Au-free Ohmic Contact Electrodes for GaN-based HEMTs Based on Ti / Al / Ni / Cu and Preparation Method Thereof

Abstract

The present invention discloses a gold-free ohmic contact electrode based on Ti / Al / Ni / Cu for GaN-based HEMT and a preparation method thereof. The electrode includes a first metal layer Ti, a second metal layer Al, a third metal Ni, and a fourth metal layer Cu arranged in sequence from bottom to top in the etched regions at both ends of the upper surface of the epitaxial layer of the GaN-based HEMT. The AlGaN / GaN epitaxy includes a substrate, a GaN buffer layer, a GaN channel layer, and an AlGaN barrier layer stacked in sequence from bottom to top. The metals are deposited by electron beam evaporation, and the third metal layer Ni of the electrode is deposited obliquely by electron beam evaporation. The third metal layer Ni of the electrode is deposited on the surface of the second metal layer Al and the side walls of the SiO2 support layer. The third metal layer Ni of the electrode wraps the fourth metal layer Cu from the bottom and side. The present invention uses Cu instead of metal Au as the cap layer and combines it with a low-temperature annealing process, which is beneficial to reducing the diffusion of Cu. Compared with the traditional ohmic contact electrode metal system, the ohmic contact electrode of the present invention helps to improve the conductivity of the electrode.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor devices, and particularly to a method for preparing a gold-free ohmic contact electrode for GaN-based HEMT based on Ti / Al / Ni / Cu. Background Art

[0002] AlGaN / GaN high electron mobility transistors (HEMTs) have attracted much attention due to their excellent characteristics such as high breakdown field strength, high electron mobility, and high electron saturation velocity. In the manufacture of HEMT devices, a multi-layer metal structure with Au as the cap layer is often used, and an ohmic contact is formed through high-temperature annealing. Au will form deep-level impurities in Si, contaminating the CMOS process line. However, the low-temperature gold-free ohmic process uses a metal compatible with the Si-CMOS process line as the cap layer on the one hand, avoiding the contamination problem brought by Au, and on the other hand, the low-temperature annealing reduces the surface roughness of the electrode, which helps to improve the breakdown voltage of the device. One of the common methods to achieve the low-temperature gold-free process is to first reduce the thickness of the barrier layer, and then prepare the source and drain electrodes and anneal them to form an ohmic contact (FIRRINCIELI A, DE JAEGER B, YOU S, et al. Au-free low temperature ohmic contacts for AlGaN / GaN power devices on 200mm Si substrates[J]).

[0003] Electron beam evaporation is a physical vapor deposition (PVD) technique that directly heats and evaporates materials using an electron beam under vacuum and transports the evaporated materials to the substrate to form a thin film. By controlling the rotation rate and tilt angle of the substrate during electron beam evaporation coating, metals can be deposited on the sidewalls of the mask layer.

[0004] Copper (Cu) has excellent thermal conductivity and plastic toughness, and Cu is often used as a metal electrode and interconnection line in the Si-CMOS process line. However, Cu and Si are prone to react with each other. In addition, during the annealing process, Cu will diffuse, diffuse into the AlGaN / GaN layer to form a Ga-Cu alloy, induce the formation of defects similar to vacancies, and increase the p-type doping on the GaN surface, inhibiting the formation of nitrogen vacancies on the GaN surface. Summary of the Invention

[0005] To solve the problems existing in the prior art, the present invention proposes a method for preparing a gold-free ohmic contact electrode for GaN-based HEMT based on Ti / Al / Ni / Cu. By using electron beam evaporation with inclined deposition, a third metal layer Ni is deposited on the surface of the second metal layer Al and the sidewalls of the SiO2 support layer, realizing that the third metal layer Ni wraps the fourth metal layer Cu from the bottom and side. At the same time, combined with the low-temperature annealing process, it is beneficial to reduce the diffusion of Cu. Using Cu with good conductivity instead of Au as the cap layer metal helps to improve the conductivity of the electrode and reduce the manufacturing cost of GaN-based HEMT devices.

[0006] The object of the present invention is achieved by at least one of the following technical solutions.

[0007] The present invention discloses a gold-free ohmic contact electrode for GaN-based HEMT based on Ti / Al / Ni / Cu. The electrode includes a first metal layer Ti, a second metal layer Al, a third metal Ni, and a fourth metal layer Cu arranged in sequence from bottom to top in the etched regions at both ends of the upper surface of the epitaxial layer of the GaN-based HEMT. The fourth metal layer Cu is located inside the third metal layer Ni. The AlGaN / GaN epitaxial layer includes a substrate, a GaN buffer layer, a GaN channel layer, and an AlGaN barrier layer stacked in sequence from bottom to top. The SiO2 support layer is above the AlGaN barrier layer.

[0008] Further, the metals of the electrode are deposited by electron beam evaporation, and the third metal layer Ni of the electrode is deposited by electron beam evaporation with inclined deposition.

[0009] Further, the third metal layer Ni of the electrode is deposited on the surface of the second metal layer Al 6 and the sidewalls of the SiO2 support layer, so that the third metal layer Ni wraps the fourth metal layer Cu from the bottom and side.

[0010] Further, the thickness of the first metal layer Ti is 1-20 nm, the thickness of the second metal layer Al is 20-50 nm, the thickness of the third metal layer Ni is 10-30 nm, and the thickness of the fourth metal layer Cu is 60-100 nm.

[0011] Further, the thickness of the SiO2 support layer is 50-150 nm.

[0012] Further, the third metal layer Ni of the electrode wraps the fourth metal layer Cu from the bottom and side.

[0013] Further, the substrate rotation speed of the electron beam evaporation with inclined deposition is 0-30 revolutions per minute and the inclined angle is 15-45°.

[0014] A method for preparing a gold-free ohmic contact electrode for GaN-based HEMT based on Ti / Al / Ni / Cu includes the following steps:

[0015] S1. Deposit a layer of SiO2 on the AlGaN / GaN epitaxial layer, define the etching window by photolithography, etch away the SiO2 in the electrode region and part of the AlGaN barrier layer, and after the etching is completed, perform surface treatment on the etched region with an acid-base solution to remove the etching residues and oxides;

[0016] S2. Prepare the first metal layer Ti, the second metal layer Al, the third metal layer Ni, and the fourth metal layer Cu of the electrode and perform annealing to form an ohmic contact, obtaining a gold-free ohmic contact electrode for GaN-based HEMT.

[0017] Furthermore, in step S2, the gas atmosphere for annealing is high-purity nitrogen, and the annealing temperature and annealing time are 450 - 650 °C and 30 s - 10 min respectively.

[0018] Compared with the prior art, the present invention has at least the following beneficial effects:

[0019] The present invention uses electron beam evaporation and inclined deposition to deposit the third metal layer Ni on the surface of the second metal layer Al and the sidewalls of the SiO2 support layer, realizing that the third metal layer Ni wraps the fourth metal layer Cu from the bottom and side. Ni has a high melting point and low atomic diffusion ability, and has good diffusion barrier characteristics. Using Ni as a barrier layer to wrap Cu from the bottom and side can not only prevent the reaction between Cu and Si, but also reduce the diffusion of Cu and avoid the reaction between Cu and GaN. At the same time, combined with the low-temperature annealing process, it is beneficial to reduce the diffusion of Cu. Using Cu with good conductivity to replace Au as the cap layer metal helps to improve the conductivity of the electrode and reduce the manufacturing cost of GaN-based HEMT devices. Brief Description of the Drawings

[0020] Figure 1 It is a schematic diagram of the etched region formed after etching and surface treatment in the embodiment of the present invention;

[0021] Figure 2 It is a schematic diagram of the third metal layer Ni deposited by electron beam evaporation and inclined deposition in the embodiment of the present invention;

[0022] Figure 3 It is a schematic diagram of the structure of the gold-free ohmic contact electrode for GaN-based HEMT in the embodiment of the present invention;

[0023] Figure 4 It is a schematic diagram of the structure of the SiO2 support layer in the embodiment of the present invention;

[0024] Figure 5 It is a schematic diagram of the current-voltage characteristic curves of electrodes with different spacings in the embodiment of the present invention;

[0025] As shown in the figure: 1 - substrate, 2 - GaN buffer layer, 3 - GaN channel layer, 4 - AlGaN barrier layer, 5 - first metal layer Ti, 6 - second metal layer Al, 7 - third metal layer Ni, 8 - fourth metal layer Cu, 9 - SiO2 support layer, 10 - photoresist layer. Detailed implementation mode

[0026] The present invention will be further described below in conjunction with the drawings and embodiments, but the implementation modes of the present invention are not limited thereto; it should be noted that for the processes or process parameters not specifically described in detail below, those skilled in the art can refer to the prior art to implement them.

[0027] Embodiment 1:

[0028] Please refer to Figures 1-4 , the Au - free ohmic contact electrode of GaN - based HEMT based on Ti / Al / Ni / Cu includes a first metal layer Ti 5, a second metal layer Al 6, a third metal layer Ni 7, and a fourth metal layer Cu 8 arranged in sequence from bottom to top in the etching regions at both ends of the upper surface of the AlGaN / GaN epitaxial layer;

[0029] As Figure 3 shown, the AlGaN / GaN epitaxial layer includes a substrate 1, a GaN buffer layer 2, a GaN channel layer 3, and an AlGaN barrier layer 4 stacked in sequence from bottom to top. An SiO2 support layer 9 is provided above the AlGaN barrier layer 4, and the SiO2 support layer 9 is located outside the electrode. In order to form a "U" - shaped third metal layer Ni when the third metal layer Ni 7 is deposited by electron beam evaporation at an inclined angle, as Figure 4 shown, a support layer on the side needs to be provided. In addition, when the electrode region is defined by the photoresist, due to various factors in the photolithography process, it is inconvenient to control the inclination angle of the photoresist sidewall, and during the stripping process, the metal deposited on the photoresist sidewall will also be stripped off. Therefore, a layer of SiO2 is inserted between the photoresist and the AlGaN / GaN epitaxial layer as a support layer. By etching SiO2, a steep inclination angle can be obtained, and the metal deposited on the sidewall of SiO2 will not be stripped off. The photoresist layer 10 is a mask for etching SiO2 and depositing metal.

[0030] Among them, the first metal layer Ti 5 is arranged in the etching regions at both ends of the upper surface of the AlGaN / GaN epitaxial layer, the second metal layer Al 6 is arranged on the first metal layer Ti 5, and the third metal layer Ni 7 wraps the bottom and side surfaces of the fourth metal layer Cu 8.

[0031] In this embodiment, the four - layer metal is deposited by electron beam evaporation. Among them, the third metal layer Ni 7 is deposited by electron beam evaporation at an inclined angle, and the deposition effect is as Figure 2 shown.

[0032] In this embodiment, the substrate rotation speed during electron beam evaporation and inclined deposition is 20 revolutions per minute and the inclination angle is 45°.

[0033] The method for preparing the Au-free ohmic contact electrode of GaN-based HEMT based on Ti / Al / Ni / Cu includes the following steps:

[0034] S1. Prepare a layer of SiO2 on the AlGaN / GaN epitaxial layer, define an etching window by photolithography, etch away the SiO2 in the electrode region and part of the AlGaN barrier layer. After the etching is completed, perform surface treatment on the etched region with an acid-base solution to remove etching residues and oxides, as Figure 1 shown, to obtain the AlGaN / GaN epitaxial layer and the SiO2 support layer 9 disposed above the AlGaN barrier layer 4;

[0035] S2. Prepare the first metal layer Ti 5, the second metal layer Al 6, the third metal layer Ni 7, and the fourth metal layer Cu 8 of the electrode and perform annealing to form an ohmic contact, obtaining the Au-free ohmic contact electrode of GaN-based HEMT, as Figure 3 shown.

[0036] In this embodiment, the thickness of the SiO2 support layer 9 is 100 nm, the thickness of the first metal layer Ti 5 is 10 nm, the thickness of the second metal layer Al 6 is 35 nm, the thickness of the third metal layer Ni 7 is 20 nm, and the thickness of the fourth metal layer Cu 8 is 80 nm.

[0037] In this embodiment, the gas atmosphere for annealing is high-purity nitrogen, and the annealing temperature and annealing time are 600 °C and 5 min, respectively.

[0038] The electrode and its preparation method provided by the embodiment of the present invention adopt electron beam evaporation and inclined deposition, deposit the third metal layer Ni on the surface of the second metal layer Al and the side wall of the SiO2 support layer, realizing that the third metal layer Ni wraps the fourth metal layer Cu from the bottom and side. At the same time, combined with the low-temperature annealing process, it is beneficial to reduce the diffusion of Cu. Using Cu with good conductivity to replace Au as the cap layer metal helps to improve the conductivity of the electrode and reduce the manufacturing cost of GaN-based HEMT devices.

[0039] The current-voltage characteristic curves of the electrodes with different spacings in this embodiment are as Figure 5 shown. The electrode size of the TLM pattern is 50 μm × 50 μm, and the electrode spacings are 10 μm, 20 μm, 30 μm, 40 μm, and 50 μm. The contact resistance of this embodiment is 0.67 Ω·mm, obtaining a good ohmic contact and having good performance.

[0040] Example 2:

[0041] The Au - free ohmic contact electrode for GaN - based HEMT based on Ti / Al / Ni / Cu includes a first metal layer Ti 5, a second metal layer Al 6, a third metal layer Ni 7, and a fourth metal layer Cu 8 arranged in sequence from bottom to top at both ends of the etched area on the upper surface of the AlGaN / GaN epitaxial layer; an SiO2 support layer 9 is further provided above the AlGaN barrier layer 4, and the SiO2 support layer 9 is located outside the electrode. In order to form a "U" - shaped third metal layer Ni when the third metal layer Ni 7 is deposited by inclined electron beam evaporation, as Figure 4 shown, a support layer located on the side is required. In addition, when the photoresist defines the electrode area, due to various factors in the photolithography process, it is inconvenient to control the inclination angle of the photoresist sidewall, and during the stripping process, the metal deposited on the photoresist sidewall will also be stripped off. Therefore, a layer of SiO2 is inserted between the photoresist and the AlGaN / GaN epitaxial layer as a support layer. By etching SiO2, a steep inclination angle can be obtained, and the metal deposited on the sidewall of SiO2 will not be stripped off. The photoresist layer 10 is a mask for etching SiO2 and depositing metal.

[0042] As Figure 3 shown, the AlGaN / GaN epitaxial layer includes a substrate 1, a GaN buffer layer 2, a GaN channel layer 3, and an AlGaN barrier layer 4 stacked in sequence from bottom to top.

[0043] Among them, the first metal layer Ti 5 is disposed at both ends of the etched area on the upper surface of the AlGaN / GaN epitaxial layer, the second metal layer Al 6 is disposed on the first metal layer Ti 5, and the third metal layer Ni 7 wraps the bottom surface and side surface of the fourth metal layer Cu 8.

[0044] In this embodiment, the fourth metal layer Cu 8 is deposited by electron beam evaporation, and the third metal layer Ni 7 is deposited by inclined electron beam evaporation. For the deposition effect, please refer to Figure 2 .

[0045] In this embodiment, the substrate rotation speed for inclined electron beam evaporation is 30 revolutions per minute and the inclination angle is 45°.

[0046] The preparation method of the aforementioned Au - free ohmic contact electrode for GaN - based HEMT based on Ti / Al / Ni / Cu includes the following steps:

[0047] S1. Prepare a layer of SiO2 on the AlGaN / GaN epitaxial layer, define the etching window by photolithography, etch away the SiO2 in the electrode area and part of the AlGaN barrier layer, and after the etching is completed, perform surface treatment on the etched area with an acid - base solution to remove the etching residues and oxides. Please refer to Figure 1, an AlGaN / GaN epitaxial layer and an SiO2 support layer 9 disposed above the AlGaN barrier layer 4 are obtained;

[0048] S2. Prepare the first metal layer Ti 5, the second metal layer Al 6, the third metal layer Ni 7, and the fourth metal layer Cu 8 of the electrode and perform annealing to form an ohmic contact, obtaining a gold-free ohmic contact electrode for GaN-based HEMT. Please refer to Figure 3 .

[0049] In this embodiment, the thickness of the SiO2 support layer is 150 nm, the thickness of the first metal layer Ti5 is 20 nm, the thickness of the second metal layer Al6 is 50 nm, the thickness of the third metal layer Ni 7 is 30 nm, and the thickness of the fourth metal layer Cu8 is 100 nm.

[0050] In this embodiment, the gas atmosphere for annealing is nitrogen, and the annealing temperature and annealing time are 650 °C and 10 min, respectively.

[0051] The contact characteristics of this embodiment are similar to those of Embodiment 1 and will not be elaborated here.

[0052] Embodiment 3:

[0053] The gold-free ohmic contact electrode for GaN-based HEMT based on Ti / Al / Ni / Cu includes a first metal layer Ti 5, a second metal layer Al 6, a third metal Ni 7, and a fourth metal layer Cu 8 arranged in sequence from bottom to top in the etched areas at both ends of the upper surface of the AlGaN / GaN epitaxial layer; an SiO2 support layer 9 is further disposed above the AlGaN barrier layer 4, and the SiO2 support layer 9 is located outside the electrode.

[0054] As Figure 3 shown, the AlGaN / GaN epitaxial layer includes a substrate 1, a GaN buffer layer 2, a GaN channel layer 3, and an AlGaN barrier layer 4 stacked in sequence from bottom to top.

[0055] Among them, the first metal layer Ti 5 is disposed in the etched areas at both ends of the upper surface of the AlGaN / GaN epitaxial layer, the second metal layer Al 6 is disposed on the first metal layer Ti 5, and the third metal layer Ni 7 wraps the bottom and side surfaces of the fourth metal layer Cu 8.

[0056] In this embodiment, the fourth metal layer Cu 8 is deposited by electron beam evaporation. Among them, the third metal layer Ni 7 is deposited by inclined electron beam evaporation. For the deposition effect, please refer to Figure 2 .

[0057] In this embodiment, the substrate rotation speed for inclined electron beam evaporation is 0 revolutions per minute and the inclination angle is 15°.

[0058] The preparation method of the gold-free ohmic contact electrode for GaN-based HEMT based on Ti / Al / Ni / Cu includes the following steps:

[0059] S1. Prepare a layer of SiO2 on the AlGaN / GaN epitaxial layer, define an etching window by lithography, etch away the SiO2 in the electrode area and part of the AlGaN barrier layer. After the etching is completed, perform surface treatment on the etched area with an acid-base solution to remove etching residues and oxides. Please refer to Figure 1 , to obtain the AlGaN / GaN epitaxial layer and the SiO2 support layer 9 disposed above the AlGaN barrier layer 4;

[0060] S2. Prepare the first metal layer Ti 5, the second metal layer Al 6, the third metal layer Ni 7 and the fourth metal layer Cu 8 of the electrode and perform annealing to form an ohmic contact, so as to obtain the gold-free ohmic contact electrode for GaN-based HEMT. Please refer to Figure 3 .

[0061] In this embodiment, the thickness of the SiO2 support layer is 50 nm, the thickness of the first metal layer Ti5 is 1 nm, the thickness of the second metal layer Al6 is 20 nm, the thickness of the third metal layer Ni 7 is 10 nm, and the thickness of the fourth metal layer Cu8 is 60 nm.

[0062] In this embodiment, the gas atmosphere for annealing is nitrogen, and the annealing temperature and annealing time are 450 °C and 10 min respectively.

[0063] The contact characteristics of this embodiment are similar to those of Embodiment 1 and will not be elaborated here.

[0064] Embodiment 4:

[0065] The gold-free ohmic contact electrode for GaN-based HEMT based on Ti / Al / Ni / Cu includes a first metal layer Ti 5, a second metal layer Al 6, a third metal Ni 7 and a fourth metal layer Cu 8 arranged in sequence from bottom to top at both ends of the etched area on the upper surface of the AlGaN / GaN epitaxial layer; a SiO2 support layer 9 is further disposed above the AlGaN barrier layer 4, and the SiO2 support layer 9 is located outside the electrode.

[0066] As Figure 3 shown, the AlGaN / GaN epitaxial layer includes a substrate 1, a GaN buffer layer 2, a GaN channel layer 3 and an AlGaN barrier layer 4 stacked in sequence from bottom to top.

[0067] Among them, the first metal layer Ti 5 is disposed at both ends of the etched area on the upper surface of the AlGaN / GaN epitaxial layer, the second metal layer Al 6 is disposed on the first metal layer Ti 5, and the third metal layer Ni 7 wraps the bottom surface and side surface of the fourth metal layer Cu 8.

[0068] In this embodiment, the fourth metal layer Cu 8 is deposited by electron beam evaporation, and the third metal layer Ni 7 is deposited by inclined electron beam evaporation. For the deposition effect, please refer to Figure 2 .

[0069] In this embodiment, the substrate rotation speed for inclined electron beam evaporation is 20 revolutions per minute and the inclination angle is 30°.

[0070] The foregoing method for preparing a gold-free ohmic contact electrode for a GaN-based HEMT based on Ti / Al / Ni / Cu includes the following steps:

[0071] S1. Prepare a layer of SiO2 on the AlGaN / GaN epitaxial layer, define an etching window by lithography, etch away the SiO2 in the electrode region and part of the AlGaN barrier layer. After the etching is completed, perform surface treatment on the etched region with an acid-base solution to remove etching residues and oxides. Please refer to Figure 1 , to obtain the AlGaN / GaN epitaxial layer and the SiO2 support layer 9 provided above the AlGaN barrier layer 4;

[0072] S2. Prepare the first metal layer Ti 5, the second metal layer Al 6, the third metal layer Ni 7, and the fourth metal layer Cu 8 of the electrode and perform annealing to form an ohmic contact, thereby obtaining a gold-free ohmic contact electrode for a GaN-based HEMT. Please refer to Figure 3 .

[0073] In this embodiment, the thickness of the SiO2 support layer is 100 nm, the thickness of the first metal layer Ti5 is 10 nm, the thickness of the second metal layer Al 6 is 40 nm, the thickness of the third metal layer Ni 7 is 30 nm, and the thickness of the fourth metal layer Cu 8 is 60 nm.

[0074] In this embodiment, the gas atmosphere for annealing is nitrogen, and the annealing temperature and annealing time are 600 °C and 30 s, respectively.

[0075] The contact characteristics of this embodiment are similar to those of Embodiment 1 and will not be elaborated here.

[0076] The above embodiments are only relatively preferred implementation manners of the present invention, and are only used to explain the present invention rather than limit the present invention. Any changes, substitutions, modifications, etc. made by those skilled in the art without departing from the spirit essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A gold-free ohmic contact electrode for GaN-based HEMT based on Ti / Al / Ni / Cu, characterized in that, It includes a first metal layer Ti(5), a second metal layer Al(6), a third metal layer Ni(7), and a fourth metal layer Cu(8) arranged in sequence from bottom to top at both ends of the upper surface of the epitaxial layer of the GaN-based HEMT. The fourth metal layer Cu(8) is located inside the third metal layer Ni(7). The AlGaN / GaN epitaxial layer includes a substrate (1), a GaN buffer layer (2), a GaN channel layer (3), and an AlGaN barrier layer (4) stacked in sequence from bottom to top. An SiO2 support layer (9) is also provided above the AlGaN barrier layer (4). The metal layer of the electrode is deposited by electron beam evaporation. The third metal layer Ni(7) of the electrode is deposited by inclined electron beam evaporation.

2. The GaN-based HEMT gold-free ohmic contact electrode based on Ti / Al / Ni / Cu according to claim 1, wherein The substrate rotation speed of the inclined electron beam evaporation deposition is 0 - 30 revolutions per minute.

3. The GaN-based HEMT gold-free ohmic contact electrode based on Ti / Al / Ni / Cu according to claim 1, characterized in that, The inclination angle of the inclined electron beam evaporation deposition is 15 - 45°.

4. The GaN-based HEMT gold-free ohmic contact electrode based on Ti / Al / Ni / Cu according to claim 1, wherein, The third metal layer Ni(7) is deposited on the surface of the second metal layer Al(6) and the side wall of the SiO2 support layer (9).

5. The GaN-based HEMT gold-free ohmic contact electrode based on Ti / Al / Ni / Cu according to claim 1, wherein The thickness of the first metal layer Ti(5) is 1 - 20 nm, the thickness of the second metal layer Al(6) is 20 - 50 nm, the thickness of the third metal layer Ni(7) is 10 - 30 nm, and the thickness of the fourth metal layer Cu(8) is 60 - 100 nm.

6. The GaN-based HEMT gold-free ohmic contact electrode based on Ti / Al / Ni / Cu according to claim 1, characterized in that, The thickness of the SiO2 support layer (9) is 50 - 150 nm.

7. A method for preparing a gold-free ohmic contact electrode of a GaN-based HEMT based on Ti / Al / Ni / Cu according to any one of claims 1-6, characterized in that, It includes the following steps: S1. Prepare a layer of SiO2 on the AlGaN / GaN epitaxial layer, lithographically define the etching window, etch away the SiO2 in the electrode area and part of the AlGaN barrier layer. After the etching is completed, perform surface treatment on the etched area with an acid-base solution to remove the etching residues and oxides. S2. Prepare the first metal layer Ti(5), the second metal layer Al(6), the third metal layer Ni(7), and the fourth metal layer Cu(8) of the electrode and perform annealing to form an ohmic contact to obtain a gold-free ohmic contact electrode for the GaN-based HEMT.

8. The preparation method of the Au-free ohmic contact electrode for GaN-based HEMT based on Ti / Al / Ni / Cu, characterized in that, In step S2, the gas atmosphere for annealing is high-purity nitrogen.

9. The preparation method of the Au-free ohmic contact electrode of GaN-based HEMT based on Ti / Al / Ni / Cu according to claim 7, characterized in that, In step S2, the annealing temperature and annealing time are 450 - 650 °C and 30 s - 10 min respectively.

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