Patent Publication Number: US-8987780-B2

Title: Graphene capped HEMT device

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to the fabrication of heterojunction transistors for high-frequency, high-power applications. 
     2. Description of the Related Art 
     A high electron mobility transistor (HEMT) is type of field effect transistor (FET) in which an electron current flows freely within a conduction channel in an un-doped semiconductor. Such a substantially unobstructed conduction channel forms adjacent to a heterojunction, i.e., a boundary between two different semiconductors.  FIG. 1  shows an example of a conventional HEMT device  100  that includes a heterostructure  102  made of two different semiconductor materials, layers  102   a  and  102   b [“High Electron Mobility Transistors ,” Laboratory for Millimeter Wave Electronics, Zurich, Switzerland (http://www.mwe.ee.ethz.ch/en/about-mwe-group/research/vision-and-aim/high-electron-mobility-transistors-hemt.html)]. The conventional HEMT device  100  can be built on any one of various different semiconductor substrates  104 . A buffer layer  106  can be inserted between the substrate  104  and the heterostructure  102 . Source and drain contacts  108  and  110 , respectively, and a gate  112  are formed on the upper layer  102   b  of the heterostructure  102 . The gate  112  modulates electron mobility within a conduction channel  114 . 
     Formation of the conduction channel  114  at the heterojunction can be understood by considering energy levels within the heterostructure. A series of plots  116  to the right of the conventional HEMT device  100  show electron concentration  118  and energy levels  120  and  122  as a function of depth below a surface  124  of the heterostructure  102 , along a spatial axis  126 . The upper semiconductor in the heterostructure  102 , layer  102   b , is a negatively doped material having a wide energy band gap. The lower semiconductor in the heterostructure  102 , layer  102   a , is an un-doped material having a narrow energy band gap. [The term “band gap” refers to the difference between the energy of conduction band electrons (free electrons) and the energy of valence band electrons (atomically bound electrons) i.e., the amount of energy needed to liberate valence electrons from atoms in the semiconductor crystal.] Because the band gaps differ, the conduction band energies  120  and  122  of the materials do not coincide. Thus, when two such semiconductors are placed in contact with one another, their energy levels are discontinuous at the boundary. Such a discontinuity gives rise to a potential well  117  that develops at the boundary (heterojunction). The potential well  117  traps unbound donor electrons from the n-doped material at the surface of the un-doped material, resulting in a peak electron concentration  118  at the heterojunction. Such trapped donor electrons are sometimes referred to as a “two-dimensional electron gas.” The location of the potential well  117  thus defines the conduction channel  114  of the HEMT. Source and drain regions at either end of the conduction channel  114  can be negatively doped or un-doped, depending on the device. 
     Because the conduction channel  114  lacks dopant impurities, electron mobility within the conduction channel  114  of a HEMT is high compared to the electron mobility in conventional FET devices. Such a high electron mobility allows a large electron current to flow within the conduction channel  114 , thereby increasing the speed of the device. A voltage applied to the gate  112  alters the conductivity within the conduction channel  114 , thereby modulating the electron current. The ability to support such a high electron current makes HEMT devices suitable for high-power, high-frequency applications such as chips used in RF communication devices (e.g., cell phones, satellite TV receivers, radar equipment, and the like). Furthermore, semiconductor materials typically used in HEMT devices include compound semiconductors that have high intrinsic electron mobility, such as, for example, gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), gallium nitride (GaN), and indium gallium nitride (InGaN), among others. GaN HEMTs are known to perform particularly well in high-power applications. 
     Graphene has drawn attention in recent years as a material for use in FETs due to its extraordinary properties, as shown in  FIGS. 2A and 2B . Graphene is a monolayer of carbon graphite atoms arranged in a honeycomb crystal lattice ( FIG. 2A ). Crystalline graphite is made of stacked sheets of graphene. Although graphene was known for many years, a single graphene sheet was not isolated until 2004, for which a Nobel prize in physics was awarded in 2010. Mechanically, graphene is one of the strongest materials ever tested, more than 100 times stronger than a comparable sheet of steel, (if steel could be made as thin as a graphene sheet). Graphene sheets are flexible and can be rolled into carbon nanotubes or formed into fullerene structures. Graphene is also very light weight, weighing only 0.77 mg per square meter. 
     Electrically, graphene has high electron mobility over a wide temperature range, lower resistance at room temperature than any known material, and low noise. Furthermore, a graphene film can be epitaxially grown on silicon carbide (SiC) by heating the SiC in a vacuum chamber to temperatures exceeding 1100C. The graphene film can then be patterned using conventional microelectronics techniques. Graphene has been studied as a material for use in microelectronics, such as in graphene field effect transistors (GFETs) [Meric, et al.,  Proceedings of the IEEE IEDM Conference , Dec. 5-7, 2011, pp. 2.1.1-2.1.4]. In a GFET, a graphene layer is used as the conducting channel to increase electron mobility for high-frequency applications. A GFET thus provides an alternative way to achieve a high electron mobility transistor without the use of a heterostructure. 
     BRIEF SUMMARY 
     A graphene capped HEMT device and a method of fabricating same are disclosed. The graphene capped HEMT device includes one or more graphene films that enhance device performance and/or reliability of an exemplary AlGaN/GaN heterostructure transistor used in high-frequency, high-energy applications. Such applications include, in particular, wireless telecommunications. 
     HEMT devices generally are advantageous because the conduction channel of the transistor resides within an un-doped material. In an un-doped material, electron mobility is enhanced because electrons experience fewer collisions. Higher electron mobility generally results in a faster device that switches on and off at high speed. Hence, such a device is useful for high-frequency signal processing. 
     The HEMT device disclosed makes use of the extraordinary material properties of graphene. One of the graphene films acts as a heat sink underneath the transistor, while the other stabilizes the source, drain, and gate regions of the transistor to prevent cracking and improve reliability during high-power operation. It is shown that a one-atom thick layer of graphene can replace a three-layer film stack previously used to prevent cracking, without otherwise degrading device performance. In addition, the HEMT device disclosed includes one or more hexagonal boron nitride adhesion layers to facilitate deposition of the compound nitride semiconductors onto the graphene films. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the drawings, identical reference numbers identify similar elements. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. 
         FIG. 1  is a side view of a conventional high electron mobility transistor (HEMT) device, including graphs of energy levels and electron concentrations associated with a generic heterostructure, according to the prior art. 
         FIG. 2A  is a pictorial view of a model of the molecular structure of a graphene crystalline sheet, according to the prior art. 
         FIG. 2B  shows a series of prior art bar graphs comparing mechanical, electrical, and thermal properties of graphene with those of other materials. 
         FIG. 3  is a cross-sectional view of a flawed AlGaN/GaN HEMT device that exhibits cracking under high-frequency operation, according to the prior art. 
         FIG. 4  is a cross section of a prior art AlGaN/GaN HEMT device that includes a multi-layer cap to reduce the cracks shown in  FIG. 3 , according to one embodiment. 
         FIG. 5  is a high-level flow diagram summarizing a processing sequence for fabricating a graphene capped HEMT device as described herein, according to two alternative embodiments. 
         FIG. 6A  is a process flow diagram showing a detailed sequence of processing steps that can be used to form a film stack for the graphene-capped HEMT as described herein, according to an exemplary embodiment. 
         FIG. 6B  is a cross-sectional view of the film stack formed by the processing steps shown in  FIG. 6A . 
         FIG. 6C  is a pictorial view of molecular structure of a hexagonal boron nitride (h-BN) crystalline film, according to the prior art. 
         FIG. 7A  is a process flow diagram showing a sequence of process steps that can be used to fabricate a graphene-capped HEMT as described herein, according to an exemplary embodiment. 
         FIGS. 7B-7D  are cross-sectional views illustrating fabrication of the transistor graphene-capped HEMT in accordance with the exemplary process flow shown in  FIG. 7A . 
         FIG. 8A  is a process flow diagram showing a sequence of process steps that can be used to form metal source, drain, and gate contacts of a graphene-capped HEMT device as described herein, according to an exemplary embodiment. 
         FIGS. 8B-8C  are cross-sectional views illustrating fabrication of the metal source, drain, and gate contacts of a graphene-capped HEMT device in accordance with the exemplary process flow shown in  FIG. 8A . 
         FIG. 9  is a cross-sectional view of a completed graphene-capped HEMT device as described herein, according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, certain specific details are set forth in order to provide a thorough understanding of various aspects of the disclosed subject matter. However, the disclosed subject matter may be practiced without these specific details. In some instances, well-known structures and methods of semiconductor processing comprising embodiments of the subject matter disclosed herein have not been described in detail to avoid obscuring the descriptions of other aspects of the present disclosure. 
     Unless the context requires otherwise, throughout the specification and claims that follow, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open, inclusive sense, that is, as “including, but not limited to.” 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same aspect. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more aspects of the present disclosure. 
     Reference throughout the specification to integrated circuits is generally intended to include integrated circuit components built on semiconducting substrates, whether or not the components are coupled together into a circuit or able to be interconnected. Throughout the specification, the terms “layer” is used in its broadest sense to include a thin film, a cap, or the like. The term “compound semiconductor” generally includes any semiconductor that has one or more materials outside column IV of the periodic table of the elements (e.g., carbon, silicon, germanium, etc.), or combinations of such materials. The term “high-frequency” in the context of operating wireless communication devices, is meant in a relative sense, and is not limited to a designated frequency range (e.g., a particular “HF band” within the range of 3-30 MHz). 
     Reference throughout the specification to conventional thin film deposition techniques for depositing silicon nitride, silicon dioxide, metals, or similar materials include such processes as chemical vapor deposition (CVD), low-pressure chemical vapor deposition (LPCVD), metal organic chemical vapor deposition (MOCVD), plasma-enhanced chemical vapor deposition (PECVD), plasma vapor deposition (PVD), atomic layer deposition (ALD), molecular beam epitaxy (MBE), electroplating, electro-less plating, and the like. Specific embodiments are described herein with reference to examples of such processes. However, the present disclosure and the reference to certain deposition techniques should not be limited to those described. For example, in some circumstances, a description that references CVD may alternatively be done using PVD, or a description that specifies electroplating may alternatively be accomplished using electro-less plating. Furthermore, reference to conventional techniques of thin film formation may include growing a film in-situ. For example, in some embodiments, controlled growth of an oxide to a desired thickness can be achieved by exposing a silicon surface to oxygen gas or to moisture in a heated chamber. 
     Reference throughout the specification to conventional photolithography techniques, known in the art of semiconductor fabrication for patterning various thin films, includes a spin-expose-develop process sequence typically followed by an etch process. Alternatively or additionally, photoresist can also be used to pattern a hard mask, which, in turn, can be used to pattern an underlying film. 
     Reference throughout the specification to conventional etching techniques known in the art of semiconductor fabrication for selective removal of polysilicon, silicon nitride, silicon dioxide, metals, photoresist, polyimide, or similar materials includes such processes as wet chemical etching, reactive ion (plasma) etching (RIE), washing, wet cleaning, pre-cleaning, spray cleaning, chemical-mechanical planarization (CMP) and the like. Specific embodiments are described herein with reference to examples of such processes. However, the present disclosure and the reference to certain deposition techniques should not be limited to those described. In some instances, two such techniques may be interchangeable. For example, stripping photoresist may entail immersing a sample in a wet chemical bath or, alternatively, spraying wet chemicals directly onto the sample. 
     Specific embodiments are described herein with reference to examples of HEMT devices that have been produced; however, the present disclosure and the reference to certain materials, dimensions, and the details and ordering of processing steps are exemplary and should not be limited to those shown. 
     In the figures, identical reference numbers identify similar features or elements. The sizes and relative positions of the features in the figures are not necessarily drawn to scale. 
       FIG. 3  is a cross-sectional view drawn from a scanning electron micrograph (SEM) image  132  of a state-of-the-art HEMT device  130  in which the heterostructure  102  has a top layer  102   a  made of aluminum gallium nitride (Al x Ga (1-x) N) on a lower layer  102   b  made of gallium nitride (GaN). The substrate  104  is either aluminum oxide (Al 2 O 3 ) or silicon carbide (SiC); the buffer layer  106  is GaN; and the T-shaped gate  112  is made of metal. The source and drain contacts  108  and  110 , respectively, are metal ohmic contacts that use metals such as Ni, Ti, Pt, Au, Cu Al, W, or combinations thereof. Each of the films has a thickness that can be in the range of about 10 nm-10 μm. The channel width ranges from about 10 nm-10 μm, while the channel length ranges from about 3 nm-3 μm. 
     Several challenges have arisen in making and operating the HEMT device  130  in high-frequency, high-energy applications. First, as shown in  FIG. 3 , a drawing of a scanning electron micrograph  132 , the AlGaN surface near the T-shaped gate  112  and near the source and drain contacts  108  and  110 , is prone to develop cracks  134  under high-power operation. Failure analysis of such cracks  134  has shown that they tend to initiate at the corners of the metal areas, where the metal meets the AlGaN layer, and can extend through substantially the full thickness of the AlGaN layer. Such cracks  134  are theorized to be associated with observed gate current leakage. In addition to reliability concerns associated with the cracks  134 , heat buildup and proper heat dissipation within the HEMT device  130  can also be as significant problem. 
       FIG. 4  shows one embodiment of an improved, multi-layer cap HEMT device  140  that has been proposed to reduce gate current leakage and also greatly reduce the cracks  134  [Fujitsu press release Jun. 24, 2009, Kawasaki, Japa (http://www.fujitsu.com/global/news/pr/archives/month/2009/20090624-01.html)]. The multi-layer cap HEMT device  140  has features in common with the conventional HEMT device  130  in that it has a heterostructure  102  constructed on a substrate  104 . In the multi-layer cap HEMT device  140 , the heterostructure is made of an upper layer  102   b  made of AlGaN on a lower layer  102   a  made of GaN and the substrate  104  is made of SiC. A three-layer cap  142  is preferably present between the gate  112  and the source and drain contacts  108  and  110 , respectively. The three-layer cap  142  includes a first layer  144  and a third layer  146 , made of n-doped GaN. A middle layer  148  is made of AlN. Above the three-layer cap  142  there is deposited a gate dielectric made of an atomic layer of SiO 2    149 , which extends underneath the T-shaped gate  112 , the gate dielectric being recessed into the top (AlGaN) layer  102   a  of the heterostructure  102 . 
     The multi-layer cap HEMT device  140  has been successful in high-energy operation in that it has not exhibited the types or quantity of cracks  134  in  FIG. 3 . In addition, the multi-layer HEMT device  140  has shown an increase in the number of carrier electrons in the conduction channel  114 . The use of SiC as the substrate material also helps to dissipate heat during high power operation. This embodiment therefore provides significant benefits over the prior art. Unfortunately, the three-layer cap  142 , in this type of structure, may have a higher dielectric constant of the gate dielectric and a slower switching speed than desired. Furthermore, the user of SiC as the substrate material is very expensive, so it would be desirable to fabricate a reliable HEMT device on a silicon substrate. 
       FIG. 5  shows a high-level flow of a fabrication process  150  for building a graphene capped HEMT  152  on a silicon substrate  104 , according to two alternative embodiments described herein. The graphene capped HEMT employs two layers of graphene at different locations to maintain both structural integrity and improved device performance under high-frequency operation. 
     In a first embodiment, at  152   a , the substrate  104 , made of silicon in this embodiment, has a layer of SiC formed thereon. 
     At  154   a , a first, epitaxial layer of graphene is then grown on the SiC. 
     Alternatively, in a second embodiment at  152   b , the silicon substrate has a layer of SiO 2  formed thereon, and the graphene layer can be formed on the SiO 2  at  154   b . The graphene layer can be formed on the layer of SiC or SiO 2  by any acceptable method, including deposition or growth in an amorphous form, a polycrystalline form, a single crystal form, or by epitaxial growth. 
     At  156 , the graphene can be capped with an adhesion layer. 
     At  158 , a heterojunction transistor is then formed that includes a second layer of graphene. 
       FIGS. 6-10  show in more detail some aspects of the fabrication process  150  for the graphene capped HEMT. 
       FIGS. 6A-6C  describe and show a film stack  160  built on the Si substrate  104  that includes first and second graphene layers  161  and  162 , respectively, and, optionally, an adhesion layer  164 . The first graphene layer  161  is designed to absorb and dissipate heat generated by the transistor, and the second graphene layer  162  is designed to prevent gate current leakage without degrading the switching speed of the graphene capped HEMT. The second graphene layer  162  also serves to dissipate heat. The detailed process flow  166  shown in  FIG. 6A  describes processing steps that can be carried out to fabricate the film stack  160 . 
     At  167 , the buffer layer  106 , made of SiO 2  in the embodiment shown, can be grown or deposited on the silicon substrate  104  using a conventional method such as, for example, thermal oxidation by heating the substrate  104  in a diffusion furnace. Alternatively, the buffer layer  106  can be made of SiC. The buffer layer  106  can have a thickness within the range of about 2 nm-100 nm. 
     At  168 , the first graphene layer  161  is formed. It can be deposited using a conventional method such as, for example, chemical vapor deposition (CVD) at a low temperature. A monolayer film is desirable, although a multi-layer graphene film can also be used. Typically, thicknesses of graphene films are in the range of about 10 Å-20 nm. The first graphene layer  161 , disposed between the heterostructure  102  and the substrate  104 , serves as a heat sink to absorb energy from the HEMT, and then spread out over a wider area to dissipate the heat, especially when the HEMT is operated in a high-frequency, high-energy mode. 
     At  170 , an optional adhesion layer  164  can be deposited on the first graphene layer  161  to facilitate subsequent deposition of the GaN lower layer of the heterostructure  102 . It is understood to persons skilled in the art of thin film fabrication that it is difficult to deposit nitride semiconductors directly onto graphene. Thus, a thin adhesion layer is recommended. 
     A desirable material for use as the adhesion layer  164  is hexagonal boron nitride (h-BN), a nitride semiconductor which has been shown to promote high-quality film growth of nitride semiconductors [Kobayashi et al.,  Nature , Vol. 484, April, 2012, p. 223-227]. Because h-BN has a hexagonal lattice structure  165  that closely matches the lattice structure of the underlying graphene film, a thin layer of single crystal h-BN can be easily formed on the graphene and can provide relief of film stress. The hexagonal lattice structure  165  of the h-BN adhesion layer  164  is shown in  FIG. 6C . The h-BN lattice is made up of hexagonal rings, each containing three boron atoms and three nitrogen atoms. H—BN is widely used in consumer products as an alternative lubricant to carbon graphite, and in high temperature equipment, for example, due to its thermal and chemical stability. H—BN is also combined with various materials to enhance their thermal, chemical, and electrical properties. Additionally or alternatively, a different adhesion layer material, for example, ZnO can be used between the GaN film and the graphene. 
     The steps  172 ,  174  entail sequential deposition of two semiconductor layers, typically compound semiconductors, to form the heterostructure  102  having a lower layer  102   a , an upper layer  102   b  and, between them, a heterojunction that exhibits energy characteristics similar to those shown in  FIGS. 1 and 3  (see plots  116 ). 
     At  172 , a lower layer  102   a  of un-doped GaN can be grown using an epitaxial deposition method such as, for example, metal-organic vapor phase epitaxy (MOVPE). Additionally, an AlN or AlGaN buffer can be grown on the h-BN surface prior to deposition of the GaN film [Kobayashi et al.,  Nature , Vol. 484, April, 2012, p. 223-227]. 
     At  174 , an upper layer  102   b  of n-doped AlGaN can be grown on the GaN film. Growth of the GaN and AlGaN films can use conventional methods known in the art of HEMT device processing. Alternatively, other intrinsic semiconductor compounds, alone or combined with or other compound semiconductor materials, can be substituted in the AlGaN/GaN heterostructure  102  such as, for example, AlN, GaAs, AlGaAs, InGaN, InP, InGaP, InN, InAlAs, InGaAs, AlGaSb, AlSb, InAs and the like. 
     At  176 , the second graphene layer  162  is formed overlying the upper layer  102   b . The second graphene layer  162  may also be considered a graphene cap, or a graphene cap layer, since it is a layer that is a cap over the top compound semiconductor layer  102   b . The complete film stack is shown in  FIG. 6B . 
       FIGS. 7A-7D  describe and show a detailed process sequence  180  for fabricating the HEMT transistor. 
     At  182 , a recessed opening  194  is made in the second graphene layer  162 , and the recessed opening extends into the underlying AlGaN layer. The recessed opening  194  can be formed using conventional patterning methods such as optical lithography and either dry plasma etching or wet chemical etching, or combinations thereof ( FIG. 7B ). 
     At  184 , a thin h-BN film  196  can be conformally deposited as a high-k gate dielectric material. H—BN is known in the art as an “outstanding gate dielectric material for GFETs, yielding interfaces nearly free of trapped charge and maintaining high mobility and carrier velocities in the graphene channel” [Meric, et al.,  Proceedings of the IEEE IEDM Conference , Dec. 5-7, 2011, pp. 2.1.1-2.1.4]. In the present case, h-BN serves as a gate dielectric in the HEMT. It is noted that in the present HEMT device, graphene is not used as a channel replacement material as described by Meric et al. with respect to the GFET device. The conduction channel  114  in the graphene-capped HEMT presented herein is located at the heterojunction, which is separate from the h-BN film and the graphene cap layers. 
     At  186 , a sacrificial film  198  can be deposited over the h-BN film  196 . The sacrificial film  198  can be made of, for example, polysilicon. The thickness of the sacrificial film  198  is nominally about 100 nm, but generally scales with the size of the electrodes, e.g., within the range of about 50 nm-450 nm ( FIG. 7C ). 
     At  188 - 190 , the sacrificial film  198  can be patterned using conventional lithography and etching techniques for a damascene inlay process. Accordingly, trenches can be created in the sacrificial film  198  for subsequent formation of the metal gate electrode, and the metal source and drain electrodes. 
     At  188 , the sacrificial film  198  can be masked using, for example, a photoresist mask or a silicon nitride hard mask, so as to expose only the desired gate area. The sacrificial film  198  can then be etched selective to the h-BN film  196  to form a gate trench  200  that extends to the surface of the h-BN film  196 . 
     At  190 , the sacrificial film  198  can be masked using, for example, a photoresist mask or a silicon nitride hard mask, so as to expose only the desired source and drain regions. The sacrificial film  198  can then be etched selective to the AlGaN film to form source and drain trenches  202  that extend to the surface of the heterostructure  102  ( FIG. 7D ). 
       FIGS. 8A-8C  describe and show a detailed process sequence  204  for fabricating metal terminals of the HEMT device, shown in its completed form in  FIG. 9 . 
     At  206 , the source, drain, and gate trenches  202  and  200 , respectively, can be filled with a desired bulk metal  220  such as, for example, Ni, Ti, Pt, Al, Cu, Ag, Au, or W ( FIG. 8B ). The thickness of the bulk metal  220  is desirably about 50 nm. Prior to depositing the desired bulk metal  220 , a liner material such as, for example, titanium, titanium nitride (TiN) or a sandwich thereof may be deposited. The liner can be used in any or all of the source, drain, and gate contacts, and is desirably very thin, in the range of about 1-8 nm. The trench fill processing step can be carried out by, for example, electroplating, electro-less plating, or other damascene techniques known in the art. If a liner is used, the liner deposition may be combined with the bulk metal fill to create a two-step metal fill process, or the liner may be grown or deposited conformally by another method (e.g., by a plasma deposition step). 
     At  208 , a planarization process is executed, such as chemical-mechanical planarization (CMP) using a slurry suitable for planarizing the surface of the bulk metal  220  ( FIG. 8C ), and stopping on the sacrificial film  198 . A more precise alternative to CMP polishing that is suitable for gold films, for example, is a smoothing process that involves use of a gas cluster ion beam (GCIB). 
     At  210 , the sacrificial film  198  remaining between the source and the gate, and the gate and the drain, can be removed ( FIG. 9 ). Consistent with the embodiment shown, removal of the sacrificial film  198  can be done using a conventional polysilicon reactive ion etch (RIE) chemistry that is selective to both metal and h-BN. 
     At  212 , a conventional passivation layer such as oxy-nitride or silicon nitride (SiN) can be applied to protect the HEMT device from environmental effects, e.g., moisture. 
     The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
     It will be appreciated that, although specific embodiments of the present disclosure are described herein for purposes of illustration, various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, the present disclosure is not limited except as by the appended claims. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.