Patent Publication Number: US-2016225913-A1

Title: Ingaaln-based semiconductor device

Description:
TECHNICAL FIELD 
     The present invention relates to a semiconductor device, and, more particularly, to a semiconductor device including an InGaAlN-based nitride semiconductor layer exhibiting favorable device properties even when the InGaAlN-based nitride semiconductor layer is a polycrystal or amorphous. 
     BACKGROUND ART 
     Because an InGaAlN-based nitride semiconductor exhibits high electron mobility and high saturation electron velocity, the InGaAlN-based nitride semiconductor attracts attention as a high-speed electronic device material which responds to a higher frequency than a conventional transistor. 
     For example, concerning InN, there have been a lot of reported cases as to electrical characteristics so far, and, while InN exhibits excellent characteristics of electron mobility of 3570 [cm 2 /Vs] and saturation electron velocity of 2.6×10 7  [cm/s], because InN is likely to cause a defect that a Fermi level is fixed in a conduction band (Non Patent Literature 1), it is not easy to realize even basic transistor operation of controlling a current using an external signal. 
     As disclosed in Non Patent Literature 2, it is typically known that the electric characteristics of InN, such as mobility, becomes poorer as a film thickness is made thinner, which is interpreted to be because defects exist more intensively on a surface or an interface than inside an InN thin film. That is, it is considered that one cause of a transistor using InN not operating is that a lot of defects occur at the interface with a layer or a substrate bonded to the InN layer, and it can be easily imagined that density of these defects depends on a difference in a lattice constant between a foundation layer or the substrate and the InN (lattice constant difference) when the InN layer is grown. 
     It is generally considered that a transistor using an InGaAlN-based nitride semiconductor such as InN as a channel does not operate unless the semiconductor layer is a single crystal, and therefore, a single crystal substrate has been used as a film formation substrate. It should be noted that Patent Literature 1 (Japanese Patent Laid-Open No. 2000-22205) discloses an invention of a semiconductor light-emitting element in which light-emitting properties is obtained by laminating a layer formed with an n-type semiconductor relatively easily obtained and a hole transporting layer made of an organic compound, using the hole transporting layer made of the organic compound in place of a p-type semiconductor of a pn-junction type LED device, and injecting a hole in the n-type semiconductor, and, while Patent Literature 1 discloses that a substrate used at that time may be a non-single crystalline substrate, the semiconductor layer is not used as a channel in the semiconductor light-emitting element. 
     For example, because single crystalline GaN and single crystalline sapphire which are currently utilized by a number of researchers as a substrate for growing InN have a lattice constant drastically different from the lattice constant of InN, it can be easily understood that if a crystal of InN is grown on such a substrate, a defect is likely to occur at an interface with the substrate. It is forecasted that a problem caused by such lattice mismatch can be solved to some extent by using an yttria-stabilized zirconia (YSZ) substrate (Non Patent Literature 3) which has a close lattice constant to that of InN. 
     However, typically, because a single crystalline substrate is expensive, a semiconductor device manufactured by growing the InGaAlN-based nitride semiconductor layer using such a substrate becomes inevitably expensive, and there are various kinds of constraints to growth conditions in order that the nitride semiconductor becomes a single crystal. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Laid-Open No. 2000-22205 
     Non Patent Literature 
     Non Patent Literature 1: C. G. Van de Walle &amp; J. Neugebauer Nature 423, 626 (2003) 
     Non Patent Literature 2: Andreas Knubel, Rolf Aidam, Volker Cimalla, Lutz Kirste, Martina Baeumler, Crenguta-Columbina Leancu, Vadim Lebedev, Jan Wallauer, Markus Walther, and Joachim Wagner, Phys. Status Solidi C 6, No. 6 (2009) 
     Non Patent Literature 3: T. Honke, H. Fujioka, J. Ohta, and M. Oshima, J. Vac. Sci. Technol. A 22, 2487 (2004) 
     Non Patent Literature 4: Jhumpa Adhikari and David A. Kofke, “Molecular simulation study of miscibility of ternary and quaternary InGaAlN alloys”, JOURNAL OF APPLIED PHYSICS, Vol. 95, p. 6129-6137 (2004). 
     SUMMARY OF INVENTION 
     Technical Problem 
     In this manner, conventionally, because the electric characteristics of InN, such as mobility, have been considered to be poorer as a film thickness of the InN is made thinner, and because it has been considered that the semiconductor device does not operate unless an InGaAlN-based nitride semiconductor layer is made of a single crystal, film forming conditions of the InGaAlN-based nitride semiconductor layer inevitably lack flexibility, which makes it difficult to even realize basic transistor operation. 
     The present invention has been made in view of such problems, and an object of the present invention is to realize a semiconductor device which drastically eliminates constraints to manufacturing conditions based on an idea completely different from the conventional idea, and which includes the InGaAlN-based nitride semiconductor layer which is inexpensive and has excellent electric characteristics. 
     Solution to Problem 
     To solve the above-described problem, the semiconductor device according to the present invention is a semiconductor device in which a polycrystalline or amorphous nitride semiconductor layer expressed with a general expression In x Ga y Al z N (where x+y+z=1.0) is provided on a substrate, and composition of the nitride semiconductor layer falls within a range of 0.3≦x≦1.0 and 0≦z&lt;0.4, the semiconductor device including the nitride semiconductor layer as a channel. 
     Preferably, the composition of the nitride semiconductor layer falls within a range of 0≦z&lt;0.2 when 0.3≦x&lt;0.7 and falls within a range of 0≦z&lt;0.1 when 0.7≦x≦1.0. 
     More preferably, the composition of the nitride semiconductor layer falls within a range of 0.5≦x≦1.0 and 0≦z&lt;0.1. 
     Further preferably, an In composition ratio of the nitride semiconductor layer is 0.99 or lower (x≦0.99). 
     In a preferred aspect, an insulating layer is provided between the substrate and the nitride semiconductor layer, and the insulating layer is one of a HfO 2  layer, an Al 2 O 3  layer and a SiO 2  layer. 
     Preferably, the nitride semiconductor layer is a film deposited through a sputtering method. For example, the nitride semiconductor layer is a film deposited through a pulsed sputtering deposition method (PSD method). 
     Preferably, the nitride semiconductor layer is a film formed at a temperature of less than 600° C. 
     In one aspect, the substrate is a non-single crystalline substrate. 
     Further, in one aspect, the substrate is an insulating substrate. For example, the substrate is a fused silica substrate. 
     In one aspect, the semiconductor device has a laminate structure in which a second nitride semiconductor layer having composition different from that of the nitride semiconductor layer is bonded on at least one principle surface of the nitride semiconductor layer. 
     In this case, the second nitride semiconductor layer may be a nitride semiconductor layer having the above-described composition. 
     For example, the semiconductor device is a field effect transistor which uses the nitride semiconductor layer as a channel, and has an ON/OFF ratio of 10 2  or higher. 
     Advantageous Effects of Invention 
     The present invention is based on new finding that when composition of an InGaAlN-based nitride semiconductor is designed to fall within an appropriate range, even when a nitride semiconductor layer is a polycrystal or amorphous, the InGaAlN-based nitride semiconductor exhibits excellent electric characteristics sufficient to realize transistor operation. According to the present invention, it is possible to provide a semiconductor device which drastically eliminates constraints to manufacturing conditions, and which includes an InGaAlN-based nitride semiconductor layer which is inexpensive and has excellent electric characteristic as a channel. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram for explaining a configuration of a transistor (semiconductor device) according to a first embodiment. 
         FIG. 2  is a diagram illustrating dependency on a film thickness of InN of a ratio between an ON current and an OFF current of a field effect transistor using an InN layer as a channel. 
         FIG. 3  is a diagram illustrating I DS -V DS  characteristics of a transistor in the case where the nitride semiconductor layer is a polycrystalline InN layer. 
         FIG. 4  is a diagram illustrating I DS -V GS  characteristics of the transistor in the case where the nitride semiconductor layer is a polycrystalline InN layer. 
         FIG. 5  is a diagram illustrating I DS -V DS  characteristics of the transistor in the case where the nitride semiconductor layer is an amorphous InN layer. 
         FIG. 6  is a diagram for explaining one aspect of a configuration of a transistor (semiconductor device) according to a second embodiment. 
         FIG. 7  is a diagram for explaining one aspect of the configuration of the transistor (semiconductor device) according to the second embodiment. 
         FIGS. 8(A) and 8(B)  are graphs illustrating I DS -V DS  characteristics and I DS -V GS  characteristics of the transistor in the case where the nitride semiconductor layer is a single crystalline InN layer having a film thickness of 2 nm, and  FIGS. 8(C) and 8(D)  are graphs illustrating I DS -V DS  characteristics and I DS -V GS  characteristics of the transistor in the case where the nitride semiconductor layer is a single crystalline InN layer having a film thickness of 5 nm. 
         FIG. 9  is a diagram in which composition of a nitride semiconductor layer of an experimentally manufactured transistor is plotted in a ternary phase diagram of In x Ga y Al z N. 
         FIG. 10  is a diagram in which composition of a nitride semiconductor layer of a transistor exhibiting an ON/OFF ratio of 10 2  or higher is plotted with “” and composition of other nitride semiconductor layers is plotted with “∘” in a ternary phase diagram of In x Ga y Al z N. 
         FIG. 11  is a diagram in which composition of a nitride semiconductor layer of a transistor exhibiting an on/off ratio of 10 3  or higher is plotted with “” and composition of other nitride semiconductor layers is plotted with “∘” in a ternary phase diagram of In x Ga y Al z N. 
         FIG. 12  is a diagram in which composition of a nitride semiconductor layer having transistor characteristics exhibiting maximum current density exceeding 5 mA/mm is plotted with “”, and composition of other nitride semiconductor layers is plotted with “∘” in a ternary phase diagram of In x Ga y Al z N. 
         FIG. 13  is a diagram illustrating electric characteristics of a transistor including a nitride semiconductor layer as a channel, the nitride semiconductor layer being expressed as In x Ga y Al z N where x=0.64, y=0 and z=0.36. 
         FIG. 14  is a diagram illustrating electric characteristics of a transistor including a nitride semiconductor layer as a channel, the nitride semiconductor layer being expressed as In x Ga y Al z N where x=0.34, y=0.33 and z=0.33. 
         FIG. 15  is a diagram illustrating electric characteristics of a transistor including a nitride semiconductor layer as a channel, the nitride semiconductor layer being expressed as In x Ga y Al z N where x=0.42, y=0.42 and z=0.16. 
         FIG. 16  is a diagram illustrating electric characteristics of a transistor including a nitride semiconductor layer as a channel, the nitride semiconductor layer being expressed as In x Ga y Al z N where x=0.3, y=0.7 and z=0. 
         FIG. 17  is a diagram illustrating electric characteristics of a transistor including a nitride semiconductor layer as a channel, the nitride semiconductor layer being expressed as In x Ga y Al z N where x=0.67, y=0.33 and z=0. 
         FIG. 18  is a diagram illustrating electric characteristics of a transistor including a nitride semiconductor layer as a channel, the nitride semiconductor layer being expressed as In x Ga y Al z N where x=0.5, y=0.5 and z=0. 
         FIG. 19  is a diagram illustrating a configuration example of a transistor having a laminate structure (heterojunction structure) in which an AlN layer and a second nitride semiconductor layer are bonded on the nitride semiconductor layer of In x Ga y Al z N. 
         FIG. 20  is a diagram illustrating a configuration example of a transistor having a bottom gate structure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of a semiconductor device according to the present invention will be described below with reference to the drawings. It should be noted that, in the description of the drawings, the same reference numerals will be assigned to the same components, if possible, and overlapped explanation will be omitted. 
     (First Embodiment: InN Layer) 
       FIG. 1  illustrates a configuration of a transistor  1   a  (semiconductor device) according to a first embodiment. The transistor  1   a  includes a substrate  2   a,  a first insulating layer  3   a,  a nitride semiconductor layer  4   a,  a second insulating layer  5   a,  a source electrode  61 , a drain electrode  62  and a gate electrode  63 . 
     The first insulating layer  3   a,  the nitride semiconductor layer  4   a  and the second insulating layer  5   a  are sequentially provided on a principal surface S 1   a  of the substrate  2   a.  The first insulating layer  3   a  is bonded to the substrate  2   a.  The nitride semiconductor layer  4   a  is bonded to the first insulating layer  3   a.  The second insulating layer  5   a  is bonded to the nitride semiconductor layer  4   a.    
     In the example illustrated in  FIG. 1 , the substrate  2   a  has insulation properties. The substrate  2   a  is a fused silica substrate. It should be noted that while the substrate  2   a  does not have to be an insulating substrate, and may be a substrate having conductive properties, when the nitride semiconductor layer  4   a  is directly formed on the substrate  2   a,  it is preferable to provide an insulating film on a surface of the substrate  2   a.    
     While the substrate  2   a  may be a single crystalline substrate, because, typically, the single crystalline substrate is expensive, the substrate  2   a  may be a non-single crystalline substrate which is less expensive. As will be described later, in the present invention, because the InGaAlN-based nitride semiconductor layer which is the nitride semiconductor layer  4   a  is a polycrystalline or amorphous film having a film thickness falling within a specific range, a single crystalline substrate does not have to be necessarily used. It should be noted that the substrate  2   a  does not have to be a so-called “crystalline” substrate, and only has to be a substrate which allows film formation through a method which will be described later, and may be a plastic substrate, or the like. 
     The first insulating layer  3   a  functions as a foundation layer of the nitride semiconductor layer  4   a,  and, for example, is a layer having a thickness between 1 nm and 20 nm. Examples of the first insulating layer  3   a  can include an amorphous HfO 2  layer, an Al 2 O 3  layer, a SiO 2  layer, or the like. Because the InGaAlN-based nitride semiconductor exhibits characteristics such as high wettability with respect to the surfaces of these insulating layers, by providing the above-described insulating layer, nucleus generation density is increased, so that it becomes possible to form a flat and high-quality polycrystalline or amorphous InGaAlN-based nitride semiconductor layer. It should be noted that when wettability of the InGaAlN-based nitride semiconductor with respect to the surface of the substrate  2   a  is sufficiently high, even if the InGaAlN-based nitride semiconductor layer is directly formed on a surface of such a substrate, it is possible to obtain a flat and high-quality InGaAlN-based nitride semiconductor layer. 
     In the present embodiment, the nitride semiconductor layer  4   a  is an InN layer provided on the substrate  2   a,  and this InN layer is a polycrystalline or amorphous film having a film thickness between 1 nm and 10 nm. It should be noted that, in the aspect illustrated in  FIG. 1 , a planar shape of the nitride semiconductor layer  4   a  is, for example, a rectangle of approximately, 50 μm×5 μm to 50 μm×10 μm. 
     While “amorphous” means, in a more limited sense, a material state which does not have long-range order as in a crystal, but has short-range order, in the present specification, amorphous also includes “cryptocrystalline” which does not have a complete crystal structure, but exhibits weak diffraction in X-ray analysis. Further, the amorphous film includes even an amorphous film which microscopically includes a microcrystal. 
     The nitride semiconductor layer  4   a  which is a group III-V compound semiconductor may have either group V polarity (N polarity) or group III polarity. The nitride semiconductor layer  4   a  can contain impurities (for example, Zn) as a dopant. Further, even a layer containing light element such as oxygen as impurities is the nitride semiconductor layer  4   a.    
     A film thickness of the nitride semiconductor layer  4   a  is between 1 nm and 10 nm. As described above, it is conventionally considered that if the film thickness of the InN is made thinner, electric characteristics such as mobility becomes poorer, there is no idea of trying to manufacture a transistor using an extremely thin film of several nanometers as a channel layer. However, the present inventors have studied characteristics of the InN layer in the case where the thickness of the InN layer is made extremely thin, and have reached a conclusion that when the thickness of the InN layer falls within the above-described thickness range, even a polycrystalline or amorphous film can exhibit electric characteristics equal to a single crystalline film, and can realize favorable transistor operation, and thus, have made the present invention. 
       FIG. 2  is a diagram illustrating dependency of a film thickness dependency of InN of a ratio between an ON current and an OFF current of a field effect transistor which uses the InN layer as a channel, the film thickness dependency being obtained through experiments by the present inventors.  FIG. 2  indicates a film thickness [nm] on a horizontal axis and indicates the ON current/OFF current ratio on a vertical axis. 
     A measurement result indicated with P 1  in  FIG. 2  is a result in the case where the nitride semiconductor layer  4   a  is polycrystalline InN, a measurement result indicated with P 2  in  FIG. 2  is a result in the case where the nitride semiconductor layer  4   a  is amorphous InN, and a measurement result indicated with P 3  in  FIG. 2  is a result in the case where the nitride semiconductor layer  4   a  is single crystalline InN. 
     Referring to  FIG. 2 , it can be understood that a favorable ON current/OFF current ratio of the nitride semiconductor layer  4   a  between approximately 10 and 10 8  can be realized in the range where the film thickness of the InN layer which is the nitride semiconductor layer  4   a  is between 1 nm and 10 nm. Further, when the film thickness of the nitride semiconductor layer  4   a  is thinner within the range between 1 nm and 10 nm, the ON current/OFF current ratio becomes more favorable. The above-described tendency does not depend on whether the InN layer which is the nitride semiconductor layer  4   a  is a single crystal, a polycrystal or amorphous. That is, by designing the film thickness of the InN layer which is the nitride semiconductor layer  4   a  in the range between 1 nm and 10 nm, even when the InN layer is a polycrystal or amorphous, it is possible to obtain electric characteristics equal to those of a single crystal. 
     It should be noted that such an InN layer is favorably a film deposited through a sputtering method because it is easy to form a film at a relatively low temperature. More preferably, the InN layer is a film deposited through a pulsed sputtering deposition method (PSD method) which has high flexibility in setting of film forming conditions. Further, because when a film is formed at a higher temperature, a size of individual grains becomes larger, which makes it difficult to obtain a flat film, it is preferable to form a film at a temperature less than 600° C. 
     While, in order to form a film of a single crystalline InN layer, it is necessary to make a diffusion length of an atom on a film formation surface sufficiently long, which inevitably requires to form a film at a relatively high temperature, in the present invention, because the InN layer does not have to be a single crystal if the film thickness falls within a range between 1 nm and 10 nm, it is possible to obtain an advantage that there is no obstacle even when a film formation temperature is set low. 
     In addition, typically, a light element such as oxygen is likely to be incorporated into a film as impurities under the influence of remaining gas within a chamber when a film is formed through the sputtering method, and, if the InN layer is a single crystal, there is a problem that such oxygen impurities act as a donor. However, if the InN layer is a polycrystal or amorphous, because oxygen impurities are incorporated into the InN layer in a state where the oxygen impurities are electrically inactive by, for example, being trapped in a grain boundary, it is possible to obtain an advantage that the oxygen impurities are less likely to act as a donor as described above. 
     As with the first insulating layer  3   a,  examples of the second insulating layer  5   a  can include an amorphous HfO 2  layer, an Al 2 O 3  layer, a SiO 2  layer, or the like. As described above, because InN has high wettability with respect to the surfaces of these insulating layers, InN has an effect of suppressing occurrence of a defect at an interface with the InN layer. It should be noted that the second insulating layer  5   a  is, for example, a layer having a thickness between approximately 1 nm and 100 nm. 
     In the example illustrated in  FIG. 1 , thicknesses of the source electrode  61 , the drain electrode  62  and the gate electrode  63  are all approximately 50 nm, and materials of the source electrode  61 , the drain electrode  62  and the gate electrode  63  are all, for example, Au. Both the source electrode  61  and the drain electrode  62  are bonded to the nitride semiconductor layer  4   a  and the second insulating layer  5   a.  The gate electrode  63  is provided on a surface of the second insulating layer  5   a  and bonded to the second insulating layer  5   a.    
     A manufacturing method of the transistor  1   a  will be described next. A wafer corresponding to the substrate  2   a  is prepared. On a surface of this wafer, the first insulating layer  3   a,  the nitride semiconductor layer  4   a  and the second insulating layer  5   a  are laminated in this order. It should be noted that layers corresponding to the first insulating layer  3   a  and the second insulating layer  5   a  may both be layers made of an oxide semiconductor. 
     If the first insulating layer  3   a  and the second insulating layer  5   a  are oxide semiconductors, these layers are both formed through, for example, an atomic layer deposition method (ALD method). An oxygen material when a film is formed through the ALD method is H 2 O, a deposition temperature is approximately 200° C., and a deposition period is approximately one and a half hours. 
     The InN layer corresponding to the nitride semiconductor layer  4   a  is formed through the pulsed sputtering deposition method (PSD method). A deposition rate of the InN layer is approximately 1 nm/min, and the thickness is set within a range between 1 nm and 10 nm. When the deposition temperature of the InN layer depends on the sputtering method, the temperature is room temperature in the case of an amorphous film, and approximately between 300° C. and 500° C. in the case of a polycrystal. That is, the temperature is lower than a typical crystal growth temperature (600° C. or higher) in the case where a film of a single crystalline InN layer is formed. 
     While a film of the InN layer corresponding to the nitride semiconductor layer  4   a  may be formed through the sputtering method other than the PSD method, and may be formed through other evaporation methods or a thin film formation methods such as an MBE method and an MOCVD method, the sputtering method is favorable because it is easy to form a film whose composition is uniform at a relatively low temperature. It should be noted that, as described above, because a size of individual grains become larger when a film of a polycrystalline nitride semiconductor layer  4   a  is formed at a higher temperature, which makes it difficult to obtain a flat film, it is preferable to form a film at a temperature of less than 600° C. 
     Contact holes respectively corresponding to the source electrode  61  and the drain electrode  62  are formed on the second insulating layer  5   a  using lithography technique. Both the source electrode  61  and the drain electrode  62  are formed through lithography after, for example, Au is vacuum evaporated. The gate electrode  63  is formed by patterning Au vacuum evaporated on a surface of the second insulating layer  5   a  through a lift-off method. 
     In this manner, the first insulating layer  3   a,  the nitride semiconductor layer  4   a  and the second insulating layer  5   a  are laminated on a surface of the wafer corresponding to the substrate  2   a  in this order, and separated into chips corresponding to the transistor  1   a  after the source electrode  61 , the drain electrode  62  and the gate electrode  63  are formed. The transistor  1   a  is manufactured through the above-described manufacturing method. 
     Transistor characteristics of the transistor  1   a  which uses the above-described InN layer as a channel will be described next with reference to  FIGS. 3 to 5 . 
       FIG. 3  illustrates I DS -V DS  characteristics of the transistor  1   a  in the case where the nitride semiconductor layer  4   a  is a polycrystalline InN layer. Here, I Ds  indicates a current flowing between a drain and a source, V DS  is a voltage between the drain and the source.  FIG. 3  indicates V DS  [V] on a horizontal axis and indicates I DS  [A] on a vertical axis. 
     The result illustrated in  FIG. 3  is I DS -V DS  characteristics in the case where V GS  which is a voltage between the gate and the source is changed in a step of −0.5 [V] within a range between 5 [V] and −8[V]. The ON current/OFF current ratio is approximately 10 5 .  FIG. 3  illustrates an aspect where I DS  approaches zero as V GS  decreases. Therefore, referring to  FIG. 3 , it can be understood that it is sufficiently possible to switch the ON current/OFF current ratio of the transistor  1   a  by controlling V GS  in the case of the polycrystalline InN. 
       FIG. 4  illustrates I DS -V GS  characteristics of the transistor  1   a  in the case where the nitride semiconductor layer  4   a  is the polycrystalline InN layer.  FIG. 4  indicates V GS  [V] on a horizontal axis and indicates I DS  [A] on a vertical axis. 
     According to the result illustrated in  FIG. 4 , it can be understood that as V GS  decreases within a range between 4 [V] and −8 [V], I DS  also decreases, and a ratio between a value of I DS  when V GS  is 4 [V] and a value of I DS  when V GS  is −8 [V] is approximately 10 5 . Therefore, referring to  FIG. 4 , it can be understood that it is sufficiently possible to control I DS  of the transistor  1   a  by controlling V GS  in the case of the polycrystalline InN. 
       FIG. 5  illustrates I DS -V DS  characteristics of the transistor  1   a  in the case where the nitride semiconductor layer  4   a  is an amorphous InN layer.  FIG. 5  indicates V DS  [V] on a horizontal axis and indicates I DS  [A] on a vertical axis. 
     The result illustrated in  FIG. 5  indicates I DS -V DS  characteristics when V GS  is changed in a step of −2 [V] within a range between 10 [V] and 0 [V].  FIG. 5  illustrates that as V GS  decreases, I DS  also approaches zero. Therefore, referring to  FIG. 5 , it can be understood that it is sufficiently possible to switch the ON current/OFF current ratio of the transistor  1   a  by controlling V GS  in the case of the amorphous InN. 
     (Second Embodiment: InN Layer) 
       FIGS. 6 and 7  are diagrams for explaining one aspect of a configuration of a transistor  1   b  (semiconductor device) according to a second embodiment. It should be noted that, also in the present embodiment, a nitride semiconductor layer  4   b  is an InN layer provided on a substrate  2   b.    
       FIG. 6(A)  is an optical microscope image illustrating a planar shape of the transistor  1   b,  and  FIG. 6(B)  is a diagram mainly illustrating a configuration of a cross-section of the transistor  1   b  along line I-I illustrated in  FIG. 6(A) . 
       FIG. 7(A)  is a TEM (Transmission Electron Microscope) lattice image illustrating a laminate structure of the transistor  1   b,    FIG. 7(B)  illustrates an electron diffraction pattern (Fourier transformed image of the TEM image) from a region indicated with InN in  FIG. 7(A) , and  FIG. 7(C)  is an electron diffraction pattern (Fourier transformed image of the TEM image) from a region indicated with YSZ in  FIG. 7(A) . It can be confirmed from  FIGS. 7(A) to 7(C)  that a single crystalline InN as the nitride semiconductor layer is epitaxially grown on a single crystalline YSZ substrate. 
     The transistor  1   b  includes the substrate  2   b,  a nitride semiconductor layer  4   b,  an insulating layer  5   b,  a source electrode  61 , a drain electrode  62  and a gate electrode  63 . The nitride semiconductor layer  4   b  and the insulating layer  5   b  are sequentially provided on a principal surface S 1   b  of the substrate  2   b.    
     In the present embodiment, the substrate  2   b  is an yttria-stabilized zirconia substrate (YSZ substrate). The YSZ substrate has relatively small in-plane lattice mismatch with not only InN but also nitride semiconductors such as InGaN, InAlN and InAlGaN which includes InN as a main component. The principal surface S 1   b  of the substrate  2   b  is bonded to the nitride semiconductor layer  4   b  and has plane indices (111). The principal surface S 1   b  is made flat at the atomic level. 
     The InN layer as the nitride semiconductor layer  4   b  is provided on the substrate  2   b.  The nitride semiconductor layer  4   b  is bonded to the substrate  2   b.  The nitride semiconductor layer  4   b  is a single crystal. The nitride semiconductor layer  4   b  is an epitaxial layer formed through epitaxial growth from the principal surface S 1   b  of the substrate  2   b.  The nitride semiconductor layer  4   b  can have any of N-polarity and group III polarity. The nitride semiconductor layer  4   b  can contain impurity Zn (zinc). The planar shape of the nitride semiconductor layer  4   b  is, for example, a rectangle of approximately 50 μm×5 μm to 50 μm×10 μm. 
     The film thickness of the InN layer which is the nitride semiconductor layer  4   b  falls between 1 nm and 10 nm. As already described with reference to  FIG. 2 , in the range where the film thickness of the InN layer which is the nitride semiconductor layer falls between 1 nm and 10 nm, the ON current/OFF current ratio of the nitride semiconductor layer is between approximately 10 and 10 8 , so that it is possible to realize a favorable ON current/OFF current ratio. Further, as the film thickness of the nitride semiconductor layer is thinner within a range between 1 nm and 10 nm, the ON current/OFF current ratio becomes more favorable. In addition, the above-described tendency does not depend on whether the InN layer which is the nitride semiconductor layer is a single crystal, a polycrystal or amorphous. 
     Therefore, in the present embodiment, while the InN layer which is the nitride semiconductor layer  4   b  is a single crystalline InN which is epitaxially grown on the single crystalline YSZ substrate, even when the InN layer is a polycrystalline or amorphous InN layer deposited on a fused silica substrate, or the like, it is possible to obtain electric characteristics equal to those of a single crystal by designing the film thickness in a range between 1 nm and 10 nm. 
     The insulating layer  5   b  is bonded to the nitride semiconductor layer  4   b.  Examples of the insulating layer  5   b  can include an amorphous HfO 2  layer, an Al 2 O 3  layer, a SiO 2  layer, or the like. As already described, because the InN has high wettability with respect to the surfaces of these insulating layers, it is possible to provide an effect of suppressing occurrence of a defect at an interface with the InN layer. It should be noted that the film thickness of the insulating layer  5   b  is, for example, between 1 nm and 100 nm. 
     In the example illustrated in  FIG. 6 , the thicknesses of the source electrode  61 , the drain electrode  62  and the gate electrode  63  are all approximately 50 nm, and materials of the source electrode  61 , the drain electrode  62  and the gate electrode  63  are all, for example, Au. Both the source electrode  61  and the drain electrode  62  are bonded to the nitride semiconductor layer  4   b  and the insulating layer  5   b.  The gate electrode  63  is provided on the surface of the insulating layer  5   b  and bonded to the insulating layer  5   b.    
     A manufacturing method of the transistor  1   b  will be described next. A wafer corresponding to the substrate  2   b  is prepared. While, in the present embodiment, this wafer is a YSZ substrate, if a polycrystalline or amorphous InN layer is formed, this wafer may be a non-single crystalline substrate or an insulating substrate (for example, a fused silica substrate). On the surface of this wafer, the nitride semiconductor layer  4   b  and the insulating layer  5   b  are laminated in this order. 
     The InN layer corresponding to the nitride semiconductor layer  4   b  is formed through the pulsed sputtering deposition method (PSD method) as in the first embodiment. A deposition rate of the InN layer is approximately 1 nm/min, and the thickness is set in a range between 1 nm and 10 nm. Because, in the present embodiment, a single crystalline InN is formed, an epitaxial temperature is between 600° C. and 700° C. However, if an amorphous InN is deposited through the sputtering method, the deposition temperature is made an ambient temperature, while, if a polycrystalline InN is deposited, the deposition temperature is made between approximately 300° C. and 500° C. That is, the deposition temperature is lower than a typical crystal growth temperature (600° C. or higher) in the case where a film of a single crystalline InN layer is formed. 
     While a film of the InN layer corresponding to the nitride semiconductor layer  4   b  may be formed through the sputtering method other than the PSD method, and may be formed through other evaporation methods or a thin film formation method such as an MBE method and an MOCVD method, the sputtering method is favorable because it is easy to form a film whose composition is uniform at a relatively low temperature. It should be noted that, as described above, a size of individual grains becomes larger as the film is formed at a higher temperature, which makes it difficult to obtain a flat film, a film of a polycrystalline nitride semiconductor layer  4   a  is preferably formed at a temperature of less than 600° C. 
     When the insulating layer  5   b  is an oxide semiconductor, for example, a film is formed through an atomic layer deposition method (ALD method). An oxygen material when the film is formed through the ALD method is H 2 O, the deposition temperature is 200° C., and a deposition period is approximately one and a half hours. 
     Contact holes respectively corresponding to the source electrode  61  and the drain electrode  62  are formed in the insulating layer  5   b  using lithography technique. Both the source electrode  61  and the drain electrode  62  are formed through lithography after, for example, Au is vacuum evaporated. The gate electrode  63  is formed by patterning Au which is vacuum evaporated on a surface of the insulating layer  5   b  using a lift-off method. 
     In this manner, on the surface of the wafer corresponding to the substrate  2   b,  the nitride semiconductor layer  4   b  and the insulating layer  5   b  are laminated in this order, and separated into chips corresponding to the transistor  1   b  after the source electrode  61 , the drain electrode  62  and the gate electrode  63  are formed. The transistor  1   b  is manufactured through the above-described manufacturing method. 
     Transistor characteristics of the transistor  1   b  which uses the above-described InN layer as a channel will be described next with reference to  FIG. 8 . 
       FIGS. 8(A) and 8(B)  illustrate I DS -V DS  characteristics ( FIG. 8(A) ) in the case where V GS  of the transistor  1   b  in the case where the nitride semiconductor layer  4   b  is a single crystalline InN layer having a film thickness of 2 nm, is changed in a step of −1 [V] within a range between +2 [v] and −2 [V], and I DS -V GS  characteristics ( FIG. 8(B) ) under V DS  of 5 [V].  FIG. 8(A)  indicates V DS  [V] on a horizontal axis and indicates I DS  [mA/mm] on a vertical axis. Further,  FIG. 8(B)  indicates V G  [V] on a horizontal axis and indicates I DS  [A] on a vertical axis. 
       FIGS. 8(C) and 8(D)  illustrate I DS -V DS  characteristics ( FIG. 8(C) ) in the case where V GS  of the transistor  1   b  in the case where the nitride semiconductor layer  4   b  is a single crystalline InN layer having a film thickness of 5 nm, is changed in a step of −2 [V] within a range between +4 [V] and −10 [V], and I DS -V GS  characteristics ( FIG. 8(D) ) under V DS  of 5 [V].  FIG. 8(C)  indicates V DS  [V] on a horizontal axis, and indicates I DS  [mA/mm] on a vertical axis.  FIG. 8(D)  indicates V G  [V] on a horizontal axis and indicates I DS  [A] on a vertical axis. 
     Referring to the results illustrated in  FIGS. 8(A) to 8(D) , it can be understood that it is sufficiently possible to switch the ON current/OFF current ratio of the transistor  1   b  by controlling V GS  in the case of a single crystalline InN. 
     Further, as described above, even when the InN layer is a polycrystal or amorphous, by designing the film thickness within a range between 1 nm and 10 nm, it is possible to obtain electric characteristics equal to those of a single crystal. Therefore, even when the nitride semiconductor layer  4   b  is a polycrystalline or amorphous InN layer, it is sufficiently possible to switch the ON current/OFF current ratio of the transistor  1   b.    
     While the principle of the present invention has been described in the preferred embodiments, a person skilled in the art would recognize that arrangement and details of the present invention can be modified without departing from such principle. 
     For example, the semiconductor device according to the present invention can be made a semiconductor device having a configuration including a laminate structure in which the nitride semiconductor layer having composition different from that of InN is bonded on at least one principle surface of the above-described InN layer, that is, a semiconductor device having a heterojunction structure. 
     The present invention is not limited to a specific configuration disclosed in the present embodiments. Therefore, all modifications and changes that result from the claims and the scope of its spirit are claimed. For example, the nitride semiconductor layers  4   a  and  4   b  of the present embodiments can be applied to semiconductor devices other than a transistor. In the case of the nitride semiconductor layer  4   a,  the first insulating layer  3   a  is also applied to such a semiconductor device along with the nitride semiconductor layer  4   a.    
     (Third Embodiment: InGaAlN Layer) 
     In the above-described first and second embodiments, the nitride semiconductor layer is made an InN layer expressed with a general expression In x Ga y Al z N (where x+y+z=1.0) where x=1. Further, it is clarified that, in the case of the InN layer, by setting the thickness within a specific thickness range, it is possible to obtain a “non-single crystalline” film exhibiting channel characteristics equal to those of a single crystal. 
     If desired electric characteristics can be obtained only within a specific thickness range (1 to 10 nm), there is a problem that flexibility of design of the semiconductor device cannot be secured. Therefore, the present inventors have further studied electric characteristics of an InGaAlN-based nitride semiconductor and found that even a “non-single crystalline film” exhibits channel characteristics equal to those of a single crystal if a composition range of the film falls within a specific composition range. 
     It has been conventionally considered that it is difficult to change the composition of In in the InGaAlN-based nitride semiconductor in a broad range in thermodynamic terms, because an ion radius of In in the InGaAlN-based nitride semiconductor is larger than those of other elements. However, such conventional knowledge is one that is applied to an InGaAlN-based nitride semiconductor obtained through a CVD method in which a film is formed at a relatively high temperature. The present inventors have pursued study based on idea that the above-described knowledge is one that is merely applied to the InGaAlN-based nitride semiconductor formed under a thermal equilibrium state, and if the InGaAlN-based nitride semiconductor is deposited through a sputtering method in which a film can be formed at a relatively low temperature, the film may be quenched while a state is kept a thermally non-equilibrium state, and film formation may stably proceed, thereby achieving the present invention. 
     In the following examples, a result of study as to how electric characteristics of the InGaAlN-based nitride semiconductor expressed with a general expression In x Ga y Al z N (where x+y+z=1.0) depends on its composition will be described. 
     A field effect transistor  1   a  having a configuration illustrated in  FIG. 1  was experimentally manufactured, and electric characteristics were evaluated with ratios between ON currents and OFF currents (ON/OFF ratios) and maximum current density of the transistor  1   a  while composition (In x Ga y Al z N) of the nitride semiconductor layer  4   a  which becomes a channel is variously changed. The substrate  2   a  is a fused silica substrate, the first insulating layer  3   a  is HfO 2  having a thickness of 20 nm, the second insulating layer  5   a  is HfO 2  which is also used as a gate insulating film and has a thickness of 20 nm. It should be noted that the substrate  2   a  may be a non-single crystalline substrate or an insulating substrate other than the fused silica substrate, and the first insulating layer  3   a  and the second insulating layer  5   a  may be an Al 2 O 3  layer or a SiO 2  layer. Further, all transistors  1   a  have a gate length of 5 μm and a channel width of 50 μm. 
     All In x Ga y Al z N layers are formed through the sputtering method (in the present embodiment, the PSD method). The deposition rate is approximately 1 nm/min. Further, the deposition temperature is made less than 600° C., and the In x Ga y Al z N layers are made polycrystalline or amorphous In x Ga y Al z N layers. 
       FIG. 9  is a diagram in which composition of the nitride semiconductor layer  4   a  of the transistor  1   a  which is experimentally manufactured is plotted in a ternary phase diagram of In x Ga y Al z N. 
       FIG. 10  is a diagram in which composition of the nitride semiconductor layer  4   a  of the transistor exhibiting an ON/OFF ratio of 10 2  or higher is plotted with “” and composition of other nitride semiconductor layer  4   a  is plotted with “∘” in a ternary phase diagram of In x Ga y Al z N among the composition illustrated in  FIG. 9 . 
     According to this result, when the composition expressed with a general expression In x Ga y Al z N (where x+y+z=1.0) falls within a range of 0.3≦x≦1.0 and 0≦z&lt;0.4, a transistor  1   a  exhibiting the ON/OFF ratio of 10 2  or higher can be obtained. 
     Further,  FIG. 11  is a diagram in which composition of the nitride semiconductor layer of a transistor exhibiting the ON/OFF ratio of 10 3  or higher is plotted with “” and composition of other nitride semiconductor layers is plotted with “∘” in a ternary phase diagram of In x Ga y Al z N. 
     According to this result, when composition expressed with a general expression In x Ga y Al z N (where x+y+z=1.0) falls within a range of 0.3≦x≦1.0 and 0≦z&lt;0.4, a transistor  1   a  exhibiting the ON/OFF ratio of 10 2  or higher can be obtained. 
     Further,  FIG. 12  is a diagram in which composition of a nitride semiconductor layer having transistor characteristics exhibiting maximum current density exceeding 5 mA/mm is plotted with “” and composition of other nitride semiconductor layers is plotted with “∘” in a ternary phase diagram of In x Ga y Al z N. 
     According to this result, when composition expressed with a general expression In x Ga y Al z N (where x+y+z=1.0) falls within a range of 0.5≦x≦1.0 and 0≦z&lt;0.1, transistor characteristics exhibiting the maximum current density exceeding 5 mA/mm can be obtained. 
     As described above, when composition of the nitride semiconductor layer expressed with a general expression In x Ga y Al z N (where x+y+z=1.0) falls within a range of 0.3≦x≦1.0 and 0≦z&lt;0.4, even when the film is a non-single crystalline film, it is possible to provide sufficient channel characteristics (having the ON/OFF ratio of 10 2  or higher) as transistor operation, and, when the composition falls within a range of 0≦z≦0.2 when 0.3≦x&lt;0.7 and 0≦z&lt;0.1 when 0.7≦x≦1.0, the ON/OFF ratio becomes further higher by an order of magnitude (10 3  or higher). Further, when the composition of the nitride semiconductor layer falls within a range of 0.5≦x≦1.0 and 0≦z&lt;0.1, a transistor having excellent transistor characteristics of maximum current density exceeding 5 mA/mm can be obtained. 
     It should be noted that, as described in the first embodiment, when InN which is a nitride semiconductor having composition of x=1.0 when the nitride semiconductor is expressed with a general expression In x Ga y Al z N is used as a channel layer, it is impossible to obtain sufficient transistor characteristics if the film thickness exceeds 10 nm. 
     Therefore, in order to secure flexibility of design of the semiconductor device, it is preferable to remove InN from the above-described composition range, that is, the In composition ratio x of the nitride semiconductor layer is preferably 0.99 or less (x≦0.99). 
     It is known that a film of an InGaAlN-based nitride semiconductor containing 1% or higher of Al or Ga, that is, a nitride semiconductor in which x≦0.99 when the nitride semiconductor is expressed with a general expression In x Ga y Al z N, becomes structurally strong, and a defect is less likely to occur (see, for example, Non Patent Literature 4). This is considered because of a phenomenon that an InGaAlN-based nitride semiconductor containing 1% or higher of Al or Ga is thermodynamically likely to be phase separated, which makes it likely to make concentration of Al or Ga locally non-uniform, resulting in suppressing propagation of dislocation. 
       FIG. 13  is a diagram illustrating electric characteristics of a transistor including a nitride semiconductor layer expressed with In x Ga y Al z N where x=0.64, y=0 and z=0.36 as a channel, and  FIG. 13(A)  illustrates I DS -V DS  characteristics in the case where V GS  is changed in a step of −1 [V] within a range between +5 [V] and −7 [V]. Further,  FIG. 13(B)  illustrates I DS -V GS  characteristics under V DS  of 1 [V]. 
     This transistor is obtained by depositing 5 nm of a channel layer having composition of In 0.64 Al 0.36 N on a fused silica substrate through a sputtering method at an ambient temperature. It should be noted that a gate insulating film is HfO 2 , a gate length is 5 μm, and a channel width is 50 μm. 
     The above-described composition falls within a range of 0.3≦x≦1.0 and 0≦z&lt;0.4, the ON/OFF ratio is 7×10 2 , and maximum current density is 0.4 mA/mm. 
       FIG. 14  illustrates electric characteristics of a transistor including a nitride semiconductor layer expressed with In x Ga y Al z N where x=0.34, y=0.33 and z=0.33 as a channel, and  FIG. 14(A)  illustrates I DS -V DS  characteristics in the case where V GS  is changed in a step of −1 [V] within a range between +5 [V] and −7 [V]. Further,  FIG. 14(B)  illustrates I DS -V GS  characteristics under V DS  of 1 [V]. 
     This transistor is obtained by depositing 5 nm of a channel layer having composition of In 0.34 Ga 0.33 Al 0.33 N on a fused silica substrate at a substrate temperature of 400° C. through the sputtering method. It should be noted that a gate insulating film is HfO 2 , a gate length is 5 μm, and a channel width is 50 μm. 
     The above-described composition also falls within a range of 0.3≦z≦1.0 and 0≦z&lt;0.4, and has the ON/OFF ratio of 1×10 3  and maximum current density of 3.4×10 −4  mA/mm. 
       FIG. 15  is a diagram illustrating electric characteristics of a transistor including a nitride semiconductor layer expressed with In x Ga y Al z  N where x=0.42, y=0.42 and z=0.16 as a channel, and  FIG. 15(A)  illustrates I DS -V DS  characteristics in the case where V GS  is changed in a step of −2 [V] within a range between +2 [V] and −6 [V]. Further,  FIG. 15(B)  illustrates I DS -V GS  characteristics under V DS  of 1 [V]. 
     This transistor is obtained by depositing 5 nm of a channel layer having composition of In 0.42 Ga 0.42 Al 0.16 N on a fused silica substrate at a substrate temperature of 400° C. through the sputtering method. It should be noted that a gate insulating film is HfO 2 , a gate length is 5 μm, and a channel width is 50 μm. 
     The above-described composition falls within a range of 0.3≦x&lt;0.7 and 0≦z&lt;0.2, and has the ON/OFF ratio of 1×10 3  and the maximum current density of 1×10 −3  mA/mm. 
       FIG. 16  is a diagram illustrating electric characteristics of a transistor including a nitride semiconductor layer expressed with In x Ga y Al z N where x=0.3, y=0.7 and z=0 as a channel, and  FIG. 16(A)  illustrates I DS -V DS  characteristics in the case where V GS  is changed in a step of −0.5 [V] within a range between +5 [V] and −9 [V]. Further,  FIG. 16(B)  illustrates I DS -V GS  characteristics under V DS  of 1 [V]. 
     This transistor is obtained by depositing 30 nm of a channel layer having composition of In 0.3 Ga 0.7 N on a fused silica substrate at a substrate temperature of 400° C. through the sputtering method. It should be noted that a gate insulating film is HfO 2 , a gate length is 5 μm, and a channel width is 50 μm. 
     The above-described composition also falls within a range of 0.3≦x&lt;0.7 and 0≦z&lt;0.2, and has the ON/OFF ratio of 1×10 6  and maximum current density of 0.5 mA/mm. 
       FIG. 17  is a diagram illustrating electric characteristics of a transistor including a nitride semiconductor layer expressed with In x Ga y Al z N where x=0.67, y=0.33 and z=0 as a channel, and  FIG. 17(A)  illustrates I DS -V DS  characteristics in the case where V GS  is changed in a step of −1 [V] within a range between +4 [V] and −9 [V]. Further,  FIG. 17(B)  illustrates I DS -V GS  characteristics under V DS  of 1 [V]. 
     This transistor is obtained by depositing 6 nm of a channel layer having composition of In 0.67 Ga 0.33 N on a fused silica substrate at a substrate temperature of 400° C. through the sputtering method. It should be noted that a gate insulating film is HfO 2 , a gate length is 5 μm and a channel width is 50 μm. 
     The above-described composition falls within a range of 0.5≦x≦1.0 and 0≦z&lt;0.1, and has an ON/OFF ratio of 1×10 4  and maximum current density of 7.5 mA/mm. 
       FIG. 18  is a diagram illustrating electric characteristics of a transistor including a nitride semiconductor layer expressed with In x Ga y Al z N where x=0.5, y=0.5 and z=0, and  FIG. 18(A)  illustrates I DS -V DS  characteristics in the case where V GS  is changed in a step of −1 [V] within a range between 0 [V] and −9 [V]. Further,  FIG. 18(B)  illustrates I DS -V GS  characteristics under V DS  of 5 [V]. 
     This transistor is obtained by depositing 45 nm of a channel layer having composition of In 0.5 Ga 0.5 N on a fused silica substrate at a substrate temperature of 400° C. through the sputtering method. It should be noted that this transistor has a ring gate structure, in which a gate insulating film is HfO 2 , a gate ring diameter is 100 μm, and a channel length is 10 μm. 
     The above-described composition also falls within a range of 0.5≦x≦1.0 and 0≦z&lt;0.1, and has an ON/OFF ratio of 1×10 8  and maximum current density of 25 mA/mm. 
     The transistor characteristics illustrated in  FIGS. 13 to 18  are characteristics of part of a number of transistors experimentally manufactured by the present inventors. As a result of performing characteristics analysis on a number of transistors, the above-described conclusion has been reached as to composition of the nitride semiconductor. 
     That is, when the composition expressed with a general expression In x Ga y Al z N (where x+y+z=1.0) falls within a range of 0.3≦x≦1.0 and 0≦z&lt;0.4, a transistor  1   a  exhibiting an ON/OFF ratio of 10 2  or higher can be obtained. 
     Further, when the composition expressed with a general expression In x Ga y Al z N (where x+y+z=1.0) falls within a range of 0.3≦x≦1.0 and 0≦z&lt;0.4, a transistor  1   a  exhibiting an ON/OFF ratio of 10 2  or higher can be obtained. 
     Still further, when the composition expressed with a general expression In x Ga y Al z N (where x+y+z=1.0) falls within a range of 0.5≦x≦1.0 and 0≦z&lt;0.1, transistor characteristics having maximum current density exceeding 5 mA/mm can be obtained. 
     While the semiconductor device according to the present invention including a nitride semiconductor layer expressed with a general expression In x Ga y Al z N (where x+y+z=1.0) as a channel has been described above, it goes without saying that the transistor configuration can be variously changed. Some examples of the transistor configuration will be described below. 
       FIG. 19  is a diagram illustrating a configuration example of a transistor  1   c  having a laminate structure (heterojunction structure) in which an AlN layer and a second nitride semiconductor layer  6   c  are bonded on the above-described nitride semiconductor layer  4   c  of In x Ga y Al z N. 
     In the example illustrated in  FIG. 19 , a substrate  2   c  is a synthetic quartz substrate. The nitride semiconductor layer  4   c  is, for example, a polycrystalline or amorphous film having a film thickness of 3 nm. An amorphous HfO 2  layer having a film thickness of 15 nm is provided on the second nitride semiconductor layer  6   c  as an insulating layer  5   c.  By placing the AlN layer between the nitride semiconductor layer  4   c  and the HfO 2  layer as the insulating layer  5   c,  as the second nitride semiconductor layer  6   c,  a favorable interface is obtained. 
       FIG. 20  is a diagram illustrating a configuration example of a transistor  1   d  having a bottom gate structure. 
     Also in the example illustrated in  FIG. 20 , a substrate  2   d  is a synthetic quartz substrate. A nitride semiconductor layer  4   d  is, for example, a polycrystalline or amorphous film having a film thickness of 3 nm. An amorphous HfO 2  layer having a film thickness between 100 nm and 150 nm is provided between the nitride semiconductor layer  4   d  and the substrate  2   d  as an insulating layer  5   d,  and a gate  63  is formed with an ITO film having a thickness of approximately 90 nm. 
     As described above, the semiconductor device according to the present invention may have a laminate structure (heterojunction structure) in which the second nitride semiconductor layer having a different composition from that of the nitride semiconductor layer is bonded on at least one principle surface of the above-described nitride semiconductor layer. 
     At this time, the second nitride semiconductor layer may be a nitride semiconductor layer expressed with the above-described general expression In x Ga y Al z N (where x+y+z=1.0). For example, a transistor may be configured such that the nitride semiconductor layer is In x1 Ga y1 Al z1 N, and the second nitride semiconductor layer is In x2 Ga y2 Al z2 N (where x2≠x1), and may be configured to have a double heterostructure in which the nitride semiconductor layer of In x1 Ga y1 Al z1 N is vertically put between the second nitride semiconductor layer of In x2 Ga y2 Al z2 N. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, it is possible to provide a semiconductor device in which constraints to manufacturing conditions are drastically eliminated, and which includes an InGaAlN-based nitride semiconductor layer which is inexpensive and has excellent electric characteristics. 
     REFERENCE SIGNS LIST 
     
         
           1   a,    1   b,    1   c,    1   d  transistor 
           2   a,    2   b,    2   c,    2   s  substrate 
           3   a  first insulating layer 
           4   a,    4   b,    4   c,    4   d  nitride semiconductor layer 
           5   a  second insulating layer 
           5   b,    5   c,    5   d  insulating layer 
           6   c  second nitride semiconductor layer 
           61  source electrode 
           62  drain electrode 
           63  gate electrode 
         S 1   a,  S 1   b  principle surface