The AlGaN/GaN structure can generate two-dimensional electron gas with high electron concentration (∼1 × 10¹³ cm⁻²) and high electron mobility (∼2000 cm²/V·s). Combined with the high critical electric field of GaN (∼3.3 MV/cm), AlGaN/GaN-based power electronic devices could achieve faster switching speeds, lower on-resistance and higher breakdown voltages (BVs) than Si-based devices. Among these devices, the power Schottky barrier diode (SBD) is particularly significant since it can fulfill the demands of different scenarios, including high voltage, high frequency switching, high temperature and high power density. They have extensive potential applications in consumer electronics, automotive electronics, new energy, industrial motors, and are even applied in ultra-high-voltage (UHV, > 10 kV) electronics.

In recent years, a large number of high-performance AlGaN/GaN SBDs have been reported, with the turn on voltages (V_on) of 0.2 V to 0.8 V and the BVs of 0.13 kV to 10.0 kV. Apparently, low V_on, low specific on-resistance (R_on,sp), and high BV with low leakage current (I_leakage) are essential for high-performance AlGaN/GaN power SBDs, especially for low-loss and high-efficiency power transmission systems. However, V_on, I_leakage, and BV are mainly determined by the Schottky contact, which is not easy to improve these three parameters at the same time. In previous reports, in order to achieve low V_on, anode recess structure has been widely used^1-8. The V_on can be further reduced to < 0.5 V with a low work function anode metal, such as tungsten (W)^6,8, or with an ohm-Schottky combined anode^1-4. However, it is important to note that this may also have some negative impacts on I_leakage and BV. Simply adopting a low-work-function metal as the Schottky electrode brings about some disadvantages, such as higher I_leakage and lower reverse BV. To use low-work-function metals while maintaining the high BV of the Schottky electrodes, a new electrode structure design is necessary. Various terminal structures have been demonstrated for optimizing the low V_on^9-12 and BV^13-27 of SBDs, including the 2.7-kV and 3.4-kV AlGaN/GaN SBDs with field plates reported in our previous work^28,29. In our previous researches, it remained challenging to significantly enhance the BV of AlGaN/GaN SBDs on Si substrates for UHV (> 10 kV)³⁰. By using high-quality GaN materials grown on SiC or sapphire substrates, high-performance AlGaN/GaN SBDs with BVs of > 9 kV and >10 kV were reported^{14,22,25,26,30}. Indeed, there still exist challenges for achieving high-performance AlGaN/GaN SBDs as it requires a suitable anode structure to ensure a V_on of < 0.5 V, as well as high-quality GaN material to achieve a BV > 10 kV. Researchers are currently actively exploring these key factors.

In this work, a double-barrier anode (DBA) structure composed of high work function (Pt, 5.65 eV) and low work function (Ta, 4.25 eV) was proposed. We innovatively combined the characteristics of these two metals and arranged them in an alternating tooth-like pattern. When utilizing them as the anode, the V_on will be determined by the metal Ta with a low work function, while the I_leakage and BV will be primarily influenced by the metal Pt with a high work function. In addition, supported by a high-quality carbon-doped GaN buffer layer on sapphire substrate, we successfully fabricated the AlGaN/GaN SBD with low V_on, low I_leakage and ultra-high BV, and demonstrated its potential for application in low-loss and high-efficiency power transmission systems.

The samples were grown by metal organic chemical vapor deposition on a 2-inch c-plane sapphire substrate. From the substrate, the device structure consists of a nucleation layer, a 3-μm carbon-doped GaN buffer layer, a 200-nm i-GaN channel layer, a 1-nm AlN spacer, a 20-nm Al_0.25Ga_0.75N barrier layer, a 2-nm GaN cap layer and a 50-nm in-situ SiN_x layer.

The detailed fabrication process is shown in the section of Method, and some process parameters and the main results have been reported in our previous work^28-30.

In order to achieve a low V_on while maintaining the reverse characteristic of the device, we have designed a new device structure different from the one in our previous work. It is known that the V_on and I_leakage of the SBD is determined by the Schottky barrier. Obviously, the work function of the anode metal plays a crucial role. One possible way is to combine the low V_on achieved through the use of low-work-function metal with the low I_leakage obtained from high-work-function metal. Therefore, in this work, an SBD structure with a double-barrier configuration was proposed. The diagram illustrating the cross section of the device is shown in Fig. 1; the anode consists of two metals (Ta and Pt). The Ta electrode presents a serrated shape and is covered by the Pt electrode. The Ta electrode is designed to exhibit a round arrangement so as to prevent premature damage. All SBD devices in this work are featured with a circular anode with a radius of 90 μm and an anode-cathode spacing (L_AC) of 85 μm. The transfer length (L_T) of the electrode beyond the anode recess is 3 μm. The width of each serrated Ta anode is 10 μm, while the angle (Angle_Ta) between them ranges from 11.25° to 30°. It is worth noting that our devices do not have any field-plates or any other terminal structures.

Figs. 2a and b show the forward and reverse I-V characteristics of SBDs with different anode structures, including the DBA structure (Angle_Ta = 11.25°, which is an optimized angle and will be discussed in Fig. 3 later) and the traditional anode structures with single metal Pt or Ta. The specific performances extracted from Fig. 2a and b have been presented in Table 1. This paper focuses on the intrinsic properties of the GaN material, so the BV is determined based on physical breakdown. The V_on was defined as the voltage at a forward current of 1 mA/mm, the R_on,sp was calculated under the consideration of a 3-μm transfer length (L_T) of the total electrode extension length, and the I_leakage was extracted at −5.0 kV. As shown in Table 1, the forward characteristics indicate that both the DBA device and the Ta-anode device have V_on values of 0.36 V (at 1 mA/mm), which is nearly half of that of the Pt-anode device (0.71 V at 1 mA/mm). As shown in Fig. 2c, d, to assess the consistency of device performance, we conducted tests on 40 DBA SBDs and analyzed the results. The V_on distribution range is 0.35 V to 0.42 V, the average value is 0.37 V, and the dominant value is 0.36 V. The working voltage at a forward current of 100 A/cm² is about 3.7 V.

The R_on values of ∼29 Ω·mm for the three devices were obtained in Fig. 2a. By calculating R_on,sp = R_on × (L_AC + L_T), the R_on,sp is only a little different among the three devices, ranging from 25.1 to 25.8 mΩ·cm². For the reverse characteristics, the I_leakage of the DBA device is 2.5 × 10⁻⁶ A/mm, which is one order of magnitude higher than that of the Pt-anode device, but is two orders of magnitude lower than that of the Ta-anode device. The BV can reach more than 10 kV for DBA and Pt-anode devices but only 7.1 kV for Ta-anode device. More than five DBA SBDs were measured; the BV values are all higher than 10 kV. The DBA device has both a small V_on of 0.3 V similar to Ta-anode device and a BV of 10 kV similar to Pt-anode device, demonstrating a good combination of the advantages of Ta and Pt in the DBA design. To investigate the stability of the device, V_on and R_on were measured after high voltage stress. When the SBD was immediately turned on after high reverse voltage (−8 kV) stress for 10 s, V_on was increased from 0.36 V to 0.49 V, and R_on increased from 25.1 mΩ·cm² to 95.1 mΩ·cm², as plotted in the red dotted line shown in Fig. 2a. However, this degradation is not permanent and can be fully recovered under a normal continuous forward voltage within a short period of time (a few minutes). The further optimization of the device structure is required. In order to suppress this degradation, it is necessary to fabricate the high-quality structures with not only reducing trap densities on the surface and at interfaces, but also minimizing the density of defects within the crystal.

It can be seen that the role of Ta electrodes is very important, therefore it is necessary to study the proportion of Ta in the DBA. The DBA SBD devices with different proportions of Ta electrodes were fabricated, with an angle between Ta electrodes (Angle_Ta) ranging from 0° (full Ta anode) to 90° (full Pt anode). Fig. 3a shows the forward I-V characteristics of devices with different Angle_Ta values; the V_on and R_on,sp are extracted in Fig. 3b. With the increase of Angle_Ta, the V_on of the device gradually increases from 0.36 V to 0.65 V, up to 0.71 V for the full Pt-anode SBD. R_on,sp shows a small fluctuation, generally near 26 mΩ·cm². Furthermore, as shown in Fig. 3a, kinks are present in all of the three forward I-V curves, after which the current exhibits a more rapid increase. Disparity in work function of Pt and Ta electrodes is the main reason accounting for this phenomenon, which will be elaborated on in the Device Simulation section. Based on these experimental results, the performance of DBA device with the optimized Angle_Ta = 11.25° is the best.

Based on the aforementioned experimental results, the power figure of merit (P-FOM = BV²/R_on,sp) of the optimized DBA device (Angle_Ta = 11.25°) was calculated to be 4.0 GW/cm². These performances indicate that the DBA structure can effectively reduce the V_on of the device while maintaining low I_leakage and high BV.

In order to reveal the specific mechanism of how the optimized DBA structure works, a series of three-dimensional simulation studies were carried out by using the Silvaco software, as shown in Fig. 4. In the simulation, the anode was set as a DBA structure combined with the metals Pt and Ta, mirroring the actual device. Ta was integrated within the surrounding Pt in a serrated shape. Fig. 4 shows the electric field distribution of the SBD under different voltage conditions. The cutplane was taken from the AlGaN/GaN interface. Here, we use V_AC to represent the anode-cathode voltage, V_on.Ta for the V_on of Ta anode, and V_on.Pt for the V_on of Pt anode. From Table 1, the values of V_on.Ta and V_on.Pt are 0.36 V and 0.71 V, respectively.

For n-type Schottky contact, under ideal conditions, the depletion region width W_d can be expressed as follows³¹:

where, ε_s denotes the relative dielectric constant of the semiconductor material, φ_B represents the Schottky barrier height, V_n is the built-in potential at the bottom of the conduction band relative to the Fermi level, V is the applied voltage, q is the electron charge, N_D is the donor concentration, φ_m is the anode metal work function, and χ is the semiconductor electron affinity. From this formula, it can be seen that W_d increases with the increase of φ_m of anode metal under ideal conditions. Then, the distribution of the electric field can reflect the formation and extension of the depletion region³¹, which affects the actual characteristics in the forward and reverse I-V curves as detailed in Fig. 5.

In Fig. 5, the forward I-V curve was divided into three parts. In part 1, the device is not turned on. At this time, as shown in Fig. 4a, the depletion region formed by Pt and Ta covered the entire area under the anode. In part 2, when V_on.Ta < V_AC < V_on.Pt, as shown in Fig. 4b, the depletion region under Ta has nearly disappeared. Despite the continued existence of the depletion region under Pt at this stage, electrons can pass through the channel under Ta to turn on the SBD, indicating that the V_on of the SBD is mainly determined by Ta. Likewise, as the proportion of Ta decreases (equivalent to an increase in Angle_Ta), the impact of Pt tends to be more prominent, resulting in the phenomenon showed in Fig. 3b. Once the Angle_Ta reaches 30°, the device V_on almost exhibits the characteristics of Pt. In part 3, when V_AC > V_on.Pt (i.e., V_AC = 0.75 V), as shown in Fig. 4c, the depletion region under Pt starts to disappear, at this stage, the overall depletion region under the whole electrode almost vanishes, allowing electrons to flow under both the Ta and Pt electrodes. As a result, the forward current consists of two parts, which explains why DBA devices exhibit kinks in the forward I-V characteristics.

When V_AC < 0 V, as shown in Fig. 4d, the depletion region formed by the neighboring Pt gradually expands until it connects, which results in a continuous depletion region at a small reverse voltage of −20 V. When applying a large reverse bias of −5 kV and −10 kV, as shown in Fig. 4e, f, the entire depletion region is uniformly and fully connected, and Pt almost dominates the depletion of the channel. At this stage, due to the lower work function of Ta, a small number of electrons can pass through Ta and generate larger I_leakage than the single-Pt-anode device. The more proportion of Ta, and the more electrons will pass through, as shown in Fig. 5b, and I_leakage increases with the proportion of Ta electrodes.

These results show that the Pt dominates the reverse depletion process in the DBA structure, so that the DBA device can maintain high BV similar to the one with a single Pt anode. At the same time, since the Ta electrode only partially covers the anode in the DBA structure, the I_leakage caused by the Ta electrode is also relatively reduced compared to the device with a full Ta anode. Please refer to Fig. 2b, the I_leakage of the optimized DBA device is one order of magnitude larger than that of the single-Pt-anode device, but two orders of magnitude lower than that of the Ta anode device, which also shows that Pt plays a dominant role under the reverse bias in the optimized DBA device.

In order to better reflect the characteristics of the DBA anode, reverse electric field simulations were conducted for a Pt-anode device, a Ta-anode device, and three Pt/Ta DBA devices with different Angle_Ta (different distances between each Ta metal) at −5kV. As shown in Fig. 6a, b, for Pt-anode and Ta-anode devices, at −5 kV, the electric field distribution in the channel of both the Pt-anode and Ta-anode devices is uniform in the cutplane (transversely) and gradually decreases along the longitudinal direction. For the DBA devices, as shown in Fig. 6c-d, and e, although the depletion regions are all fully connected finally, the electric field distribution in the channel is not uniform in the cutplane (transversely). The fluctuation reflects the influence from the arrangement of Ta, the shorter the distance between each Ta metal is, the smaller the fluctuation of the electric field distribution will be, but it is clear that the Pt dominates the connection of the depletion region.

By combining the advantage of Ta under the forward bias and that of Pt under the reverse bias, the DBA structure was employed to realize the low-V_on and high-BV SBD. Fig. 7 plots the benchmark of BV versus V_on and R_on,sp for the state-of-art GaN-based lateral SBDs. The DBA SBD of this work not only showed a high BV of > 10 kV among GaN-based lateral SBDs on any substrates, but also achieved the low V_on of 0.36 V and low R_on,sp of 25.1 mΩ·cm². This research has shown that using the right metal combinations and geometric configurations in the SBD anode can harness the specific advantages of each metal, leading to enhanced device performance across various aspects. This may reflect a new approach to achieving breakthroughs in device performance.

In summary, in this work, the low-V_on and high-BV SBD device was realized by adopting the DBA structure and high-quality GaN material grown on sapphire substrate. The V_on of the DBA SBD is 0.36 V, while the BV remains at 10 kV. Combined with the R_{on, sp} of 25.1 mΩ·cm², the highest P-FOM of the device could reach as high as 4.0 GW/cm². These results demonstrate the feasibility of combining metal anodes and successfully utilizing the advantages of each metal and provide a feasible solution to achieve a low-turn-on AlGaN/GaN SBD together with ultra-high BV, which is expected to promote the applications of GaN materials in the field of UHV electronics.

Acknowledgments The authors acknowledge Corenergy. Inc. for the epitaxy of the wafers.This work is supported by National Key R & D Project grant No. 2022YFE0122700), National High-Tech R & D Project (grant No. 2015AA033305), Jiangsu Provincial Key R & D Program (grant No. BK2015111), China Postdoctoral Science Foundation (grant No. 2023M731583), Jiangsu Provincial Innovation and Entrepreneurship Doctor Program, the Research and Development Funds from State Grid Shandong Electric Power Company and Electric Power Research Institute.

Declaration of competing interest The authors declare no competing interests.