WO2021094875A1 - Semiconductor device having a diode structure - Google Patents

Semiconductor device having a diode structure Download PDF

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Publication number
WO2021094875A1
WO2021094875A1 PCT/IB2020/060381 IB2020060381W WO2021094875A1 WO 2021094875 A1 WO2021094875 A1 WO 2021094875A1 IB 2020060381 W IB2020060381 W IB 2020060381W WO 2021094875 A1 WO2021094875 A1 WO 2021094875A1
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Prior art keywords
ill
nitride
nitride layer
polarization
graded
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PCT/IB2020/060381
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French (fr)
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Xiaohang Li
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King Abdullah University Of Science And Technology
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    • H—ELECTRICITY
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    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/86—Types of semiconductor device ; Multistep manufacturing processes therefor controllable only by variation of the electric current supplied, or only the electric potential applied, to one or more of the electrodes carrying the current to be rectified, amplified, oscillated or switched
    • H01L29/861—Diodes
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    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/20—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only AIIIBV compounds
    • H01L29/2003—Nitride compounds
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    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
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    • H01L29/201—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only AIIIBV compounds including two or more compounds, e.g. alloys
    • H01L29/205—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only AIIIBV compounds including two or more compounds, e.g. alloys in different semiconductor regions, e.g. heterojunctions
    • H—ELECTRICITY
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    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/66007—Multistep manufacturing processes
    • H01L29/66075—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials
    • H01L29/66083—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials the devices being controllable only by variation of the electric current supplied or the electric potential applied, to one or more of the electrodes carrying the current to be rectified, amplified, oscillated or switched, e.g. two-terminal devices
    • H01L29/66196—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials the devices being controllable only by variation of the electric current supplied or the electric potential applied, to one or more of the electrodes carrying the current to be rectified, amplified, oscillated or switched, e.g. two-terminal devices with an active layer made of a group 13/15 material
    • H01L29/66204—Diodes
    • H01L29/66219—Diodes with a heterojunction, e.g. resonant tunneling diodes [RTD]
    • H—ELECTRICITY
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    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/0304—Inorganic materials including, apart from doping materials or other impurities, only AIIIBV compounds
    • H01L31/03044—Inorganic materials including, apart from doping materials or other impurities, only AIIIBV compounds comprising a nitride compounds, e.g. GaN
    • H—ELECTRICITY
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    • H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
    • H01L31/10—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors characterised by potential barriers, e.g. phototransistors
    • H01L31/101—Devices sensitive to infrared, visible or ultraviolet radiation
    • H01L31/102—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier
    • H01L31/109—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier the potential barrier being of the PN heterojunction type
    • H—ELECTRICITY
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    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/005—Processes
    • H01L33/0062—Processes for devices with an active region comprising only III-V compounds
    • H01L33/0075—Processes for devices with an active region comprising only III-V compounds comprising nitride compounds
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    • H01L33/02—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/04—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a quantum effect structure or superlattice, e.g. tunnel junction
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    • H01L33/02—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/16—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a particular crystal structure or orientation, e.g. polycrystalline, amorphous or porous
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    • H01L33/02—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/26—Materials of the light emitting region
    • H01L33/30—Materials of the light emitting region containing only elements of Group III and Group V of the Periodic Table
    • H01L33/32—Materials of the light emitting region containing only elements of Group III and Group V of the Periodic Table containing nitrogen
    • H—ELECTRICITY
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    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/36—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the electrodes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/544—Solar cells from Group III-V materials

Definitions

  • Embodiments of the disclosed subject matter generally relate to semiconductor devices having a two-dimensional diode structure formed in two adjacent heterojunctions of Ill-nitride materials or a three-dimensional diode structure in two adjacent polarization graded Ill-nitride layers.
  • Ill-nitride materials i.e. , materials with nitride in combination with a Group-Ill material, i.e., at least one of aluminum, boron, gallium, and indium
  • a Group-Ill material i.e., at least one of aluminum, boron, gallium, and indium
  • Heterostructures of Ill-nitride materials can be used for a number of different devices, including diodes, optoelectronic devices, such as visible and ultraviolet light emitting diodes (LEDs), laser diodes, as well as high-power devices, such as high electron mobility transistors (HEMTs).
  • These devices typically include one of the layers of the heterojunction being n-type or p-type doped. This can be achieved by impurity doping by adding silicon (for n-type doping) or magnesium (for p-type doping) to the Ill-nitride layer.
  • Ill-nitride materials are particularly interesting due to the spontaneous and piezoelectric polarization that occurs when a Ill-nitride heterojunction is grown along the metal-polar (0001) and N-polar (000 ⁇ ) directions c-axis, which produces an electric field oriented parallel to the growth axis.
  • the spontaneous and piezoelectric polarization of Ill-nitride heterojunctions have been exploited to produce polarization doping due to the polarization at the interface of the two layers of the heterojunction. This achieves similar, but better performing results, compared to impurity doping because polarizing doping does not exhibit the impurity scattering that occurs in impurity-doped Ill-nitride heterojunctions.
  • Polarization-doped Ill-nitride heterojunctions have been found to produce two-dimensional electron gas (2DEG) or two-dimensional hole gas (2DHG) channels, which has been exploited to form, among other structures, quantum wells.
  • 2DEG two-dimensional electron gas
  • 2DHG two-dimensional hole gas
  • an N-polar gallium nitride (GaN) layer grown on an N-polar aluminum nitride layer (AIN) forms a 2DEG channel near the heterojunction interface, in the N-polar GaN layer.
  • Whether a 2DEG or 2DHG channel is formed and the location of the channel depends upon the difference in polarization between the two layers, the band offset of the two layers, and the thickness of the layers. Specifically, the polarization difference between the two layers should be non-zero. The conduction band offset or the valence band offset of the two layers should be non-zero.
  • Ill- nitride semiconductors with a 2DEG or 2DHG channel induced by polarization doping have typically been limited to high electron mobility transistors (HEMT), see Reference [6]
  • HEMT high electron mobility transistors
  • the combination of the 2DEG and the 2DHG to form a lateral diode has been extensively studied for the AIGaAs based compound semiconductors, an example of which is described in Reference [7]
  • impurity doping and metal- insulator-semiconductor structures have been utilized.
  • no such diode device has been achieved on the Ill-nitride (e.g., GaN) based structures comprising 2DEG and 2DHG.
  • a semiconductor device which includes a first heterojunction comprising a first Ill-nitride layer arranged directly on a second Ill-nitride layer.
  • the first and second Ill-nitride layers are both either N-polar or metal-polar.
  • the semiconductor device also includes a second heterojunction, arranged directly laterally adjacent to the first heterojunction so that there is an interface between the first and second heterojunctions, comprising a third Ill-nitride layer arranged directly on a fourth Ill-nitride layer.
  • the third and fourth Ill-nitride layers are both either N-polar or metal-polar.
  • the first, second, third, and fourth Ill- nitride layers are directly adjacent to the interface. Due to a polarization difference between the first and second Ill-nitride layers, a band offset of the first and second Ill-nitride layers, and a thickness of the first and second Ill-nitride layers, the first heterojunction has a two-dimensional electron gas (2DEG) channel in one of the first and second Ill-nitride layers.
  • 2DEG two-dimensional electron gas
  • the second heterojunction Due to a polarization difference between the third and fourth Ill-nitride layers, a band offset of third and fourth Ill-nitride layers, and a thickness of the third and fourth Ill-nitride layers, the second heterojunction has a two-dimensional hole gas (2DHG) channel in one of the third and fourth Ill-nitride layers.
  • the 2DEG and 2DHG channels are aligned in a common horizontal plane at the interface or are arranged in horizontal planes that are within 50 nm of each other in a vertical direction at the interface.
  • a method for forming a semiconductor device comprising a first Ill-nitride layer arranged directly on a second Ill-nitride layer is formed.
  • the first and second Ill- nitride layers are both either N-polar or metal-polar.
  • a second heterojunction comprising a third Ill-nitride layer arranged directly on a fourth Ill-nitride layer is formed.
  • the third and fourth Ill-nitride layers are both either N-polar or metal-polar.
  • the first and second heterojunctions are arranged directly adjacent to each other so that a interface exists between the first and second heterojunctions.
  • the first, second, third, and fourth Ill-nitride layers are directly adjacent to the interface.
  • the first heterojunction has a two-dimensional electron gas, 2DEG, channel in one of the first and second Ill-nitride layers.
  • the second heterojunction has a two-dimensional hole gas, 2DHG, channel in one of the third and fourth Ill-nitride layers.
  • the 2DEG and 2DHG channels are aligned in a common horizontal plane at the interface or are arranged in horizontal planes that are within 50 nm of each other in a vertical direction at the interface.
  • a semiconductor device which comprises a first polarization graded Ill-nitride layer with a bottom side arranged on a first substrate and a top side opposite of the bottom side.
  • the first polarization graded Ill-nitride layer comprises an alloy of first and second Ill-nitride elements.
  • a composition of the alloy of the first polarization graded Ill-nitride layer changes from a first Ill-nitride composition at the bottom side to a second Ill-nitride composition at the top side so that a portion of the first polarization graded Ill-nitride layer between the top and bottom sides has a deceasing amount of one of the first and second Ill-nitride elements and an increasing amount of the other one of the first and second Ill-nitride elements moving from the bottom side to the top side of the first polarization graded Ill-nitride layer.
  • the semiconductor device also comprises a second polarization graded Ill-nitride layer with a bottom side arranged on a second substrate and a top side opposite of the bottom side.
  • the second polarization graded Ill-nitride layer comprises an alloy of third and fourth Ill-nitride elements.
  • a composition of the alloy of the second polarization graded Ill-nitride layer changes from a third Ill-nitride composition at the bottom side to a fourth Ill-nitride composition at the top side so that a portion of the second polarization graded Ill- nitride layer between the top and bottom sides has a deceasing amount of one of the third and fourth Ill-nitride elements and an increasing amount of the other one of the third and fourth Ill-nitride elements moving from the bottom side to the top side of the second polarization graded Ill-nitride layer.
  • the first and second polarization graded Ill-nitride layers are directly adjacent to each other so that there is an interface between the first and second polarization graded Ill-nitride layers.
  • a bulk of the first polarization graded Ill-nitride layer has three-dimensional electron gas and a bulk of the second polarization graded Ill-nitride layer has three-dimensional hole gas.
  • Figure 1 A is a schematic, cross-sectional side view of a semiconductor device according to embodiments
  • Figure 1 B is a schematic, cross-sectional top view of a semiconductor device according to embodiments
  • Figure 1C is a schematic, cross-sectional side view of a semiconductor device according to embodiments.
  • Figure 2A is a schematic, cross-sectional side view of a forward biased semiconductor device according to embodiments
  • Figure 2B is a schematic, cross-sectional side view of a reversed biased semiconductor device according to embodiments
  • Figure 3A is a schematic, cross-sectional side view of a forward biased semiconductor device with additional potential sources according to embodiments;
  • Figure 3B is a schematic, cross-sectional side view of a reversed biased semiconductor device with additional potential sources according to embodiments
  • Figure 4 is a flow diagram of a method for forming a semiconductor device according to embodiments.
  • Figure 5 is a schematic, cross-sectional side view of a semiconductor device according to embodiments. DETAILED DESCRIPTION
  • the term horizontal should be understood as a direction perpendicular to the growth direction of the layers, whereas vertical should be understood as the direction growth direction of the layers.
  • the term lateral should be understood as being adjacent in the horizontal direction, and the term on top of should be understood as referring to an arrangement in the vertical direction (i.e., the growth direction of the layers).
  • FIGS 1A-1C are schematic diagrams of semiconductor devices 100A and 100B according to embodiments.
  • the semiconductor devices 100A and 100B can be used as part of an optoelectronic device or can be used as diodes that do not emit light.
  • the semiconductor devices 100A and 100B include a first heterojunction 105A or 105B comprising a first Ill-nitride layer 110A or 11 OB arranged directly on a second Ill-nitride layer 115A or 115B.
  • the first 110A or 11 OB and second 115A or 115B Ill-nitride layers are both either N-polar or metal-polar.
  • a second heterojunction 120A or 120B is arranged directly laterally adjacent to the first heterojunction 105A or 105B so that there is an interface 135A or 135B between the first 105A or 105B and second 120A or 120B heterojunctions.
  • the second heterojunction 120A or 120B comprises a third Ill-nitride layer 125A or 125B arranged directly on a fourth Ill-nitride layer 130A or 130B.
  • the third 125A or 125B and fourth 130A or 130B Ill-nitride layers are both either N-polar or metal-polar.
  • the phrase being arranged directly on should understood as there being no intentionally formed Ill-nitride materials between the heterojunction or the layers and the interface. It should be recognized, however, a de minimis amount of material can be present, such as material that is formed during a growth process (as discussed below) or some type of adhesive to join the heterojunctions together (as also discussed below).
  • the first 105A or 105B, second 110A or 11 OB, third 125A or 125B, and fourth 130A and 130B Ill-nitride layers are directly adjacent to the interface 135A or 135B.
  • the phrase arranged directly adjacent should be understood as there being no intentionally formed Ill-nitride materials between the heterojunction or the layers and the interface. It should be recognized, however, a de minimis amount of material can be present, such as material that is formed during a growth process (as discussed below) or some type of adhesive to join the heterojunctions together (as also discussed below).
  • the first heterojunction 105A or 105B has a two-dimensional electron gas (2DEG) channel 140A or 140B in one of the first and second Ill-nitride layers.
  • 2DEG two-dimensional electron gas
  • the second heterojunction 120A or 120B has a two-dimensional hole gas (2DHG) channel 145A or 145B in one of the third 125A or 125B and fourth 130A or 130B Ill-nitride layers.
  • 2DHG two-dimensional hole gas
  • the 2DEG 140A or 140B and 2DHG 145A or 145B channels are aligned in a common horizontal plane at the interface 135A or 135B or are arranged in horizontal planes that are within 50 nm of each other in a vertical direction at the interface 135A or 135B.
  • the 2DEG and 2DHG channels are arranged in the same horizontal plane, there is no barrier to prevent the flow of electrons and holes across the interface 135A or 135B.
  • a slight misalignment i.e. , up to 50 nm
  • a misalignment between the 2DEG and 2DHG channels can be intentionally formed to introduce quantum confinement for higher efficiency radiative recombination or as a carrier transport barrier.
  • the horizontal alignment (or slight misalignment up to 50 nm) of the 2DEG and 2DHG channels results in the formation of a functional equivalent to a p-n junction, and accordingly an area running horizontally across the semiconductor device 105A or 105B can be referred to as a sheet diode because the diode functionality exists mainly in this area and not in the bulk material, as in conventional p-n diodes formed by intentional doping.
  • the top view of the semiconductor device 100A or 100B in Figure 1B illustrates that the 2DEG and 2DHG channels are formed across the semiconductor device 100A or 100B in a direction perpendicular to the direction in which the electrons or holes flow across the interface 135A or 135B.
  • the Ill-nitride layers of the semiconductor device 105A illustrated in Figure 1A have approximately the same height, whereas the Ill-nitride layers 110B and 125B in Figure 1C have different heights. This difference is due to the different polarization differences, thicknesses, and band offsets of the Ill-nitride layers of each heterojunction.
  • Ill-nitride layers 110A and 125A can GaN layers and Ill-nitride layers 115A and 130A can be AIN layers.
  • Ill-nitride layers 110B and 130B can be AIN layers and Ill-nitride layers 115B and 125B are GaN layers. It should be recognized that these are non-limiting examples and that any of the heterojunctions listed in Table 1 can be employed, so long as one heterojunction forms a 2DEG channel and the other heterojunction forms a 2DHG channel.
  • the upper Ill-nitride layers 110A or 110B and 125A or 125B need not terminate at the same height.
  • the 2DEG and 2DHG channels are formed by the heterojunction and exist regardless of whether a potential is applied across the semiconductor device 105A or 105B.
  • the heterojunctions 105A or 105B having the 2DEG channel can be arranged on the right side of the device and the heterojunctions 120A or 120B can be formed on the left side of the device.
  • Figures 2A and 2B respectively illustrate the semiconductor device 100A of Figure 1A with an applied forward and reverse bias by a first potential source 205A or 205B, respectively.
  • a first potential source 205A applies a forward bias via contact pads 210A and 210B, as illustrated in Figure 2A
  • the 2DEG 140A and 2DHG 140B channels recombine into photons near the interface 135A. These generated photons can be used to produce light, such as for a light emitting diode or a laser diode.
  • the semiconductor device 100A can be used as a photodetector or a solar cell.
  • the diode formed from the 2DEG 140A and 2DHG 140B channels can act as an inverter.
  • Figures 2A and 2B are illustrated using the semiconductor device 100A of Figure 1A, the application of forward and reverse biases is equally applicable to the semiconductor device 105B, which would operate in a similar manner to semiconductor device 105A under the application of forward and reverse biases.
  • the application of the current by the first potential source 205A would be a reverse biasing of the device and the application of current by the first potential source 205B would be a forward biasing.
  • second potential source 305A and third potential source 310A are respectively coupled to the upper surface of Ill-nitride layers 110A and 125A via contact pads 315A and 320A, respectively.
  • first and second potential sources 305A and 305B creates electron and hole deprived regions, and accordingly these potential sources 305A and 305B act as a gate to control the flow of electrons and holes across the interface 135A.
  • the second and third potential sources 305A and 310A should be applied in a region proximate to the interface 135A so as to create the electron and hole deprived regions in each heterojunction at the interface 135A.
  • FIG. 3A The semiconductor device of Figure 3A is illustrated having a forward bias 205A, and potential sources 305A and 305B can also be employed when the device is under reverse bias 205B, which is illustrated in Figure 3B.
  • Figures 3A and 3B are illustrated using the semiconductor device 100A of Figure 1A, the application of forward and reverse biases, as well as additional potential sources, is equally applicable to the semiconductor device 105B, which would operate in a similar manner to semiconductor device 105A under the application of forward and reverse biases and with the additional potential sources.
  • FIG 4 is a flow diagram of a method for forming the semiconductor devices 100A and 110B.
  • a first heterojunction 105A or 105B is formed comprising a first Ill-nitride layer 110A or 110B arranged directly on a second Ill-nitride layer 115A or 115B (step 410).
  • the first 110A or 110B and second 115A or 115B Ill-nitride layers are both either N-polar or metal-polar.
  • a second heterojunction 120A or 120B is formed comprising a third Ill-nitride layer 125A or 125B arranged directly on a fourth Ill-nitride layer 130A or 130B (step 410).
  • the third 125A or 125B and fourth 130A or 130B Ill-nitride layers are both either N-polar or metal-polar.
  • the first 105A or 105B and second 120A or 120B heterojunctions are arranged directly adjacent to each other so that a interface 135A or 135B exists between the first 105A or 105B and second 120A or 120B heterojunctions.
  • the first 105A or 105B, second 110A or 110B, third 125A or 125B, and fourth 130A or 130B Ill-nitride layers are directly adjacent to the interface 135A or 135B.
  • the first heterojunction 105A or 105B has a 2DEG channel 140A or 140B in one of the first and second Ill-nitride layers.
  • the second heterojunction 120A or 120B has a 2DHG channel 145A or 145B in one of the third 125A or 125B and fourth 130A or 130B Ill-nitride layers.
  • the 2DEG 140A or 140B and 2DHG 145A or 145B channels are aligned in a common horizontal plane at the interface 135A or 135B or are arranged in horizontal planes that are within 50 nm of each other in a vertical direction at the interface 135A or 135B.
  • the method has been described as forming the first heterojunction 105A or 105B having the 2DEG channel and then forming the second heterojunction 120A or 120B having the 2DHG channel.
  • the second heterojunction 120A or 120B having the 2DHG channel can be formed before the first heterojunction 105A or 105B having the 2DEG channel.
  • references to forming the first and second heterojunctions should not be interpreted as the first heterojunction being formed before the second heterojunction and instead should be understood that either heterojunction can be formed first.
  • the heterojunctions can be formed in any number of ways, including separately forming the two heterojunctions and then physically attaching the two heterojunctions at the interface using, for example, an adhesive.
  • Another way to form the two heterojunctions is by selective area growth in which the different layers are all grown in the same growth chamber by selectively covering different parts of the device as it is grown. For example, a portion of a substrate can be covered so that the substrate has an exposed portion and a covered portion. The second Ill-nitride layer can then be grown on the exposed portion of the substrate. The covered portion of the substrate can then be uncovered and the second Ill-nitride layer can be covered and then the fourth Ill-nitride layer can be grown. While the second Ill-nitride layer is still covered, the third Ill-nitride layer can be grown on the fourth Ill-nitride layer. The second Ill-nitride layer can then be uncovered, the fourth Ill-nitride layer can be covered, and then the first Ill- nitride layer can be grown on the second Ill-nitride layer.
  • a semiconductor device 500 includes a first polarization graded Ill-nitride layer 505 with a bottom side 510 arranged on a substrate 515 and a top side 520 opposite of the bottom side 510.
  • the first polarization graded Ill-nitride layer 505 comprises an alloy of first and second Ill- nitride elements.
  • a composition of the alloy of the first polarization graded Ill-nitride layer changes from a first Ill-nitride composition at the bottom side 510 to a second Ill-nitride composition at the top side 520 so that a portion of the first polarization graded Ill-nitride layer 505 between the top 520 and bottom 510 sides has a deceasing amount of one of the first and second Ill-nitride elements and an increasing amount of the other one of the first and second Ill-nitride elements moving from the bottom side 510 to the top side 520 of the first polarization graded Ill-nitride layer 505.
  • the semiconductor device 500 also includes a second polarization graded Ill-nitride layer 525 with a bottom side 530 arranged on the substrate 515 and a top side 540 opposite of the bottom side 530.
  • the second polarization graded Ill- nitride layer 525 comprises an alloy of third and fourth Ill-nitride elements.
  • a composition of the alloy of the second polarization graded Ill-nitride layer 525 changes from a third Ill-nitride composition at the bottom side 530 to a fourth Ill- nitride composition at the top side 540 so that a portion of the second polarization graded Ill-nitride layer 505 between the top 540 and bottom 530 sides has a deceasing amount of one of the third and fourth Ill-nitride elements and an increasing amount of the other one of the third and fourth Ill-nitride elements moving from the bottom side 530 to the top side 540 of the second polarization graded Ill- nitride layer 525.
  • the first 505 and second 525 polarization graded Ill-nitride layers are directly adjacent to each other so that there is an interface 545 between the first 505 and second 525 polarization graded Ill-nitride layers. Due to the polarization grading, 3DEG is formed in the bulk of one of the first 505 and second 525 polarization graded Ill-nitride layers and 3DHG is formed in the bulk of the other one of the first 505 and second 525 polarization graded Ill-nitride layers.
  • the polarization graded Ill-nitride layer having 3DHG functions in a similar manner to an impurity p- doped layer and the polarization graded Ill-nitride layer having 3DEG functions in a similar manner to an impurity n-doped layer.
  • the bulk material of the first 505 and second 525 polarization graded Ill-nitride layers form a p-n junction at interface 545.
  • the Ill-nitride alloys of the first 505 and second 525 graded Ill- nitride layers can be the same Ill-nitride alloys or different Ill-nitride alloys.
  • this can involve grading the first layer 505 from Ill-polar AIN to Ill- polar GaN and grading the second layer 525 from Ill-polar GaN to Ill-polar AIN.
  • 3DHG would be formed in the first graded Ill-nitride layer 505 and 3DEG would be formed in second graded Ill-nitride layer 525. It should be recognized that this is a non-limiting example and that the grading can from one Ill- nitride to a second Ill-nitride as described in Table 1 above.
  • Non-limiting examples include: the first graded Ill-nitride layer 505 graded from AI0.2Ga0.sN to Alo.5Gao.5N and the second graded Ill-nitride layer 525 being graded from Alo.5Gao.5N to Alo.2Gao.8N; the first graded Ill-nitride layer 505 graded from Alo.4Gao.6N to Alo.5Gao.5N and the second graded Ill-nitride layer 525 being graded from Alo.5Gao.5N to Alo.2Gao.8N; and the first graded Ill-nitride layer 505 graded from AI0.2Ga0.sN to Alo.5Gao.5N and the second graded Ill-nitride layer 525 being graded from lno.1Gao.9N to lno.2Gao.8N.
  • the first graded Ill-nitride layer 505 is n-type and the second graded Ill-nitride layer 525 is p-type.
  • Figure 5 illustrates the top sides 520 and 540 of the first 505 and second 525 graded Ill-nitride layers being terminated at the same height, this need not be the case and the top sides 520 and 540 can terminate at different heights.
  • the polarization doping concentration profile (s ) is a change in polarization ( dP ) relative to the change in position of the particular polarization within the layer dl, which is reflected in the following equation: dP
  • equation (1) can be rewritten as: dP dx s P, dx dl (2)
  • the first term of equation (2) reflects the polarization change rate with respect to alloy composition, which will be referred to as the composition-polarization change rate and can be represented by the following equation: k _ dP dx (3)
  • the graded Ill-nitride layer is a p-type layer and when k is negative, the graded Ill-nitride layer is n-type.
  • k positive
  • the graded Ill-nitride layer is a p-type layer and when k is negative, the graded Ill-nitride layer is n-type.
  • the second term of equation (2) reflects the grading speed (i.e. , the speed at which the composition of x changes over the thickness of the layer) and can be represented by the following equation: dx
  • the semiconductor device 500 can be forward or reversed biased, or additional potential sources added in a similar manner to that described above in connection with semiconductor device 100A and will operate in a similar manner.
  • Figure 5 illustrates a forward bias being applied via a voltage source 545 coupled to the top side 520 of the first polarization graded Ill-nitride layer 505 via contact 550A and to the top side 540 of the second polarization graded Ill-nitride layer 525 via a contact 550B.
  • Reverse biasing would have a similar arrangement to that illustrated in Figure 2B.
  • the additional potential sources can be applied to the first 505 and second 525 graded Ill-nitride layers via respective contacts, at the respective top sides 520 and 540, proximate to the interface 545 so that the application of current from the potential sources forms depletion regions on each side of the interface 545.
  • the formation of the 2DEG and 2DHG channels that are aligned in a common horizontal plane at the interface or slightly misaligned in the vertical direction at the interface can be referred to as a sheet diode because the p-n junction is formed by the channels themselves instead of in the bulk of the material.

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Abstract

A semiconductor device includes a first heterojunction having a first III-nitride layer arranged directly on a second III-nitride layer. The first and second III-nitride layers are both either N-polar or metal-polar. The semiconductor device also includes a second heterojunction, arranged directly laterally adjacent to the first heterojunction so that there is an interface between the first and second heterojunctions, having a third III-nitride layer arranged directly on a fourth III-nitride layer. The third and fourth III-nitride layers are both either N-polar or metal-polar. The III-nitride layers are directly adjacent to the interface. Due to a polarization difference between the first and second III-nitride layers, a band offset of the first and second III-nitride layers, and a thickness of the first and second III-nitride layers, the first heterojunction has a 2DEG channel in one of the first and second III-nitride layers. Due to a polarization difference between the third and fourth III-nitride layers, a band offset of third and fourth III-nitride layers, and a thickness of the third and fourth III-nitride layers, the second heterojunction has 2DHG channel in one of the third and fourth III-nitride layers. The 2DEG and 2DHG channels are aligned in a common horizontal plane at the interface or are arranged in horizontal planes that are within 50 nm of each other in a vertical direction at the interface.

Description

SEMICONDUCTOR DEVICE HAVING A DIODE STRUCTURE
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent Application No. 62/934,044, filed on November 12, 2019, entitled “USE OF POLARIZATION- INDUCED TWO-DIMENSIONAL ELECTRON GAS AND TWO-DIMENSIONAL HOLE GAS OF MI-NITRIDE SEMICONDUCTORS FOR MAKING A SHEET PN JUNCTION,” the disclosure of which is incorporated herein by reference in its entirety.
BACKGROUND
TECHNICAL FIELD
[0002] Embodiments of the disclosed subject matter generally relate to semiconductor devices having a two-dimensional diode structure formed in two adjacent heterojunctions of Ill-nitride materials or a three-dimensional diode structure in two adjacent polarization graded Ill-nitride layers.
DISCUSSION OF THE BACKGROUND
[0003] Ill-nitride materials (i.e. , materials with nitride in combination with a Group-Ill material, i.e., at least one of aluminum, boron, gallium, and indium) have a wurtzite crystal structure, which has advantageous properties, including a large bandgap range. Heterostructures of Ill-nitride materials can be used for a number of different devices, including diodes, optoelectronic devices, such as visible and ultraviolet light emitting diodes (LEDs), laser diodes, as well as high-power devices, such as high electron mobility transistors (HEMTs). These devices typically include one of the layers of the heterojunction being n-type or p-type doped. This can be achieved by impurity doping by adding silicon (for n-type doping) or magnesium (for p-type doping) to the Ill-nitride layer.
[0004] Ill-nitride materials are particularly interesting due to the spontaneous and piezoelectric polarization that occurs when a Ill-nitride heterojunction is grown along the metal-polar (0001) and N-polar (000Ϊ) directions c-axis, which produces an electric field oriented parallel to the growth axis. The spontaneous and piezoelectric polarization of Ill-nitride heterojunctions have been exploited to produce polarization doping due to the polarization at the interface of the two layers of the heterojunction. This achieves similar, but better performing results, compared to impurity doping because polarizing doping does not exhibit the impurity scattering that occurs in impurity-doped Ill-nitride heterojunctions.
[0005] Polarization-doped Ill-nitride heterojunctions have been found to produce two-dimensional electron gas (2DEG) or two-dimensional hole gas (2DHG) channels, which has been exploited to form, among other structures, quantum wells. For example, under certain conditions, an N-polar gallium nitride (GaN) layer grown on an N-polar aluminum nitride layer (AIN) forms a 2DEG channel near the heterojunction interface, in the N-polar GaN layer.
[0006] The formation of 2DEG or 2DHG channels in polarization-doped Ill- nitride heterojunctions is discussed in a number of publications, including references [1], [2], [3], [4], [5]
[0007] Whether a 2DEG or 2DHG channel is formed and the location of the channel depends upon the difference in polarization between the two layers, the band offset of the two layers, and the thickness of the layers. Specifically, the polarization difference between the two layers should be non-zero. The conduction band offset or the valence band offset of the two layers should be non-zero. Ill- nitride semiconductors with a 2DEG or 2DHG channel induced by polarization doping have typically been limited to high electron mobility transistors (HEMT), see Reference [6] The combination of the 2DEG and the 2DHG to form a lateral diode has been extensively studied for the AIGaAs based compound semiconductors, an example of which is described in Reference [7] To achieve 2DEG and 2DHG in the AIGaAs based structures, many methods such as impurity doping and metal- insulator-semiconductor structures have been utilized. However, no such diode device has been achieved on the Ill-nitride (e.g., GaN) based structures comprising 2DEG and 2DHG.
[0008] In contrast to 2DEG and 2DHG channels, which are two-dimensional channels formed in a specific portion of the material, it is known to form 3DEG and 3DHG in the bulk of the material using polarization grading (see References 8 and 9). Polarization grading involves changing the relative composition of two or more materials as a layer is grown. Forming 3DEG and 3DHG in materials using polarization grading avoids the impurity scattering that occurs due to impurity doping, which in turn results in a higher carrier mobility. However, the lateral diode comprising 3DEG and 3DHG induced by polarization doping has not been proposed. [0009] Thus, it would be desirable to provide a semiconductor device that can provide higher carrier mobility compared to semiconductor devices formed by impurity doping. SUMMARY
[0010] According to an embodiment, there is a semiconductor device, which includes a first heterojunction comprising a first Ill-nitride layer arranged directly on a second Ill-nitride layer. The first and second Ill-nitride layers are both either N-polar or metal-polar. The semiconductor device also includes a second heterojunction, arranged directly laterally adjacent to the first heterojunction so that there is an interface between the first and second heterojunctions, comprising a third Ill-nitride layer arranged directly on a fourth Ill-nitride layer. The third and fourth Ill-nitride layers are both either N-polar or metal-polar. The first, second, third, and fourth Ill- nitride layers are directly adjacent to the interface. Due to a polarization difference between the first and second Ill-nitride layers, a band offset of the first and second Ill-nitride layers, and a thickness of the first and second Ill-nitride layers, the first heterojunction has a two-dimensional electron gas (2DEG) channel in one of the first and second Ill-nitride layers. Due to a polarization difference between the third and fourth Ill-nitride layers, a band offset of third and fourth Ill-nitride layers, and a thickness of the third and fourth Ill-nitride layers, the second heterojunction has a two-dimensional hole gas (2DHG) channel in one of the third and fourth Ill-nitride layers. The 2DEG and 2DHG channels are aligned in a common horizontal plane at the interface or are arranged in horizontal planes that are within 50 nm of each other in a vertical direction at the interface.
[0011] According to another embodiment, there is a method for forming a semiconductor device. A first heterojunction comprising a first Ill-nitride layer arranged directly on a second Ill-nitride layer is formed. The first and second Ill- nitride layers are both either N-polar or metal-polar. A second heterojunction comprising a third Ill-nitride layer arranged directly on a fourth Ill-nitride layer is formed. The third and fourth Ill-nitride layers are both either N-polar or metal-polar. The first and second heterojunctions are arranged directly adjacent to each other so that a interface exists between the first and second heterojunctions. The first, second, third, and fourth Ill-nitride layers are directly adjacent to the interface. The first heterojunction has a two-dimensional electron gas, 2DEG, channel in one of the first and second Ill-nitride layers. The second heterojunction has a two-dimensional hole gas, 2DHG, channel in one of the third and fourth Ill-nitride layers. The 2DEG and 2DHG channels are aligned in a common horizontal plane at the interface or are arranged in horizontal planes that are within 50 nm of each other in a vertical direction at the interface.
[0012] According to a further embodiment, there is a semiconductor device, which comprises a first polarization graded Ill-nitride layer with a bottom side arranged on a first substrate and a top side opposite of the bottom side. The first polarization graded Ill-nitride layer comprises an alloy of first and second Ill-nitride elements. A composition of the alloy of the first polarization graded Ill-nitride layer changes from a first Ill-nitride composition at the bottom side to a second Ill-nitride composition at the top side so that a portion of the first polarization graded Ill-nitride layer between the top and bottom sides has a deceasing amount of one of the first and second Ill-nitride elements and an increasing amount of the other one of the first and second Ill-nitride elements moving from the bottom side to the top side of the first polarization graded Ill-nitride layer. The semiconductor device also comprises a second polarization graded Ill-nitride layer with a bottom side arranged on a second substrate and a top side opposite of the bottom side. The second polarization graded Ill-nitride layer comprises an alloy of third and fourth Ill-nitride elements. A composition of the alloy of the second polarization graded Ill-nitride layer changes from a third Ill-nitride composition at the bottom side to a fourth Ill-nitride composition at the top side so that a portion of the second polarization graded Ill- nitride layer between the top and bottom sides has a deceasing amount of one of the third and fourth Ill-nitride elements and an increasing amount of the other one of the third and fourth Ill-nitride elements moving from the bottom side to the top side of the second polarization graded Ill-nitride layer. The first and second polarization graded Ill-nitride layers are directly adjacent to each other so that there is an interface between the first and second polarization graded Ill-nitride layers. A bulk of the first polarization graded Ill-nitride layer has three-dimensional electron gas and a bulk of the second polarization graded Ill-nitride layer has three-dimensional hole gas.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:
[0014] Figure 1 A is a schematic, cross-sectional side view of a semiconductor device according to embodiments;
[0015] Figure 1 B is a schematic, cross-sectional top view of a semiconductor device according to embodiments;
[0016] Figure 1C is a schematic, cross-sectional side view of a semiconductor device according to embodiments;
[0017] Figure 2A is a schematic, cross-sectional side view of a forward biased semiconductor device according to embodiments;
[0018] Figure 2B is a schematic, cross-sectional side view of a reversed biased semiconductor device according to embodiments;
[0019] Figure 3A is a schematic, cross-sectional side view of a forward biased semiconductor device with additional potential sources according to embodiments; [0020] Figure 3B is a schematic, cross-sectional side view of a reversed biased semiconductor device with additional potential sources according to embodiments
[0021] Figure 4 is a flow diagram of a method for forming a semiconductor device according to embodiments; and
[0022] Figure 5 is a schematic, cross-sectional side view of a semiconductor device according to embodiments. DETAILED DESCRIPTION
[0023] The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of polar and semi-polar wurtzite Ill-nitrides.
[0024] Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.
[0025] In the following discussion, the term horizontal should be understood as a direction perpendicular to the growth direction of the layers, whereas vertical should be understood as the direction growth direction of the layers. Similarly, the term lateral should be understood as being adjacent in the horizontal direction, and the term on top of should be understood as referring to an arrangement in the vertical direction (i.e., the growth direction of the layers).
[0026] Figures 1A-1C are schematic diagrams of semiconductor devices 100A and 100B according to embodiments. The semiconductor devices 100A and 100B can be used as part of an optoelectronic device or can be used as diodes that do not emit light. As illustrated, the semiconductor devices 100A and 100B include a first heterojunction 105A or 105B comprising a first Ill-nitride layer 110A or 11 OB arranged directly on a second Ill-nitride layer 115A or 115B. The first 110A or 11 OB and second 115A or 115B Ill-nitride layers are both either N-polar or metal-polar. A second heterojunction 120A or 120B is arranged directly laterally adjacent to the first heterojunction 105A or 105B so that there is an interface 135A or 135B between the first 105A or 105B and second 120A or 120B heterojunctions. The second heterojunction 120A or 120B comprises a third Ill-nitride layer 125A or 125B arranged directly on a fourth Ill-nitride layer 130A or 130B. The third 125A or 125B and fourth 130A or 130B Ill-nitride layers are both either N-polar or metal-polar. The phrase being arranged directly on should understood as there being no intentionally formed Ill-nitride materials between the heterojunction or the layers and the interface. It should be recognized, however, a de minimis amount of material can be present, such as material that is formed during a growth process (as discussed below) or some type of adhesive to join the heterojunctions together (as also discussed below).
[0027] The first 105A or 105B, second 110A or 11 OB, third 125A or 125B, and fourth 130A and 130B Ill-nitride layers are directly adjacent to the interface 135A or 135B. The phrase arranged directly adjacent should be understood as there being no intentionally formed Ill-nitride materials between the heterojunction or the layers and the interface. It should be recognized, however, a de minimis amount of material can be present, such as material that is formed during a growth process (as discussed below) or some type of adhesive to join the heterojunctions together (as also discussed below). [0028] Due to a polarization difference between the first 110A or 110B and second 115 or 115B Ill-nitride layers, a band offset of the first 110A or 110B and second 115A or 115B Ill-nitride layers, and a thickness of the first 110A or 110B and second 115A or 115B Ill-nitride layers, the first heterojunction 105A or 105B has a two-dimensional electron gas (2DEG) channel 140A or 140B in one of the first and second Ill-nitride layers. Due to a polarization difference between the third 125A or 125B and fourth 130A or 130B Ill-nitride layers, a band offset of third 125A or 125B and fourth 130A or 130B Ill-nitride layers, and a thickness of the third 125A or 125B and fourth 130A or 130B Ill-nitride layers, the second heterojunction 120A or 120B has a two-dimensional hole gas (2DHG) channel 145A or 145B in one of the third 125A or 125B and fourth 130A or 130B Ill-nitride layers.
[0029] As discussed in the Background section, whether a 2DEG or 2DHG channel is formed and the location of the channel depends upon the difference in polarization between the two layers, the band offset of the two layers, and the thickness of the layers. Table 1 below lists the combinations of Ill-nitride layers that result in the formation of a 2DEG or 2DHG channel. Using the information in Table 1, those skilled in the art would easily be able to determine the necessary band offset and thickness of the layers to form the 2DEG or 2DHG channel. Although Table 1 lists binary compositions for the heterojunction with specific strain, the heterojunction can also be formed by ternary/ternary heterojunctions or ternary/binary heterojunctions. Those skilled in the art would be able to identify, based on the compositions of the two layers and the strain between the two layers, Ill-nitride ternary/ternary heterojunctions or Ill-nitride ternary/binary heterojunctions that form a 2DEG or 2DHG channel in one of the layers of the heterojunction. [0030]
Figure imgf000013_0001
TABLE 1 [0031] The 2DEG 140A or 140B and 2DHG 145A or 145B channels are aligned in a common horizontal plane at the interface 135A or 135B or are arranged in horizontal planes that are within 50 nm of each other in a vertical direction at the interface 135A or 135B. When the 2DEG and 2DHG channels are arranged in the same horizontal plane, there is no barrier to prevent the flow of electrons and holes across the interface 135A or 135B. In practice, however, a slight misalignment (i.e. , up to 50 nm) may occur between the 2DEG and 2DHG channels, which will not prevent the flow of the electrons and holes across the interface 135A or 135B. Further, a misalignment between the 2DEG and 2DHG channels can be intentionally formed to introduce quantum confinement for higher efficiency radiative recombination or as a carrier transport barrier.
[0032] It should be recognized that the horizontal alignment (or slight misalignment up to 50 nm) of the 2DEG and 2DHG channels results in the formation of a functional equivalent to a p-n junction, and accordingly an area running horizontally across the semiconductor device 105A or 105B can be referred to as a sheet diode because the diode functionality exists mainly in this area and not in the bulk material, as in conventional p-n diodes formed by intentional doping.
[0033] The top view of the semiconductor device 100A or 100B in Figure 1B illustrates that the 2DEG and 2DHG channels are formed across the semiconductor device 100A or 100B in a direction perpendicular to the direction in which the electrons or holes flow across the interface 135A or 135B. The Ill-nitride layers of the semiconductor device 105A illustrated in Figure 1A have approximately the same height, whereas the Ill-nitride layers 110B and 125B in Figure 1C have different heights. This difference is due to the different polarization differences, thicknesses, and band offsets of the Ill-nitride layers of each heterojunction.
[0034] In one non-limiting example of the device 105A in Figure 1A, Ill-nitride layers 110A and 125A can GaN layers and Ill-nitride layers 115A and 130A can be AIN layers. In one non-limiting example of device 105B in Figure 1C, Ill-nitride layers 110B and 130B can be AIN layers and Ill-nitride layers 115B and 125B are GaN layers. It should be recognized that these are non-limiting examples and that any of the heterojunctions listed in Table 1 can be employed, so long as one heterojunction forms a 2DEG channel and the other heterojunction forms a 2DHG channel. Furthermore, it should be recognized that the upper Ill-nitride layers 110A or 110B and 125A or 125B need not terminate at the same height. It should also be recognized that the 2DEG and 2DHG channels are formed by the heterojunction and exist regardless of whether a potential is applied across the semiconductor device 105A or 105B. Additionally, the heterojunctions 105A or 105B having the 2DEG channel can be arranged on the right side of the device and the heterojunctions 120A or 120B can be formed on the left side of the device.
[0035] Figures 2A and 2B respectively illustrate the semiconductor device 100A of Figure 1A with an applied forward and reverse bias by a first potential source 205A or 205B, respectively. When a the first potential source 205A applies a forward bias via contact pads 210A and 210B, as illustrated in Figure 2A, the 2DEG 140A and 2DHG 140B channels recombine into photons near the interface 135A. These generated photons can be used to produce light, such as for a light emitting diode or a laser diode. When the first potential source 205B applies a reverse bias via contact pads 210A and 210B, as illustrated in Figure 2B, incoming light impinging upon Ill-nitride layers 110A and 125A can excite electron and hole pairs, which can be separated to form photo current. Thus, under reverse bias, the semiconductor device 100A can be used as a photodetector or a solar cell. Moreover, under reverse bias, the diode formed from the 2DEG 140A and 2DHG 140B channels can act as an inverter. Although Figures 2A and 2B are illustrated using the semiconductor device 100A of Figure 1A, the application of forward and reverse biases is equally applicable to the semiconductor device 105B, which would operate in a similar manner to semiconductor device 105A under the application of forward and reverse biases. Should the heterojunctions 105A or 105B be arranged on the right side of the device and the heterojunctions 120A or 120B be arranged on the left side of the device, the application of the current by the first potential source 205A would be a reverse biasing of the device and the application of current by the first potential source 205B would be a forward biasing.
[0036] As discussed above, providing a horizontal misalignment between the 2DEG and 2DHG channels can introduce quantum confinement that results in a higher efficiency radiative recombination or as a carrier transport barrier. This can also be achieved by providing additional potential sources to the devices of Figures 2A and 2B, which is illustrated in Figures 3A and 3B. Specifically, second potential source 305A and third potential source 310A are respectively coupled to the upper surface of Ill-nitride layers 110A and 125A via contact pads 315A and 320A, respectively. Thus, application of current by the first and second potential sources 305A and 305B creates electron and hole deprived regions, and accordingly these potential sources 305A and 305B act as a gate to control the flow of electrons and holes across the interface 135A. As will be recognized by those skilled in the art, the second and third potential sources 305A and 310A should be applied in a region proximate to the interface 135A so as to create the electron and hole deprived regions in each heterojunction at the interface 135A.
[0037] The semiconductor device of Figure 3A is illustrated having a forward bias 205A, and potential sources 305A and 305B can also be employed when the device is under reverse bias 205B, which is illustrated in Figure 3B. Although Figures 3A and 3B are illustrated using the semiconductor device 100A of Figure 1A, the application of forward and reverse biases, as well as additional potential sources, is equally applicable to the semiconductor device 105B, which would operate in a similar manner to semiconductor device 105A under the application of forward and reverse biases and with the additional potential sources.
[0038] Figure 4 is a flow diagram of a method for forming the semiconductor devices 100A and 110B. A first heterojunction 105A or 105B is formed comprising a first Ill-nitride layer 110A or 110B arranged directly on a second Ill-nitride layer 115A or 115B (step 410). The first 110A or 110B and second 115A or 115B Ill-nitride layers are both either N-polar or metal-polar. A second heterojunction 120A or 120B is formed comprising a third Ill-nitride layer 125A or 125B arranged directly on a fourth Ill-nitride layer 130A or 130B (step 410). The third 125A or 125B and fourth 130A or 130B Ill-nitride layers are both either N-polar or metal-polar. The first 105A or 105B and second 120A or 120B heterojunctions are arranged directly adjacent to each other so that a interface 135A or 135B exists between the first 105A or 105B and second 120A or 120B heterojunctions. The first 105A or 105B, second 110A or 110B, third 125A or 125B, and fourth 130A or 130B Ill-nitride layers are directly adjacent to the interface 135A or 135B. The first heterojunction 105A or 105B has a 2DEG channel 140A or 140B in one of the first and second Ill-nitride layers. The second heterojunction 120A or 120B has a 2DHG channel 145A or 145B in one of the third 125A or 125B and fourth 130A or 130B Ill-nitride layers. The 2DEG 140A or 140B and 2DHG 145A or 145B channels are aligned in a common horizontal plane at the interface 135A or 135B or are arranged in horizontal planes that are within 50 nm of each other in a vertical direction at the interface 135A or 135B.
[0039] For ease of explanation, but not limitation, the method has been described as forming the first heterojunction 105A or 105B having the 2DEG channel and then forming the second heterojunction 120A or 120B having the 2DHG channel. However, the second heterojunction 120A or 120B having the 2DHG channel can be formed before the first heterojunction 105A or 105B having the 2DEG channel. Thus, references to forming the first and second heterojunctions should not be interpreted as the first heterojunction being formed before the second heterojunction and instead should be understood that either heterojunction can be formed first.
[0040] The heterojunctions can be formed in any number of ways, including separately forming the two heterojunctions and then physically attaching the two heterojunctions at the interface using, for example, an adhesive.
[0041] Another way to form the two heterojunctions is by selective area growth in which the different layers are all grown in the same growth chamber by selectively covering different parts of the device as it is grown. For example, a portion of a substrate can be covered so that the substrate has an exposed portion and a covered portion. The second Ill-nitride layer can then be grown on the exposed portion of the substrate. The covered portion of the substrate can then be uncovered and the second Ill-nitride layer can be covered and then the fourth Ill-nitride layer can be grown. While the second Ill-nitride layer is still covered, the third Ill-nitride layer can be grown on the fourth Ill-nitride layer. The second Ill-nitride layer can then be uncovered, the fourth Ill-nitride layer can be covered, and then the first Ill- nitride layer can be grown on the second Ill-nitride layer.
[0042] The discussion above involved heterojunctions formed from two separate layers of Ill-nitride polar materials. 3DEG and 3DHG can be formed in the bulk of the material using polarization grading, which is described in References [10] and [11] As described in Reference [10], assuming a ternary wurtzite Ill-nitride alloy AxCi-xN, polarization grading involves changing the relative compositions of the two Ill-nitride elements from an initial composition ^xinitCi-xinitN to a final composition at the top of the layer of AXfinalC1.XfinalN , wherein A and C are different group-ill elements (i.e. , different ones of aluminum, boron, gallium, and indium). An example of this is illustrated in Figure 5.
[0043] As illustrated in Figure 5, a semiconductor device 500 includes a first polarization graded Ill-nitride layer 505 with a bottom side 510 arranged on a substrate 515 and a top side 520 opposite of the bottom side 510. The first polarization graded Ill-nitride layer 505 comprises an alloy of first and second Ill- nitride elements. A composition of the alloy of the first polarization graded Ill-nitride layer changes from a first Ill-nitride composition at the bottom side 510 to a second Ill-nitride composition at the top side 520 so that a portion of the first polarization graded Ill-nitride layer 505 between the top 520 and bottom 510 sides has a deceasing amount of one of the first and second Ill-nitride elements and an increasing amount of the other one of the first and second Ill-nitride elements moving from the bottom side 510 to the top side 520 of the first polarization graded Ill-nitride layer 505.
[0044] The semiconductor device 500 also includes a second polarization graded Ill-nitride layer 525 with a bottom side 530 arranged on the substrate 515 and a top side 540 opposite of the bottom side 530. The second polarization graded Ill- nitride layer 525 comprises an alloy of third and fourth Ill-nitride elements. A composition of the alloy of the second polarization graded Ill-nitride layer 525 changes from a third Ill-nitride composition at the bottom side 530 to a fourth Ill- nitride composition at the top side 540 so that a portion of the second polarization graded Ill-nitride layer 505 between the top 540 and bottom 530 sides has a deceasing amount of one of the third and fourth Ill-nitride elements and an increasing amount of the other one of the third and fourth Ill-nitride elements moving from the bottom side 530 to the top side 540 of the second polarization graded Ill- nitride layer 525.
[0045] The first 505 and second 525 polarization graded Ill-nitride layers are directly adjacent to each other so that there is an interface 545 between the first 505 and second 525 polarization graded Ill-nitride layers. Due to the polarization grading, 3DEG is formed in the bulk of one of the first 505 and second 525 polarization graded Ill-nitride layers and 3DHG is formed in the bulk of the other one of the first 505 and second 525 polarization graded Ill-nitride layers. The polarization graded Ill-nitride layer having 3DHG functions in a similar manner to an impurity p- doped layer and the polarization graded Ill-nitride layer having 3DEG functions in a similar manner to an impurity n-doped layer. Thus, the bulk material of the first 505 and second 525 polarization graded Ill-nitride layers form a p-n junction at interface 545.
[0046] As illustrated, the first graded Ill-nitride layer 505 is graded from Ax. · L -c. .Ή to AX . ,C -X . ,N and the second graded Ill-nitride layer 525 is graded from
Figure imgf000021_0001
In one embodiment, xinit = 1 , xtinai = 0, yinit = 1, and ynnai = 0. The Ill-nitride alloys of the first 505 and second 525 graded Ill- nitride layers can be the same Ill-nitride alloys or different Ill-nitride alloys. In a non limiting example, this can involve grading the first layer 505 from Ill-polar AIN to Ill- polar GaN and grading the second layer 525 from Ill-polar GaN to Ill-polar AIN. In this non-limiting example, 3DHG would be formed in the first graded Ill-nitride layer 505 and 3DEG would be formed in second graded Ill-nitride layer 525. It should be recognized that this is a non-limiting example and that the grading can from one Ill- nitride to a second Ill-nitride as described in Table 1 above. Other non-limiting examples include: the first graded Ill-nitride layer 505 graded from AI0.2Ga0.sN to Alo.5Gao.5N and the second graded Ill-nitride layer 525 being graded from Alo.5Gao.5N to Alo.2Gao.8N; the first graded Ill-nitride layer 505 graded from Alo.4Gao.6N to Alo.5Gao.5N and the second graded Ill-nitride layer 525 being graded from Alo.5Gao.5N to Alo.2Gao.8N; and the first graded Ill-nitride layer 505 graded from AI0.2Ga0.sN to Alo.5Gao.5N and the second graded Ill-nitride layer 525 being graded from lno.1Gao.9N to lno.2Gao.8N. In each of these non-limiting examples, the first graded Ill-nitride layer 505 is n-type and the second graded Ill-nitride layer 525 is p-type. Although Figure 5 illustrates the top sides 520 and 540 of the first 505 and second 525 graded Ill-nitride layers being terminated at the same height, this need not be the case and the top sides 520 and 540 can terminate at different heights. [0047] As described in Reference [10], the polarization doping concentration profile (s ) is a change in polarization ( dP ) relative to the change in position of the particular polarization within the layer dl, which is reflected in the following equation: dP
Op = - dl (1)
[0048] Because the change in polarization is based on the composition (x) of the wurtzite Ill-nitride alloy at a particular position within the layer, equation (1) can be rewritten as: dP dx s P, dx dl (2)
[0049] The first term of equation (2) reflects the polarization change rate with respect to alloy composition, which will be referred to as the composition-polarization change rate and can be represented by the following equation: k _ dP dx (3)
[0050] When k is positive, the graded Ill-nitride layer is a p-type layer and when k is negative, the graded Ill-nitride layer is n-type. Thus, so long as one of the graded Ill-nitride layers has a positive k and the other one of the graded Ill-nitride layers has a negative k, a p-n junction will be formed by the two adjacent graded Ill- nitride layers.
[0051] The second term of equation (2) reflects the grading speed (i.e. , the speed at which the composition of x changes over the thickness of the layer) and can be represented by the following equation: dx
[0052] n9 = Έ
[0053] Additional details for forming the graded layers 505 and 525 can be found in Reference [10] [0054] The semiconductor device 500 can be forward or reversed biased, or additional potential sources added in a similar manner to that described above in connection with semiconductor device 100A and will operate in a similar manner. Figure 5 illustrates a forward bias being applied via a voltage source 545 coupled to the top side 520 of the first polarization graded Ill-nitride layer 505 via contact 550A and to the top side 540 of the second polarization graded Ill-nitride layer 525 via a contact 550B. Reverse biasing would have a similar arrangement to that illustrated in Figure 2B. The additional potential sources can be applied to the first 505 and second 525 graded Ill-nitride layers via respective contacts, at the respective top sides 520 and 540, proximate to the interface 545 so that the application of current from the potential sources forms depletion regions on each side of the interface 545. [0055] As discussed above, the formation of the 2DEG and 2DHG channels that are aligned in a common horizontal plane at the interface or slightly misaligned in the vertical direction at the interface can be referred to as a sheet diode because the p-n junction is formed by the channels themselves instead of in the bulk of the material. Further, two polarization graded layers with 3DEG and 3DHG, respectively, form a lateral diode arrangement. These arrangements are referred to as a lateral diode because the electrons and holes move laterally between two adjacent structures. This contrasts with conventional diodes that are arranged with one layer arranged on top of the other layer so that the electrons and holes move horizontally between the layers.
[0056] For ease of explanation and not limitation, certain embodiments have been described above with binary Ill-nitrides. However, these embodiments are equally applicable to alloys of Ill-nitrides. [0057] The disclosed embodiments provide methods for semiconductor devices having a lateral arrangement. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.
[0058] Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.
[0059] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.
References
[1] “Polarization-Induced Hole Doping in Wide-Band-Gap Uniaxial Semiconductor Heterostructures”, Simon et al., SCIENCE, 01 JAN 2010 : 60-64.
[2] Wood, C. & Jena, D. (2008). Polarization effects in semiconductors: From ab initio theory to device applications. 10.1007/978-0-387-68319-5.
[3] “Polarization Effects in Group Ill-Nitride Materials and Devices”, Dissertation by Qiyuan Wei, Arizona State University, May 2012.
[4] “Polarization-engineering in group Ill-nitride heterostructures: New opportunities for device design”, Debdeep Jena et al., Phys. Status Solidi A 208, No. 7, 1511-1516 (2011).
[5] “Study of Two-Dimensional Electron Gases for GaN-Based Heterostructure” by Rahman, Md Saifur et al., Journal of Electrical Engineering and Electronic Technology (2017).
[6] “Lateral-Polarity Structure of AIGaN Quantum Wells: A Promising Approach to Enhancing the Ultraviolet Luminescence” by Guo, Wei et al., Advanced Functional Materials (2018).
[7] "Lateral Two-Dimensional p-i-n Diode in a Completely Undoped GaAs/AIGaAs Quantum Well”, Dai, Truong et al., Japanese Journal of Applied Physics. 52. 4001- 10.7567/JJAP.52.014001 (2013).
[8] “Three-dimensional hole gas induced by polarization in (OOOI)-oriented metal-face Ill- nitride structure”, L. Zhang et al., Appl. Phys. Lett. 97, 062103 (2010) [9] “Controlling a three dimensional electron slab of graded AlxGai xN”, Adhikari, Rajdeep et al., Applied Physics Letters. 108. 10.1063/1.4939788 (2015).
[10] WO 2019/111153 by Xiaohang Li et al., published June 13, 2019.
[11] “GaN based Heterojunction Bipolar Transistors”, PowerPoint by John Simon, EE 666, April 7, 2005.

Claims

WHAT IS CLAIMED IS:
1. A semiconductor device (100A, 100B), comprising: a first heterojunction (105A, 105B) comprising a first Ill-nitride layer (110A,
11 OB) arranged directly on a second Ill-nitride layer (115 A , 115B), wherein the first (110A, 11 OB) and second (115 A , 115B) Ill-nitride layers are both either N-polar or metal-polar; and a second heterojunction (120A, 120B), arranged directly laterally adjacent to the first heterojunction (105A, 105B) so that there is an interface (135A, 135B) between the first (105A, 105B) and second (120A, 120B) heterojunctions, comprising a third Ill-nitride layer (125A, 125B) arranged directly on a fourth Ill-nitride layer (130A, 130B), wherein the third (125A, 125B) and fourth (130A, 130B) Ill-nitride layers are both either N-polar or metal-polar, wherein the first (105A, 105B), second (110A, 110B), third (125A, 125B), and fourth (130A, 130B) Ill-nitride layers are directly adjacent to the interface (135A, 135B), wherein, due to a polarization difference between the first (110A, 110B) and second (115 A , 115B) Ill-nitride layers, a band offset of the first (110A, 11 OB) and second (115 A , 115B) Ill-nitride layers, and a thickness of the first (110A, 11 OB) and second (115 A , 115B) Ill-nitride layers, the first heterojunction (105A, 105B) has a two-dimensional electron gas, 2DEG, channel (140A, 140B) in one of the first and second Ill-nitride layers, wherein, due to a polarization difference between the third (125A, 125B) and fourth (130A, 130B) Ill-nitride layers, a band offset of third (125A, 125B) and fourth (130A, 130B) Ill-nitride layers, and a thickness of the third (125A, 125B) and fourth (130A, 130B) Ill-nitride layers, the second heterojunction (120A, 120B) has a two- dimensional hole gas, 2DHG, channel (145A, 145B) in one of the third (125A, 125B) and fourth (130A, 130B) Ill-nitride layers, and wherein the 2DEG (140A, 140B) and 2DHG (145A, 145B) channels are aligned in a common horizontal plane at the interface (135A, 135B) or are arranged in horizontal planes that are within 50 nm of each other in a vertical direction at the interface (135A, 135B).
2. The semiconductor device of claim 1 , further comprising: a first current source coupled to the first and third Ill-nitride layers, wherein application of current from the current source to the first and third Ill-nitride layers causes a flow of electrons from towards a negative terminal of the first current source.
3. The semiconductor device of claim 2, further comprising: a second current source coupled to the first Ill-nitride layer; and a third current source coupled to the third Ill-nitride layer.
4. The semiconductor device of claim 1 , wherein the 2DEG and 2DHG channels are aligned in a c-plane orientation at the interface.
5. The semiconductor device of claim 1 , wherein the 2DEG and 2DHG channels are within 50 nm of alignment in the c-plane orientation at the interface.
6. The semiconductor device of claim 2, wherein a positive terminal of the first current source is coupled to the third Ill-nitride layer and the negative terminal of the first current source is coupled to the first Ill-nitride layer, and wherein the semiconductor device is a light emitting diode or laser diode.
7. The semiconductor device of claim 2, wherein the negative terminal of the first current source is coupled to the third Ill-nitride layer and a positive terminal of the first current source is coupled to the first Ill-nitride layer, and wherein the semiconductor device is a solar cell, photodetector, or inverter.
8. The semiconductor device of claim 1, wherein the second and fourth Ill-nitride layers have a different thickness in a direction perpendicular to the interface.
9. The semiconductor device of claim 1 , wherein the 2DEG and 2DHG channels extend along a plane perpendicular to the interface.
10. A method for forming a semiconductor device (100A,100B), the method comprising: forming (405) a first heterojunction (105A, 105B) comprising a first Ill-nitride layer (110A, 110B) arranged directly on a second Ill-nitride layer (115 A , 115 B) , wherein the first (110A, 11 OB) and second (115A, 115B) Ill-nitride layers are both either N-polar or metal-polar; and forming (410) a second heterojunction (120A, 120B) comprising a third Ill- nitride layer (125A, 125B) arranged directly on a fourth Ill-nitride layer (130A, 130B), wherein the third (125A, 125B) and fourth (130A, 130B) Ill-nitride layers are both either N-polar or metal-polar, wherein the first (105A, 105B) and second (120A, 120B) heterojunctions are arranged directly adjacent to each other so that a interface (135A, 135B) exists between the first (105A, 105B) and second (120A, 120B) heterojunctions, wherein the first (105A, 105B), second (110A, 110B), third (125A, 125B), and fourth (130A, 130B) Ill-nitride layers are directly adjacent to the interface (135A, 135B), wherein the first heterojunction (105A, 105B) has a two-dimensional electron gas, 2DEG, channel (140A, 140B) in one of the first and second Ill-nitride layers, wherein the second heterojunction (120A, 120B) has a two-dimensional hole gas, 2DHG, channel (145A, 145B) in one of the third (125A, 125B) and fourth (130A, 130B) Ill-nitride layers, and wherein the 2DEG (140A, 140B) and 2DHG (145A, 145B) channels are aligned in a common horizontal plane at the interface (135A, 135B) or are arranged in horizontal planes that are within 50 nm of each other in a vertical direction at the interface (135A, 135B).
11. The method of claim 10, wherein the first and second heterojunctions are formed separately and mechanically attached to each other at the interface.
12. The method of claim 10, wherein the second and fourth Ill-nitride layers are formed prior to forming the first and third Ill-nitride layers.
13. The method of claim 10, wherein the first and second heterojunctions are formed by selective area growth by: covering a portion of a substrate so that the substrate has an exposed portion and a covered portion; growing the second Ill-nitride layer on the exposed portion of the substrate; uncovering the covered portion of the substrate; covering the second Ill-nitride layer and growing the fourth Ill-nitride layer; growing the third Ill-nitride layer on the fourth Ill-nitride layer; uncovering the second Ill-nitride layer; covering the fourth Ill-nitride layer and growing the first Ill-nitride layer on the second Ill-nitride layer.
14. A semiconductor device (500), comprising: a first polarization graded Ill-nitride layer (505) with a bottom side (510) arranged on a substrate (515) and a top side (520) opposite of the bottom side (510), wherein the first polarization graded Ill-nitride layer (505) comprises an alloy of first and second Ill-nitride elements, and wherein a composition of the alloy of the first polarization graded Ill-nitride layer (505) changes from a first Ill-nitride composition at the bottom side (510) to a second Ill-nitride composition at the top side (520) so that a portion of the first polarization graded Ill-nitride layer (505) between the top (520) and bottom (510) sides has a deceasing amount of one of the first and second Ill-nitride elements and an increasing amount of the other one of the first and second Ill-nitride elements moving from the bottom side (510) to the top side (520) of the first polarization graded Ill-nitride layer (505); and a second polarization graded Ill-nitride layer (525) with a bottom side (530) arranged on the substrate (515) and a top side (540) opposite of the bottom side (530), wherein the second polarization graded Ill-nitride layer (525) comprises an alloy of third and fourth Ill-nitride elements, and wherein a composition of the alloy of the second polarization graded Ill-nitride layer (525) changes from a third Ill-nitride composition at the bottom side (530) to a fourth Ill-nitride composition at the top side (540) so that a portion of the second polarization graded Ill-nitride layer (505) between the top (540) and bottom (530) sides has a deceasing amount of one of the third and fourth Ill-nitride elements and an increasing amount of the other of the third and fourth Ill-nitride elements moving from the bottom side (530) to the top side (540) of the second polarization graded Ill-nitride layer (525), wherein the first (505) and second (525) polarization graded Ill-nitride layers are directly adjacent to each other so that there is an interface (545) between the first (505) and second (525) polarization graded Ill-nitride layers, wherein a bulk of the first polarization graded Ill-nitride layer (505) has three- dimensional electron gas and a bulk of the second polarization graded Ill-nitride layer (525) has three-dimensional hole gas.
15. The semiconductor device of claim 14, further comprising: a first current source coupled to the top side of the first and second polarization graded Ill-nitride layers, wherein application of current from the current source to the top side of the first and second polarization graded Ill-nitride layers causes a flow of electrons from towards a negative terminal of the first current source.
16. The semiconductor device of claim 15, further comprising: a second current source coupled to the first polarization graded Ill-nitride layer proximate to the interface; and a third current source coupled to the second polarization graded Ill-nitride layer proximate to the interface.
17. The semiconductor device of claim 14, wherein the first and second polarization graded Ill-nitride layers comprise a first group-ill element A and a second group Ill-element C, wherein the first polarization graded Ill-nitride layer is g sraded from Ax xi. nitC 1_x xi.m .tN to AX xfri.nal ,C 1_X xfi.nal ,N and the second g sraded Ill-nitride lay Jer is graded from
Figure imgf000033_0001
18. The semiconductor device of claim 14, wherein the first and second polarization graded Ill-nitride layers comprise a same Ill-nitride alloy.
19. The semiconductor device of claim 14, wherein the first and second polarization graded Ill-nitride layers comprise different Ill-nitride alloys.
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