A partial p-gan cap layer and reverse gradient barrier enhanced radio frequency hemt and a preparation method thereof

Abstract

The present application relates to a kind of partial P-GaN cap layer and reverse gradient barrier enhanced radio frequency HEMT and its preparation method, in the structure, by the insertion of AlGaN reverse component gradual change barrier layer, and the setting of P-GaN cap layer, undoped GaN cap layer and air cavity structure below gate, in the improved part of the band of barrier layer, the electron concentration near gate region is reduced, the gate leakage current is reduced, on the basis of relieving current collapse, the gate voltage swing range is widened, the linearity of transconductance is improved, the gate contact resistance is reduced, the range of depletion region below gate is reduced, the direct current and radio frequency characteristics of device are improved, while the parasitic capacitance below gate cap is reduced to the greatest extent, the radio frequency characteristics of device are improved.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor devices, and in particular to a partial P-GaN cap layer and reverse gradient barrier enhanced radio frequency HEMT and a preparation method thereof. Background Art

[0002] Millimeter wave devices are becoming increasingly important in military defense and production life due to their high reliability, portability, stability and other advantages. High electron mobility transistors are an important component of millimeter wave devices and have been a research hotspot for domestic academic institutions and semiconductor companies in recent decades. To realize the full potential of millimeter wave 5G and other communication systems, more linear and efficient transistors will need to be selected than current technologies. Although gallium nitride HEMTs provide excellent power density, their limited linearity and efficiency, especially in complex modulation, will affect the overall performance of the system and require significant digital pre-distortion compensation and significant power reversal. At present, nonlinearity and gain compression problems based on AlGaN / GaN HEMTs still exist. How to improve the linearity and RF characteristics of RF HEMT devices is one of the problems to be solved in this invention.

[0003] On the other hand, one of the main challenges of GaN heteroepitaxy is the growth of a semi-insulating buffer layer due to the natural background n-doping of GaN grown by metal-organic chemical vapor deposition. Due to process conditions, electrically active buffer traps often exist near the AlGaN and GaN heterojunction channels. Both intentional and unintentional doping can lead to severe dispersion of drain current under dynamic conditions, known as current collapse, and increase gate leakage current, both of which are bottlenecks for improving the performance of power RF devices. Reducing current collapse and gate leakage current in RF HEMT devices is one of the issues that the present invention urgently addresses. Summary of the Invention

[0004] In response to the technical problems existing in the prior art, the primary purpose of the present invention is to provide a partial P-GaN cap layer and a reverse-gradient barrier enhanced radio frequency HEMT and a method for preparing the same. The device is provided with an AlGaN barrier layer with a gradient Al composition and inserted into the AlGaN barrier layer. The AlGaN upper barrier layer, the AlGaN barrier layer with a reverse-gradient Al composition, the AlGaN lower barrier layer, and the GaN channel layer together form a special heterojunction. On the one hand, the insertion of the reverse-gradient barrier layer reduces the background electron concentration, creating resistance in the middle of the barrier layer, which to some extent alleviates the trap capture problem, reduces current collapse, and reduces gate leakage current. On the other hand, the use of a higher Al composition in the AlGaN lower layer improves the confinement of the two-dimensional electron gas.

[0005] Furthermore, the present invention utilizes a unique capping layer structure combined with a T-type gate structure. This capping layer employs an undoped GaN capping layer positioned on either side of a P-GaN capping layer. The P-GaN capping layer reduces the depletion region under the gate, resulting in better DC characteristics for the device. The undoped GaN capping layer bears a portion of the gate voltage, improving the linearity of the transconductance and further reducing gate leakage current. Simultaneously, the T-type gate structure reduces gate contact resistance, enhancing gate control.

[0006] In order to achieve the above object, the present invention adopts at least the following technical solutions:

[0007] In one aspect, the present invention provides a partially doped P-GaN and reverse gradient barrier enhanced radio frequency HEMT device, comprising:

[0008] A substrate; an AlGaN buffer layer located on the substrate; a GaN channel layer located on the AlGaN buffer layer; and an AlGaN barrier layer located on the GaN channel layer, wherein the AlGaN barrier layer comprises, in sequence along a growth direction, an AlGaN lower barrier layer, an AlGaN reverse composition graded barrier layer, and an AlGaN upper barrier layer, wherein the AlGaN upper barrier layer has an Al composition of 21%, the AlGaN lower barrier layer has an Al composition of 26%, and the composition of the AlGaN reverse composition graded barrier layer changes linearly from 26% to 21% along the growth direction.

[0009] a cap layer structure located on the AlGaN barrier layer, the cap layer structure consisting of a P-GaN cap layer and a first undoped GaN cap layer and a second undoped GaN cap layer, the first undoped GaN cap layer and the second undoped GaN cap layer being located on either side of the P-GaN cap layer, the first undoped GaN cap layer having a width equal to that of the second undoped GaN cap layer, and the P-GaN cap layer having a width greater than that of the first and second undoped GaN cap layers;

[0010] a Schottky contact gate electrode located on the cap layer structure, the Schottky contact electrode consisting of a gate foot contact electrode and a gate cap contact electrode, the gate foot contact electrode being in contact with the cap layer structure;

[0011] An air dielectric is provided between the gate cap contact electrode and the AlGaN upper barrier layer, and the air dielectric is located on both sides of the cap layer structure and the gate foot contact electrode;

[0012] an ohmic contact source electrode and a drain electrode located on the AlGaN barrier layer;

[0013] A passivation layer is filled between the Schottky contact gate electrode and the source electrode and the drain electrode, and is located on the surface of the AlGaN upper barrier layer and on both sides of the air medium.

[0014] Furthermore, the width of the first and second undoped GaN cap layers is 50-80 nm, and the width of the P-GaN cap layer 21 is 80-150 nm.

[0015] Furthermore, the passivation layer consists of a first passivation layer and a second passivation layer. The first passivation layer is located on the surface of the AlGaN upper barrier layer, and the thickness of the first passivation layer is equal to the thickness of the cap layer structure.

[0016] Furthermore, the second passivation layer is located on the first passivation layer, and the thickness of the second passivation layer is equal to the thickness of the gate foot contact electrode.

[0017] Furthermore, the doping concentration of the P-GaN cap layer is 3×10 17 cm -3 ;

[0018] The P-GaN cap layer and the undoped GaN cap layer are formed by an epitaxial growth process, or the P-GaN cap layer is formed in the undoped GaN cap layer by an ion implantation process.

[0019] Furthermore, the thickness of the GaN channel layer is 35 nm; the thickness of the AlGaN barrier layer is 15 nm; the thickness of the AlGaN upper barrier layer is 5 nm; and the thickness of the AlGaN lower barrier layer is 5 nm.

[0020] Furthermore, the thickness of the AlGaN reverse composition graded barrier layer is 5 nm.

[0021] Another aspect of the present invention provides a method for preparing a partially doped P-GaN and reverse gradient barrier enhanced radio frequency HEMT device, comprising the following steps:

[0022] growing an AlGaN buffer layer on the substrate;

[0023] epitaxially growing a GaN channel layer on the AlGaN buffer layer;

[0024] epitaxially growing an AlGaN lower barrier layer with an Al composition of 26% on the GaN channel layer;

[0025] Epitaxially growing an AlGaN reverse composition graded barrier layer on the AlGaN lower barrier layer, wherein the composition of the AlGaN reverse composition graded barrier layer changes linearly from 26% to 21% along the growth direction;

[0026] epitaxially growing an AlGaN upper barrier layer with an Al composition of 21% on the AlGaN reverse composition graded barrier layer;

[0027] Depositing a first passivation layer on the AlGaN upper barrier layer using a plasma enhanced chemical vapor deposition (PECVD) process;

[0028] forming a growth window for a cap layer structure in the first passivation layer, and forming a P-GaN cap layer and first and second undoped GaN cap layers in the growth window, wherein the first undoped GaN cap layer and the second undoped GaN cap layer are respectively located on either side of the P-GaN cap layer, the width of the first undoped GaN cap layer is equal to the width of the second undoped GaN cap layer, and the width of the P-GaN cap layer is greater than the widths of the first and second undoped GaN cap layers;

[0029] Depositing a second passivation layer on the cap layer structure and the first passivation layer, and forming a gate foot contact electrode window in the second passivation layer;

[0030] Depositing metal in the gate foot contact electrode window to form a gate foot contact electrode;

[0031] Etching the second passivation layer and the first passivation layer to form air cavity windows located on both sides of the gate foot contact electrode and the cap layer structure, and depositing photoresist in the air cavity windows;

[0032] forming a source electrode, a drain electrode, and a gate cap contact electrode, wherein the gate cap contact electrode and the gate foot contact electrode constitute a T-type gate structure;

[0033] The photoresist is removed to form an air cavity containing an air medium. The air cavity is located between the gate cap contact electrode and the AlGaN upper barrier layer and on both sides of the cap layer structure and the gate foot contact electrode.

[0034] Furthermore, a growth window for a cap layer structure is formed in the first passivation layer. The step of forming a P-GaN cap layer and an undoped GaN cap layer in the growth window includes etching the first passivation layer to form a P-GaN cap layer window, epitaxially growing a P-GaN cap layer in the window, and then etching both sides of the P-GaN cap layer to form an undoped GaN window, and epitaxially growing an undoped GaN cap layer in the window.

[0035] Furthermore, a growth window for a cap layer structure is formed in the first passivation layer. The step of forming a P-GaN cap layer and an undoped GaN cap layer in the growth window includes etching the first passivation layer to form a growth window for the cap layer structure, epitaxially growing an undoped GaN cap layer in the growth window, and then using an ion implantation process to implant ions into the undoped GaN cap layer to form a P-GaN cap layer.

[0036] Furthermore, the width of the first and second undoped GaN cap layers is 50-80 nm, and the width of the P-GaN cap layer 21 is 80-150 nm.

[0037] Furthermore, the thickness of the cap layer structure is equal to the thickness of the first passivation layer;

[0038] The thickness of the AlGaN reverse composition graded barrier layer is 5 nm;

[0039] The substrate is a semi-insulating 4H-SiC substrate or a 6H-SiC substrate, and its thickness is reduced to 50-100 μm.

[0040] Compared with the prior art, the present invention has at least the following beneficial effects:

[0041] The present invention introduces an AlGaN barrier layer with a reversely graded Al composition into the AlGaN barrier layer. This improves the conduction band difference and potential difference of the AlGaN barrier layer as a whole, reduces the impact of electrically active buffer traps present near the AlGaN and GaN heterojunction channels on device performance, and alleviates current collapse. The introduction of the reversely graded Al composition layer increases a portion of the conduction band in the barrier layer. This increase in the conduction band requires higher energy for electrons to cross the barrier layer, thereby reducing gate leakage current and improving device reliability.

[0042] On the other hand, a special cap layer structure and air cavity structure are used under the T-type gate. The use of the P-type GaN layer in the cap layer structure reduces the range of P-type GaN in the traditional structure, reduces the depletion range of the channel, and improves the DC characteristics of the device; the introduction of undoped GaN cap layers on both sides of the P-type GaN produces a partial potential drop, requiring a higher voltage to turn on the device. The undoped GaN cap layer bears part of the gate voltage, widens the gate voltage swing range, thereby improving the linearity of the transconductance and reducing the gate contact resistance. The shortening of the P-GaN cap layer reduces the range of the depletion region under the gate and improves the DC and RF characteristics of the device; at the same time, the introduction of the air cavity also minimizes the parasitic capacitance under the gate cap, directly improving the RF characteristics of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 FIG. 1 is a schematic cross-sectional structural diagram of an enhanced radio frequency HEMT according to an embodiment of the present invention.

[0044] Figure 2 This is a schematic diagram of the cross-sectional structure of a traditional P-GaN HEMT structure.

[0045] Figure 3 1 is a comparison diagram of transfer curves and transconductance characteristic curves of an enhancement mode RF HEMT structure according to an embodiment of the present invention and a conventional P-GaN HEMT structure.

[0046] Figure 4 1 is a graph comparing the gate leakage current of an enhancement mode RF HEMT structure according to an embodiment of the present invention and a conventional P-GaN HEMT structure.

[0047] Figure 5 1 is a conduction band diagram and electron concentration distribution diagram of an enhancement mode RF HEMT structure according to an embodiment of the present invention and a conventional P-GaN HEMT structure.

[0048] Figure 6 1 is a conduction band diagram and potential distribution diagram of the vertical region from the top of the GaN cap layer to the bottom of the GaN channel layer of the enhancement mode RF HEMT structure according to one embodiment of the present invention. DETAILED DESCRIPTION

[0049] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings of the present invention. The described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, other embodiments obtained by ordinary technicians in this field without making creative work are all within the scope of protection of the present invention. The experimental methods described in the following examples are all conventional methods unless otherwise specified; the reagents and materials, unless otherwise specified, can be obtained from public commercial channels.

[0050] Spatially relative terms such as "below," "beneath," "below," "above," "upper," etc. are used in this specification to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures.

[0051] In addition, the use of terms such as "first," "second," and the like to describe various elements, layers, regions, sections, and the like is not intended to be limiting. The use of "having," "containing," "including," and "comprising" are open-ended terms that indicate the presence of stated elements or features, but do not exclude additional elements or features, unless the context clearly indicates otherwise.

[0052] In this specification, the width refers to the direction between the source and the drain, and the thickness refers to the direction from the gate to the substrate.

[0053] The present invention is further described in detail below. One embodiment of the present invention provides a partially doped P-GaN and reverse gradient barrier enhanced RF HEMT, such as Figure 1 As shown, the device includes a substrate 11, which is preferably a SiC substrate. In a preferred embodiment, a semi-insulating 4H-SiC or 6H-SiC substrate is selected. The thickness of the SiC substrate can be reduced to 50-100 μm.

[0054] The AlGaN buffer layer 12 is 2 μm thick and is located on the substrate 11. A stack of layers consisting of a GaN channel layer 13, an AlGaN lower barrier layer 14, an AlGaN reverse composition graded barrier layer 15, an AlGaN upper barrier layer 16, and a P-GaN cap layer 21 is located on the AlGaN buffer layer 12.

[0055] The source 17, drain 18, and cap layer structure are located on the surface of the AlGaN upper barrier layer 16, and the cap layer structure is located between the source 17 and drain 18. The cap layer structure consists of a P-GaN cap layer 21 and a first undoped GaN cap layer and a second undoped GaN cap layer 22 located on both sides of the P-GaN cap layer 21. The doping concentration of the P-GaN cap layer 21 is 3×10 17 cm -3 The width of the first undoped GaN cap layer is equal to the width of the second undoped GaN cap layer, and the widths of the first and second undoped GaN cap layers are 50 to 80 nm. The setting of the widths of the first and second undoped GaN cap layers can enable the gate to bear more gate voltage, generate more potential drops, require a higher voltage to turn on the device, expand the gate voltage swing range, improve the transconductance characteristics, and thus improve the linearity of the device. At the same time, the peak value of the transconductance is reduced due to the widening of the gate length. If the widths of the first and second undoped GaN cap layers are smaller than this width range, the gate voltage swing range will be too small, the transconductance range will be too short, and the transconductance range will be affected.

[0056] The width of the P-GaN cap layer is greater than the width of the first and second undoped GaN cap layers located on either side of it, and the thickness of the P-GaN cap layer is equal to the thickness of the first and second undoped GaN cap layers. The width of the P-GaN cap layer 21 is 80 to 150 nm. The provision of a wider P-GaN cap layer can expand the depletion range of the gate relative to the channel layer, ensuring a higher threshold voltage while improving the device's frequency characteristics. If the width of the P-GaN cap layer is too large and exceeds the above range, it will lead to a decrease in DC characteristics and the output power of the device. If the width of the P-GaN cap layer is less than the above width range, the depletion range of the gate relative to the channel will be too small, resulting in an excessively low threshold voltage of the device. In a preferred embodiment, the width of the P-GaN cap layer 21 is selected to be 100 nm, the width of the first and second undoped GaN cap layers is 70 nm, and the total width of the cap layer structure is 240 nm. This width setting ensures that while achieving optimal RF characteristics, a higher threshold voltage, output current density, and transconductance are obtained.

[0057] Among them, the Al component in the AlGaN buffer layer 12 is 5%. The GaN channel layer 13 and the AlGaN lower barrier layer 14, the AlGaN reverse component layer 15, and the AlGaN upper barrier layer 16 form a special heterojunction. The thickness of the GaN channel layer 13 is 35nm. The thickness of the AlGaN lower barrier layer 14 is 5nm, and its Al component is 26%. The thickness of the AlGaN reverse component graded barrier layer 15 is 5nm, and the Al component changes linearly from 26% to 21% along the growth direction. The thickness of the AlGaN upper barrier layer 16 is 5nm, and its Al component is 21%. The introduction of the AlGaN lower barrier layer 14 with an Al component of 26% into the heterojunction increases the spontaneous and piezoelectric polarization effects and improves the density of the two-dimensional electron gas (2DEG). As Figure 5 As shown in the figure, the introduction of the AlGaN reverse composition graded barrier layer, on the one hand, uses the reverse composition graded AlGaN layer as a resistive buffer layer to suppress the electrically active buffer traps at the AlGaN / GaN heterojunction, which to some extent alleviates the trap capture problem and suppresses the current collapse effect. On the other hand, the conduction band difference and potential difference between AlGaN and GaN increase, and charges require more energy to reach the gate region, thus suppressing gate leakage current. This special heterostructure enhances the DC and RF performance of the device and reduces gate leakage current.

[0058] The gate foot contact electrode 19 and the gate cap contact electrode 24 form a T-shaped gate structure and are located above the cap layer structure. The width of the gate cap contact electrode 24 is greater than that of the gate foot contact electrode 19. The gate metal is preferably a Ni / Au composite metal layer.

[0059] An air cavity containing an air dielectric is provided between the gate cap contact electrode 24 and the AlGaN upper barrier layer 16. The first passivation layer 20 and the second passivation layer 23 are located between the source 17, the drain 18 and the air cavity. The first passivation layer and the second passivation layer are preferably made of silicon nitride material. The air cavity isolates the first passivation layer 20, the second passivation layer 23 from the contact with the gate foot contact electrode 19 and the undoped GaN cap layer 22. Compared with other dielectric structures, this setting reduces parasitic capacitance and improves the RF performance of the device. The thickness of the first passivation layer 20 is equal to the thickness of the P-GaN cap layer 21 and the undoped GaN cap layer 22, and its thickness is 50nm. The thickness of the second passivation layer 23 is equal to the thickness of the gate foot contact electrode 19, and its thickness is 60 to 100nm.

[0060] In a preferred embodiment, the cap layer structure is formed using an epitaxial growth process. For example, a wet etching process or an ICP etching process is used to etch the first passivation layer 20 to form a P-GaN cap layer growth window. Subsequently, a P-GaN cap layer 21 is epitaxially grown in the cap layer growth window using a metal organic chemical vapor deposition (MOCVD) process, and then both sides of the P-GaN cap layer are etched to form undoped GaN growth windows, in which an undoped GaN cap layer 22 is epitaxially grown.

[0061] In another preferred embodiment, the cap layer structure is formed using an ion implantation process. For example, a wet etching process or an ICP etching process is used to etch the first passivation layer 20 to form a growth window for the cap layer structure. An undoped GaN cap layer is epitaxially grown in the growth window. Subsequently, a mask pattern is formed on the undoped GaN cap layer, and ion implantation is performed to form a P-GaN cap layer in the undoped GaN cap layer.

[0062] The cap layer structure of the present invention reduces the size of the P-GaN cap layer area, reduces the area of ​​the depletion region below, reduces the gate contact resistance, and improves the DC and RF performance of the device; on the other hand, Figure 3 As shown, the undoped GaN cap layer 22 bears part of the gate voltage, resulting in a partial potential drop, requiring a higher voltage to turn on the device, increasing the gate voltage swing range, improving the transconductance characteristics and thus improving the linearity of the device. The undoped GaN cap layer 22 directly increases the conduction band difference between the gate contact electrode 19 and the AlGaN upper barrier layer 16. Electrons require more energy to cross the barrier layer to reach the triangular barrier layer, thereby reducing the gate leakage current and improving the reliability of the device.

[0063] Based on the above-mentioned enhanced RF HEMT device, the following is a detailed description of the device preparation method, which includes the following steps:

[0064] First, a semi-insulating 4H-SiC substrate is selected for substrate cleaning. The chamber temperature is set to a high-temperature cleaning temperature of approximately 1000°C, and hydrogen is introduced to remove surface contaminants and form steps.

[0065] Next, the metal organic chemical vapor deposition (MOCVD) process was used to grow the Al atomic layer. When the substrate temperature was 960°C and the reaction chamber pressure was 50 torr, trimethylaluminum (TMAl) was introduced at a flow rate of 250 sccm. The temperature was then reduced to 860°C, the pressure in the pressure chamber was changed to 100 torr, and NH3 was introduced at a flow rate of 5 slm for 6 minutes. When the substrate temperature was 750°C, a pulsed laser deposition process was used to grow the AlGaN film. The flow rate of TMAl was controlled to be constant at 40 sccm, and the flow rate of the gallium source was also controlled at 40 sccm, resulting in an AlGaN buffer layer with a composition of 5% and a growth thickness of 2 μm.

[0066] A GaN channel layer with a thickness of 35 nm was epitaxially grown on the AlGaN buffer layer. Unintentional doping was used, and H2, NH3 and gallium sources were introduced. The growth temperature was set to 900-920°C, the pressure was 5300-5500Pa, the gallium source flow rate was 220sccm, the H2 flow rate was 500sccm, and the NH3 flow rate was 5000sccm.

[0067] An AlGaN lower barrier layer with an Al component of 26% is epitaxially grown on the GaN channel layer. Unintentional doping is used, and NH3, H2, gallium source and aluminum source are introduced. The growth temperature is set to 900-920°C and the thickness is 5nm.

[0068] An AlGaN barrier layer with graded composition is epitaxially grown on the AlGaN lower barrier layer. NH3 and H2, trimethylaluminum (TMAl) and trimethylgallium (TMGa) are introduced by unintentional doping. The gradient layer is adjusted from Al to H2 by tilting the mass flow controllers of TMAl and TMGa. 0.26 Ga 0.74 N, to about Al 0.21 Ga 0.79 N ends, and the growth temperature is 1080℃.

[0069] A 5nm-thick AlGaN upper barrier layer with an Al composition of 21% is then epitaxially grown on the AlGaN reverse-graded-composition layer. This layer is unintentionally doped with NH3, H2, aluminum, and gallium sources at a temperature of 900-920°C. The AlGaN upper barrier layer, the AlGaN reverse-graded-composition barrier layer, the AlGaN lower barrier layer, and the GaN channel layer form a unique heterojunction.

[0070] A first SiN passivation layer is deposited on the AlGaN upper barrier layer using plasma-enhanced chemical vapor deposition (PECVD) at a temperature of 250°C. SiH₄ is ​​used as the silicon source, and N₂O as the nitrogen source. The SiH₄ flow rate is set to 150 sccm, the N₂O flow rate is set to 800 sccm, and the power is set to 20-30 W. After the first SiN passivation layer is grown, its parameters can be adjusted using equipment such as an ellipsometer to ensure that various parameters, such as leakage current, meet specified requirements.

[0071] Then, a wet etching method is used to etch a growth window for the P-GaN cap layer on the first SiN passivation layer. Preferably, the length of the P-GaN cap layer window is 100 nm.

[0072] Then, a P-GaN layer is epitaxially grown in the P-GaN cap layer window, with a thickness of 50 nm, the same as the first SiN passivation layer. The doping concentration of P-type GaN is 3×10 17 cm -3 The growth temperature range is 1000~1100℃ and the pressure range is 5300~5500Pa.

[0073] Continue to use the wet etching method to etch the growth windows of the undoped GaN cap layer on both sides of the P-GaN cap layer. Preferably, the length of the undoped GaN cap layer windows on both sides is 70 nm.

[0074] Then, an undoped GaN cap layer is epitaxially grown in the undoped GaN cap layer window, and the thickness thereof is the same as that of the first SiN passivation layer, that is, 50 nm.

[0075] A second SiN passivation layer is then deposited over the first SiN passivation layer, undoped GaN cap layer, and P-GaN cap layer using plasma-enhanced chemical vapor deposition (PECVD). The thickness of the passivation layer is set to 60-100 nm at a temperature of 250°C. SiH₄ is ​​used as the silicon source, and N₂O as the nitrogen source. The SiH₄ flow rate is set to 150 sccm, the N₂O flow rate is set to 800 sccm, and the power is set to 20-30 W. After the second SiN passivation layer is grown, its parameters can be adjusted using equipment such as an ellipsometer to ensure that its leakage current and other parameters meet specified requirements.

[0076] Next, a photoresist layer is spin-coated and the gate foot growth window is formed by photolithography. Electron beam evaporation is used to deposit Ni / Au metal. The metal is then stripped by immersion in acetone solution, rinsed with ultrapure water for 2 minutes, and dried with nitrogen, forming a gate foot contact electrode with a thickness of 60 to 100 nm.

[0077] Continue to use the wet etching method to etch air cavity windows on both sides of the gate contact electrode. Preferably, the window length is 0.1 μm and the thickness is 110-150 nm. Spin-coat photoresist in the air cavity window to fill the window.

[0078] Continue to spin-coat the photoresist layer and photolithography to form the growth window of the source and drain. Use electron beam evaporation process to deposit metal Ti / Al / Ni / Au with a vacuum degree of less than 2×10 -6 Pa, with an evaporation rate of 2 to 3 angstroms per second and a power range of 150 to 200 W. The epitaxial wafer is then immersed in an acetone solution for 10 to 20 minutes to remove the metal other than the source and drain electrodes. This is followed by annealing at 800 to 900°C for 30 to 40 seconds, using nitrogen as the protective gas. Preferably, the epitaxial wafer is subjected to an ohmic contact annealing in ammonia at 850°C for 30 seconds to form source and drain electrodes with a thickness of 110 to 150 nm.

[0079] Finally, a photoresist layer was spin-coated and the gate cap growth window was formed using photolithography. Ni / Au metal was deposited using electron beam evaporation. The metal was then stripped by immersion in acetone, rinsed with ultrapure water for 2 minutes, and dried with ammonia, forming a 100nm thick gate cap contact electrode.

[0080] Finally, the photoresist in the air cavity window is removed to form the air cavity structure, and the Figure 1 The device shown was fabricated.

[0081] Figure 2 This is a schematic diagram of the cross-sectional structure of a traditional P-GaN HEMT structure. Figure 2 As shown in the figure, the traditional P-GaN HEMT structure includes, from bottom to top, a substrate, an AlGaN buffer layer, a GaN channel layer, a barrier layer, a source, a drain and a P-GaN cap layer located on the barrier layer, the P-GaN cap layer located between the source and the drain, a gate located on the P-GaN cap layer, located between the source and the P-GaN cap layer, and a passivation layer located between the drain and the P-GaN cap layer. The barrier layer is an AlGaN barrier layer with a fixed composition of 21%.

[0082] like Figure 3As shown, compared with conventional P-GaN HEMT devices, the drain current density of the cap layer structure and reverse gradient barrier enhanced HEMT device of the present invention is increased by 70% from 0.445 A / mm to 0.758 A / mm, and the peak transconductance is increased by 15%. The transconductance linearity is also higher. This is because, on the one hand, the T-type gate of the present invention reduces the gate foot length and gate contact resistance. The cap layer structure and T-type gate reduce the gate contact resistance, reduce the depletion range of the channel layer, and improve the DC characteristics of the device. On the other hand, the cap layer structure of the present invention further reduces the range of the depletion region below the gate, thereby improving the DC characteristics of the device. In addition, the undoped GaN cap layer bears part of the gate voltage, which improves the transconductance linearity of the device to a certain extent. The threshold voltage is reduced from 1.31 V to 1.05 V, which still remains at a high level.

[0083] like Figure 4 and Figure 5 As shown in the figure, compared with the traditional P-GaN HEMT device, the gate leakage current density of the cap layer structure and the reverse gradient barrier enhanced AlGaN / GaN HEMT device of the present invention is gs The voltage drop is reduced by 1 to 5 orders of magnitude over the range of -2 to 7V, demonstrating excellent gate leakage current suppression. This is because the insertion of the AlGaN reverse-composition graded barrier layer raises a portion of the barrier layer's conduction band, reducing the electron concentration near the gate, thereby reducing gate leakage current. Compared to conventional P-GaN HEMT devices, the cap layer structure and reverse-gradient barrier-enhanced AlGaN / GaN HEMT devices of the present invention exhibit an overall decrease in electron concentration in the barrier layer, an overall increase in electron concentration in the channel layer, and a continuous decrease in electron concentration in the buffer layer with increasing depth. This is because the T-shaped gate and cap layer structure of the present invention reduces gate contact resistance and narrows the channel depletion range, resulting in an overall increase in electron concentration in the channel layer and improved DC characteristics of the device. The negative polarization effect of the reverse-gradient barrier generates holes that neutralize the electron concentration in the barrier layer, reducing the electron concentration there and lowering gate and buffer layer leakage current.

[0084] like Figure 6 As shown, the introduction of the undoped GaN cap layer in the cap layer structure and the reverse gradient barrier enhanced RF device of the present invention produces a partial potential drop, requiring a higher voltage to turn on the device. The introduction of the undoped GaN cap layer bears part of the gate voltage, widening the swing range of the gate voltage.

[0085] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A partial P-GaN cap layer and reverse gradient barrier enhanced radio frequency HEMT device, characterized in that: It includes: substrate; an AlGaN buffer layer located on the substrate; a GaN channel layer located on the AlGaN buffer layer; an AlGaN barrier layer located on the GaN channel layer, the AlGaN barrier layer sequentially comprising an AlGaN lower barrier layer, an AlGaN reverse composition graded barrier layer, and an AlGaN upper barrier layer along the growth direction, wherein the AlGaN upper barrier layer has an Al composition of 21%, the AlGaN lower barrier layer has an Al composition of 26%, and the composition of the AlGaN reverse composition graded barrier layer changes linearly from 26% to 21% along the growth direction; a cap layer structure located on the AlGaN barrier layer, the cap layer structure consisting of a P-GaN cap layer and a first undoped GaN cap layer and a second undoped GaN cap layer of equal thickness, the first undoped GaN cap layer and the second undoped GaN cap layer being located on either side of the P-GaN cap layer, respectively, the width of the first undoped GaN cap layer being equal to the width of the second undoped GaN cap layer, and the width of the P-GaN cap layer being greater than the widths of the first and second undoped GaN cap layers; a Schottky contact gate electrode located on the cap layer structure, the Schottky contact gate electrode consisting of a gate foot contact electrode and a gate cap contact electrode, the gate foot contact electrode being in contact with the cap layer structure; An air dielectric is provided between the gate cap contact electrode and the AlGaN upper barrier layer, and the air dielectric is located on both sides of the cap layer structure and the gate foot contact electrode; an ohmic contact source electrode and a drain electrode located on the AlGaN barrier layer; A passivation layer is filled between the Schottky contact gate electrode and the source electrode and the drain electrode, and is located on the surface of the AlGaN upper barrier layer and on both sides of the air medium.

2. The enhanced radio frequency HEMT device according to claim 1, wherein: The width of the first and second undoped GaN cap layers is 50-80 nm, and the width of the P-GaN cap layer is 80-150 nm.

3. The enhanced radio frequency HEMT device according to claim 2, wherein: The passivation layer consists of a first passivation layer and a second passivation layer, wherein the first passivation layer is located on the surface of the AlGaN upper barrier layer and has a thickness equal to that of the cap layer structure; The second passivation layer is located on the first passivation layer, and the thickness of the second passivation layer is equal to the thickness of the gate foot contact electrode.

4. The enhanced radio frequency HEMT device according to any one of claims 1 to 3, characterized in that: The doping concentration of the P-GaN cap layer is 3×10 17 cm -3 ; The P-GaN cap layer and the undoped GaN cap layer are formed by an epitaxial growth process, or the P-GaN cap layer is formed in the undoped GaN cap layer by an ion implantation process.

5. The enhancement mode RF HEMT device according to claim 4, wherein: The thickness of the GaN channel layer is 35 nm; the thickness of the AlGaN barrier layer is 15 nm; the thickness of the AlGaN upper barrier layer is 5 nm; and the thickness of the AlGaN lower barrier layer is 5 nm.

6. The enhanced radio frequency HEMT device according to claim 4, wherein: The thickness of the AlGaN reverse composition graded barrier layer is 5 nm.

7. A method for preparing a partial P-GaN cap layer and a reverse gradient barrier enhanced radio frequency HEMT device, characterized in that: The steps include: growing an AlGaN buffer layer on the substrate; epitaxially growing a GaN channel layer on the AlGaN buffer layer; epitaxially growing an AlGaN lower barrier layer with an Al composition of 26% on the GaN channel layer; Epitaxially growing an AlGaN reverse composition graded barrier layer on the AlGaN lower barrier layer, wherein the composition of the AlGaN reverse composition graded barrier layer changes linearly from 26% to 21% along the growth direction; epitaxially growing an AlGaN upper barrier layer with an Al composition of 21% on the AlGaN reverse composition graded barrier layer; Depositing a first passivation layer on the AlGaN upper barrier layer using a plasma enhanced chemical vapor deposition (PECVD) process; forming a growth window for a cap layer structure in the first passivation layer, and forming a P-GaN cap layer and first and second undoped GaN cap layers in the growth window, wherein the first undoped GaN cap layer and the second undoped GaN cap layer are respectively located on either side of the P-GaN cap layer, the width of the first undoped GaN cap layer is equal to the width of the second undoped GaN cap layer, and the width of the P-GaN cap layer is greater than the widths of the first and second undoped GaN cap layers; Depositing a second passivation layer on the cap layer structure and the first passivation layer, and forming a gate foot contact electrode window in the second passivation layer; Depositing metal in the gate foot contact electrode window to form a gate foot contact electrode; Etching the second passivation layer and the first passivation layer to form air cavity windows located on both sides of the gate foot contact electrode and the cap layer structure, and depositing photoresist in the air cavity windows; forming a source electrode, a drain electrode, and a gate cap contact electrode, wherein the gate cap contact electrode and the gate foot contact electrode constitute a T-type gate structure; The photoresist is removed to form an air cavity containing an air medium. The air cavity is located between the gate cap contact electrode and the AlGaN upper barrier layer and on both sides of the cap layer structure and the gate foot contact electrode.

8. The method for preparing an enhanced radio frequency HEMT device according to claim 7, wherein: A growth window for a cap layer structure is formed in the first passivation layer. In the step of forming a P-GaN cap layer and an undoped GaN cap layer in the growth window, the steps include etching the first passivation layer to form a P-GaN cap layer window, epitaxially growing a P-GaN cap layer in the window, and then etching both sides of the P-GaN cap layer to form an undoped GaN window, and epitaxially growing an undoped GaN cap layer in the window.

9. The method for preparing an enhanced radio frequency HEMT device according to claim 7, wherein: A growth window for a cap layer structure is formed in the first passivation layer. The steps of forming a P-GaN cap layer and an undoped GaN cap layer in the growth window include etching the first passivation layer to form the growth window for the cap layer structure, epitaxially growing an undoped GaN cap layer in the growth window, and then using an ion implantation process to implant ions into the undoped GaN cap layer to form a P-GaN cap layer.

10. The method for preparing an enhanced radio frequency HEMT device according to any one of claims 7 to 9, characterized in that: The width of the first and second undoped GaN cap layers is 50 to 80 nm, and the width of the P-GaN cap layer is 80 to 150 nm; The thickness of the cap layer structure is equal to the thickness of the first passivation layer; The thickness of the AlGaN reverse composition graded barrier layer is 5 nm; The substrate is a semi-insulating 4H-SiC substrate or a 6H-SiC substrate, and its thickness is reduced to 50-100 μm.

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