vapor Deposition (Plasma Enhanced) Chemical Vapor Deposition (PECVD) or other epitaxial method suitable for growing nitride materials.

In order to achieve the above object, according to another aspect of the present invention, a nitrogen-facing nitride transistor structure is provided, comprising: a substrate; a nucleation layer formed on the substrate; a buffer layer, Is located on the nucleation layer, the buffer layer has at least one intermediate layer; a barrier layer, the barrier layer is located above the buffer layer; a channel layer, the channel layer is tied to the barrier layer a source electrode, a drain electrode and a gate three metal electrodes are disposed on the channel layer, wherein the gate position is between the source and the drain, the source and the drain The metal electrode is an ohmic contact, and the gate metal electrode is in Schottky contact; wherein the growth of the intermediate layer is compared to form a growth ratio of the buffer layer which is lower than the growth ratio of the buffer layer.

The nitrogen as described above faces the nitride crystal structure, wherein the substrate is selected from the group consisting of sapphire, tantalum carbide, niobium, gallium nitride, aluminum nitride or other substrate material suitable for epitaxial growth of nitride; The nitrogen is oriented to a nitride crystal structure, wherein the nucleation layer may be gallium nitride, aluminum nitride, aluminum gallium nitride or other nitride material which can effectively reduce stress and defects of the buffer layer induced by the substrate; The nitrogen is oriented to a nitride crystal structure, wherein the buffer layer may be a gallium nitride or other nitride material; and the nitrogen as described above faces the nitride crystal structure, wherein the barrier layer may be aluminum gallium nitride or indium nitride Aluminum or other nitride material having a larger energy gap than the material of the channel layer; as described above, the nitrogen faces the nitride transistor structure, wherein the channel layer may be gallium nitride or other energy gap is less than a nitride material having a gap of a material of the barrier layer; as described above, the nitrogen faces the nitride crystal structure, wherein an interface between the barrier layer and the channel layer must form a two-dimensional electron gas; as described above, the nitrogen faces the nitride transistor a structure, wherein the nitride intermediate layer may be gallium nitride, aluminum gallium nitride or other nitride material; as described above, the nitrogen faces the nitride crystal structure, wherein the nitride intermediate layer has a growth ratio of less than five The buffer layer grows five to three ratios; as described above, the nitrogen faces the nitride transistor structure, wherein the transistor structure can be an enhancement or depletion element.

The nitrogen as described above faces the nitride crystal structure, wherein the epitaxial growth is selected from the group consisting of chemical vapor deposition (CVD), and organometallic chemical vapor deposition (Metal Organic Chemical). Vapor Deposition (MOCVD), Molecular Beam Epitaxy (MBE), Hydride Vapor Phase Epitaxy (HVPE), Plasma Enhanced Chemical Vapor Deposition (PECVD) Or other epitaxial methods suitable for growing nitride materials.

The above summary and the following detailed description and drawings are intended to further illustrate the manner, means and effects of the present invention in achieving its intended purpose. Other purposes and advantages of this creation will be explained in the following description and drawings.

100, 200‧‧‧ substrate

101, 201‧‧‧ nucleation layer

102, 205‧‧‧ channel layer

103, 203‧‧‧ middle layer

104, 204‧‧‧ barrier layer

105, 206‧‧‧ source

106, 207‧‧ ‧ bungee

107, 208‧‧‧ gate

112, 215‧‧‧Two-dimensional electronic gas

The first figure is a schematic diagram of a gallium-facing nitride transistor structure of the present invention; The second figure is a schematic diagram of a nitrogen-oriented nitride crystal structure of the present invention; the third figure is a result of measuring the electrical conductivity of a nitride transistor having a low five-three ratio intermediate layer; the fourth figure is the invention A result of electrical measurement of a nitride transistor having a low five-three ratio intermediate layer.

The embodiments of the present invention are described by way of specific examples, and those skilled in the art can readily understand the advantages and effects of the present invention from the disclosure of the present disclosure.

Please refer to the first figure for a schematic diagram of a gallium-facing nitride transistor structure of the present invention. As shown in FIG. 1 , the present invention provides a gallium-oriented nitride transistor structure, comprising the following composition: a substrate 100. In this embodiment, sapphire is used as an epitaxial substrate, but gallium carbide, germanium, gallium nitride can also be used. Aluminum nitride or other substrate material suitable for nitride epitaxial growth; thereafter epitaxial growth of the nucleation layer 101 on the substrate 100 by organometallic chemical vapor phase epitaxy, and gallium nitride is used in this embodiment. As the nucleation layer 101, depending on the epitaxial method and different substrates, aluminum nitride, aluminum gallium nitride or other nitride materials which can effectively reduce the stress and defects of the substrate-initiating channel layer 102 can be selected; The channel layer 102 is grown on the nucleation layer 101. In this embodiment, gallium nitride is used as the channel layer 102, but other energy gaps may be used instead of the barrier layer 104. According to the characteristics of the present invention, the channel layer 102 is grown at a ratio of five to three. In this embodiment, the channel layer 102 is grown at a ratio of 848 to 848. The value of the five-three ratio can be determined by the epitaxial device and the program. Differently, the channel layer 102 of the present invention should not be limited to a ratio of five to three ratios. In the process of growing the channel layer 102, the invention reduces the ratio of five to three to between 71 and 346 to grow one. The lower five or three ratio intermediate layer 103, this embodiment uses gallium nitride and aluminum gallium nitride as the lower five-three ratio intermediate layer 103, but other nitride materials may also be used, and the value of the five-three ratio may depend on the epitaxial device and The program is changed differently, and should not limit the low-fifth ratio intermediate layer 103 of the present invention to a ratio of five to three ratios; and after the low-three-three ratio intermediate layer 103 process described above, the fifth-to-three ratio is increased to 848. The channel layer 102; thereafter, the barrier layer 104 is grown on the channel layer 102. In this embodiment, aluminum gallium nitride is used as the barrier layer 104, but indium aluminum nitride or other energy gaps may be used to be larger than the channel layer 102. a nitride material of the gap, and the boundary between the barrier layer 104 and the channel layer 102 Dimensional electron gas 112 to be formed. Thereafter, three metal electrodes are formed on the barrier layer 104. The source 105, the drain 106 and the gate 107 complete the fabrication of the transistor device, wherein the gate 107 is located between the source 105 and the drain 106, the source. The metal electrode of the drain is in ohmic contact, the gate metal electrode is a Schottky contact; and a voltage is provided by the gate electrode, and the current switching characteristic and current magnitude are controlled by adjusting the voltage; wherein the transistor structure It can be an enhanced or depleted component.

Please refer to the second figure, which is a schematic diagram of a nitrogen-facing nitride transistor structure of the present invention. As shown in Figure 2, the present invention provides a nitrogen-oriented The nitride crystal structure includes the following composition: a substrate 200. This embodiment uses sapphire as the epitaxial substrate, but may also use tantalum carbide, tantalum, gallium nitride, aluminum nitride or the like as a nitride epitaxial. a grown substrate material; thereafter, epitaxial growth of the nucleation layer 201 is performed on the substrate 200 by an organometallic chemical vapor phase epitaxy method. In this embodiment, gallium nitride is used as the nucleation layer 201, but the epitaxial method is different. Alternatively, a different substrate may be used, and aluminum nitride, aluminum gallium nitride or other nitride material which can effectively reduce the stress and defects of the substrate-initiated buffer layer 202 may be selected; thereafter, the buffer layer 202 is grown on the nucleation layer 201, The embodiment uses gallium nitride as the buffer layer 202, but other nitride materials may be used. According to the features of the present invention, the buffer layer 202 is grown by a ratio of five to three. In this embodiment, the buffer layer 202 is grown at a ratio of 848 to 848. The value of the five-three ratio may vary according to the epitaxial device and the program, and the buffer layer 202 of the present invention should not be limited to a ratio of five to three. In the process of growing the buffer layer 202, the present invention will be five. The ratio is reduced to 7 Between 1 and 346, the intermediate layer 203 is grown by a lower five to three ratio. In this embodiment, gallium nitride and aluminum gallium nitride are used as the lower five to three intermediate layer 203, but other nitride materials may also be used. The value can be changed according to the epitaxial device and the program, and should not limit the low five to three intermediate layer 203 of the present invention to grow five to three ratios; and after the low five or three intermediate layer 203 process described above Then, the fifth to third ratio is raised to the 848 growth buffer layer 202; thereafter, the barrier layer 204 is grown on the buffer layer 202. In this embodiment, aluminum gallium nitride is used as the barrier layer 204, but indium aluminum nitride can also be used. Or other nitride material having a larger energy gap than the channel layer 205; and subsequently growing the channel layer 205 over the barrier layer 204. The embodiment uses gallium nitride as the channel layer 205, but other nitride materials having a smaller energy gap than the barrier layer 204 can be used; and the interface between the channel layer 205 and the barrier layer 204 must form a two-dimensional electron gas 215. . Thereafter, three metal electrodes are fabricated on the channel layer 205, and the source 206 and the drain 207 gate 208 complete the fabrication of the transistor device. The gate 208 is located between the source 206 and the drain 207, and the source and the gate are The metal electrode of the pole is an ohmic contact, and the gate metal electrode is a Schottky contact; and a voltage is provided by the gate electrode, and the current switching characteristic and current magnitude of the channel are controlled by adjusting the voltage; wherein the transistor structure can be Enhanced or depleted components.

Please refer to the third figure, which is a result of measuring the electrical conductivity of a nitride transistor having a low five-three ratio intermediate layer. As shown in Figure 3, 2DEG electron mobility (Mobility) and sheet concentration (Sheet Concentration) graphs including room temperature (RT) and low temperature 80K, which compares the general nitride crystal structure with low The electrical measurement results of the nitride crystal structure of the five-three-nitride intermediate layer, wherein the ratio of the five to three ratios of the lower five to three nitride intermediate layers are 177, 141 and 71 respectively; as can be seen from the figure, with the lower five three intermediate reduction ratio of the nitride layer fifty-three ratio, and low electron mobility at room temperature have a significant increase, the electron mobility at room temperature may be 1481cm ² / Vs was increased to 1815cm ² / Vs; cryogenic 80K electron mobility is more 6349cm ² /Vs is greatly increased to 13420cm ² /Vs. Because the low temperature measurement can make the test piece not affected by the external environmental thermal disturbance, it can show the result of structural improvement; however, compared with the electron mobility, the carrier concentration From 1.4x10 ¹³ cm ^-2 to 1.2x10 ¹³ cm ^-2 , but still above 10 ¹³ cm ^-2 , still enough to meet the needs of gallium nitride transistor components.

Please refer to the fourth figure, which is a result of measuring the electrical conductivity of a nitride transistor having a low five-three ratio intermediate layer. As shown in FIG. 4, please refer to FIG. 4 again, which shows the electrical measurement results of the nitride crystal structure with the low five-three ratio nitride intermediate layer proposed by the present invention, including room temperature (Room Temperature, RT). And 2eG electron mobility (Mobility) and sheet concentration (Sheet Concentration) map of low temperature 80K, which compares the general nitride crystal structure with the nitride crystal structure with a low five-three ratio nitride intermediate layer The measurement results, wherein the lower five to three nitride intermediate layers are respectively gallium nitride and aluminum gallium nitride, and the five to three ratios are 71 and 346 respectively; as can be seen from the figure, when using aluminum gallium nitride as the lower five when more than three nitride intermediate layer, the electron mobility at room temperature may be 1481cm ² / Vs was increased to 1800cm ² / Vs; cryogenic 80K electron mobility is increased from 6349cm ² / Vs sharply to 14800cm ² / Vs, also shows improved structure The result of the latter; and in the measurement of the concentration of the carrier, the use of aluminum gallium nitride as the lower five-three-nitride intermediate layer can increase the carrier concentration from 1.4x10 ¹³ cm ^-2 to 2.1x10 ¹³ cm ^-2 , Shows the use of aluminum gallium nitride as the middle of the lower five to three ratio nitride The layer can simultaneously increase the carrier concentration.

The above-described embodiments are merely illustrative of the features and functions of the present invention and are not intended to limit the scope of the technical content of the present invention. Any person skilled in the art can modify and change the above embodiments without departing from the spirit and scope of the creation. Therefore, the scope of protection of this creation should be as listed in the scope of the patent application described later.

100‧‧‧Substrate

101‧‧‧ nucleation layer

102‧‧‧Channel layer

103‧‧‧Intermediate

104‧‧‧Barrier layer

105‧‧‧ source

106‧‧‧汲polar

107‧‧‧ gate

112‧‧‧Two-dimensional electronic gas

Claims (17)

A gallium-facing nitride transistor structure includes: a substrate; a nucleation layer formed on the substrate; a channel layer disposed on the nucleation layer, the channel layer having at least An intermediate layer; a barrier layer, the barrier layer is above the channel layer; a source, a drain and a gate three metal electrodes are placed on the barrier layer, the gate The position is between the source and the drain, the source is in ohmic contact with the metal electrode of the drain, and the gate metal electrode is in Schottky contact; wherein the growth of the intermediate layer is formed to form the The growth of the channel layer is lower than the ratio of five to three, and the growth ratio of the middle layer is between 71 and 177. The gallium according to claim 1 is a nitride-oriented crystal structure, wherein the substrate is selected from one of sapphire, tantalum carbide, niobium, gallium nitride, and aluminum nitride. The gallium-facing nitride transistor structure according to claim 1, wherein the nucleation layer, the channel layer, the intermediate layer, and the barrier layer comprise a nitride. The gallium according to claim 3 is a nitride-oriented crystal structure, wherein the nucleation layer is one of gallium nitride, aluminum nitride, and aluminum gallium nitride. A gallium-oriented nitride transistor structure as described in claim 3, wherein the channel layer is gallium nitride. The gallium according to claim 3 is a nitride-oriented crystal structure, wherein the barrier layer is one of aluminum gallium nitride and indium aluminum nitride. The gallium according to claim 3 is a nitride-oriented crystal structure, wherein the intermediate layer is one of gallium nitride and aluminum gallium nitride. The gallium-oriented nitride transistor structure according to claim 1, wherein the interface between the barrier layer and the channel layer forms a two-dimensional electron gas. A nitrogen-facing nitride transistor structure includes: a substrate; a nucleation layer formed on the substrate; a buffer layer disposed on the nucleation layer, the buffer layer having at least An intermediate layer; a barrier layer, the barrier layer is located above the buffer layer; a channel layer, the channel layer is above the barrier layer; a source, a drain and a gate a metal electrode is disposed on the channel layer, wherein the gate position is between the source and the drain, the source is in ohmic contact with the metal electrode of the drain, and the gate metal electrode is Xiao The base contact is formed; wherein the growth of the intermediate layer is compared to form a growth ratio of the buffer layer, and the growth ratio of the intermediate layer is between 71 and 346. The nitrogen according to claim 9 is a nitride-oriented crystal structure, wherein the substrate is selected from one of sapphire, tantalum carbide, niobium, gallium nitride, and aluminum nitride. The nitrogen according to claim 9 is for a nitride crystal structure, wherein the nucleation layer, the buffer layer, the channel layer, the intermediate layer, and the barrier layer comprise a nitride. The nitrogen according to claim 11 is a nitride-oriented crystal structure, wherein the nucleation layer is one of gallium nitride, aluminum nitride, and aluminum gallium nitride. The nitrogen according to claim 11 is a nitride-oriented crystal structure, wherein the buffer layer is gallium nitride. The nitrogen according to claim 11 is a nitride-oriented crystal structure, wherein the barrier layer is one of aluminum gallium nitride and indium aluminum nitride. The nitrogen according to claim 11 is a nitride-oriented crystal structure, wherein the channel layer is gallium nitride. The nitrogen according to claim 11 is a nitride-oriented crystal structure, wherein the intermediate layer is one of gallium nitride and aluminum gallium nitride. The nitrogen according to claim 11 is a nitride-oriented crystal structure, wherein an interface between the barrier layer and the channel layer must form a two-dimensional electron gas.