Achievement of low turn-on voltage in Ga₂O₃ Schottky and heterojunction hybrid rectifiers using W/Au anode contact

Reducing switching losses in Ga₂O₃ rectifiers is essential to achieve high efficiency, better thermal management, reliable high-frequency operation, cost savings, enhanced system reliability, and competitive advantages in power electronic applications.^1–5 The total power loss in rectifiers is composed of on-state, switching, and off-state losses,^6–9 which also depend on temperature.^10,11 A low turn-on voltage (V_on) and a low specific on-resistance (R_on) contribute to reducing on-state losses in Schottky rectifiers. Compared to PN junction diodes, Schottky Barrier Diodes (SBDs) offer a significant advantage in reverse recovery time, thereby minimizing switching losses.^12,13 Off-state losses, another component of power losses, are primarily governed by reverse leakage current, which can be exacerbated by Schottky barrier lowering and tunneling effects under high reverse bias conditions.^12,13

While most of the focus has been on achieving high breakdown voltage (V_B), especially using heterojunction NiO/Ga₂O₃ rectifiers,^14–47 there is also a need for intermediate breakdown devices with a low R_on. To achieve this in Schottky rectifiers requires the use of metal contacts with lower work functions relative to the usual Ni^1–4 or oxidized metal contacts.^48,49 Li et al.¹ reported a vertical β-Ga₂O₃ heterojunction barrier Schottky (HJBS) diode incorporating tungsten (W), a low work function metal, for the Schottky contact. These devices exhibited a low V_on of 0.48 V and a blocking voltage from 756 to 1347 V. A systematic study was conducted on the spacing width of the p-type region in the β-Ga₂O₃ HJBS diode to elucidate its modulation effect on forward characteristics, including V_on, Schottky barrier height, R_on, and reverse leakage current.

We have previously shown that the electrical and structural properties of sputter-deposited W Schottky contacts with gold (Au) overlayers on n-type Ga₂O₃ exhibit substantial stability up to 500 °C.^16,17 However, reverse leakage in the diode structures significantly increases (by a factor of 2) when subjected to annealing at temperatures between 550 and 600 °C. The sputter deposition process induces near-surface damage, resulting in a reduced Schottky barrier height of 0.71 eV in the as-deposited state. This barrier height increases to 0.81 eV following a 60-s anneal at 500 °C.^16,17 Although this value is lower compared to the conventional Ni/Au Schottky barrier height of 1.07 eV, W exhibits superior thermal stability. This was corroborated by Auger electron spectroscopy analysis of the contact and interfacial region, which revealed minimal changes in contact morphology.

There have been a number of recent studies that concluded Ga₂O₃ diodes exhibited switching properties comparable to commercial SiC diodes only under certain operating conditions.^12,13,50 Under heavy load conditions, buck converter efficiencies with Ga₂O₃ diodes surpassed those with Si fast-recovery diodes but were lower than those with SiC diodes. Future applications will necessitate improvements in the trade-off between on-resistance and capacitive charge. Jahdi et al.⁵⁰ assessed the performance of β-Ga₂O₃ rectifiers in heavy-duty Modular Multilevel Converter (MMC)-based Voltage Source Converters (VSCs), comparing them to Si-IGBTs and SiC-FETs for High Voltage Direct Current (HVDC) and Medium Voltage Direct Current (MVDC) converter station applications. They found that Ga₂O₃ offers potential advantages where on-resistance is low. Of potential interest are electric vehicle (EV) charging applications. The range of voltages of interest typically falls within the following categories:

• Level 1 charging (standard AC charging) at 120 V (North America). This is the standard household outlet voltage, suitable for overnight charging.

• Level 2 charging (faster AC charging at 208–240 V, common in residential, commercial, and public charging stations). This provides faster charging compared to level 1 and is commonly used in homes with a dedicated EV charger or in public charging stations.

• DC fast charging (DCFC or level 3 charging) at 400–800 V (and up to 1000 V in some cases). This type of charging allows for very rapid charging, significantly reducing the time needed to charge an EV.

• Ultra-fast DC charging up to 1500 V (emerging technologies). These systems are designed for extremely fast charging, potentially bringing charging times down to just a few minutes. These voltage ranges are crucial for designing EV charging infrastructure and ensuring compatibility with various EV models.

In this paper, we report on the achievement of simultaneous low V_on (0.22–1.50) and high V_B (600–2400 V) in Ga₂O₃ Schottky and heterojunction hybrid rectifiers using tungsten as the anode metal.

The rectifiers were fabricated on thick (10 μm) lightly doped (8 × 10¹⁵ cm⁻³) epitaxial layers grown by halide vapor phase epitaxy on heavily doped (8 × 10¹⁸ cm⁻³) bulk substrates with (001) orientation, produced using the edge-defined film-fed technique. Ohmic contacts were established on the wafer backsides by depositing e-beam evaporated Ti/Au, followed by annealing at 550 °C for 3 min.^26,28,29 SBDs, HJDs, and the hybrid diodes were all fabricated on the same wafer, with their schematic structure illustrated in Fig. 1.

A bilayer of W/Au served as the Schottky contact and established contact with the p⁺-doped NiO, which was deposited via sputtering.^16,17 A 20 nm tungsten layer was deposited by DC sputtering using a 3-in. pure tungsten target. The DC power was set to 75 W, and the process pressure was maintained at 5 mTorr in a pure argon (Ar) ambient. Following the tungsten deposition, an 80 nm gold (Au) layer was deposited via e-beam evaporation to reduce the contact’s sheet resistance and prevent tungsten oxidation. The deposition conditions for the bilayer NiO have been previously documented.³⁰ Consequently, the hybrid contact structure comprises regions exhibiting Schottky behavior and regions forming a heterojunction rectifier. The ratio of these two regions varied from purely Schottky to pure heterojunction by adjusting the diameter of the Schottky contact opening of the NiO, which ranged from 20 to 80 μm, while the W/Au diameter remained constant at 100 μm. The Schottky contact area percentage was thus varied from 4% to 64%.

Current density–voltage (J–V) characteristics were measured in air at 25 °C, utilizing an Agilent 4156C parameter analyzer. High reverse bias device breakdown characteristics were analyzed with a Glassman high-voltage power supply. A mega-ohm resistor was integrated into the setup, and its voltage drop was subtracted. In addition, before each breakdown test, contact integrity was verified by conducting a forward sweep up to 6 V and a reverse sweep up to −100 V, confirming the J–V characteristics. The on-resistance was derived from the voltage–current (dV/dI) derivative extracted from the J–V characteristics, with corrections applied to account for the resistance contributions from external circuit components, including cables, chuck, and probe, which collectively amounted to 10 Ω, as determined through interconnected I–V measurements. The on–off ratio was determined, in which the bias was switched from 6 V forward to a range of reverse bias voltages up to −100 V.

The forward J–V characteristics for devices with different Schottky contact area percentages within the hybrid contact are shown in Fig. 2. It also shows the corresponding R_on values of various hybrid devices. Pure Schottky diodes exhibit the highest forward current and the lowest R_on values (5 mΩ∙cm²) due to their lower effective barrier height relative to heterojunction devices.

The turn-on voltages of devices with different Schottky contact area percentages at 100 mA/cm², ranging from 0.22 V for Schottky rectifiers to 1.50 V for pure heterojunction rectifiers are shown in Fig. 3. These are significantly lower compared to the values from 0.66 V for Schottky rectifiers to 1.77 V for pure heterojunction rectifiers when conventional Ni is used as the anode metal.^26,28,29 This indicated that the lower work function of the W translates to lower V_on values. Table I summarizes the comparison between W and Ni anode devices as a function of percentage areas of the Schottky contact metal.

The reverse current–voltage (I–V) characteristics determine V_B, and the resultant on/off ratio for devices with different diameters of the Schottky contact within the hybrid devices is shown in Fig. 4. The breakdown voltages ranged from 600 V for pure SBD to 2400 V for pure HJD. These are sufficient for the first three classes of vehicle charging discussed earlier. The on/off ratios were in the range of 10⁶to 10¹⁰. The trend of lower reverse current density in heterojunction rectifiers is also evident in Fig. 5, which shows V_B and reverse leakage current density at −100 V bias as a function of both SBD percentage diameter and area. There was more of a linear correlation between diameter and V_B since the breakdown of the devices was located at the periphery of the Schottky contact. On the other hand, reverse leakage current density depends more on the area of the Schottky contact because the source of the leakage current was from the lower barrier height Schottky contact part of the hybrid device.

Similar plots for V_on and forward current density as a function of either SBD diameter or area are shown in Fig. 6. There is a huge gap between small Schottky percentages and pure heterojunction devices regardless of diameter or area. This shows the dominant effect of the low work function Schottky contact for reducing V_on. A better correlation between the SBD area and the forward current density can also be observed, showing that the forward characteristics below 6 V depend primarily on the contribution from the Schottky component.

The switching performance of devices with different Schottky contact area percentages is shown in Fig. 7. The results are summarized in Table II, where it is seen that the reverse recovery time, t_rr, is in the range of 31.2 to 34.4 nanoseconds across the range of SBD areas and a generally higher reverse recovery current, I_rr, for the HJD end of the range of contact percentages. Figure 8 shows the Baliga’s figure-of-merit (V_B²/R_on) and energy switching losses of hybrid devices with varying percentages of diameter and area of the Schottky contact. The former is significantly larger for the HJDs because of their larger V_B and is >1 GW cm⁻² for pure heterojunctions. Note that the switching energy losses for SBDs are generally less than those of the HJDs due to the higher forward current density. In addition, the similarity of the energy loss of pure HJD and the hybrid device with a low SBD percentage indicated the trade-off between the higher leakage current, which leads to a higher off-state energy loss, and the higher forward current density due to the lower V_on of the Schottky contact.

A summary of V_on results as a function of breakdown voltages in various types of vertical Schottky Ga₂O₃ rectifiers has been reported in the literature (Fig. 9).^1,18,51–59 The use of W contacts in this work extends the range of accessible V_on values achievable in Ga₂O₃ rectifiers. This shows the continuing maturity of the technology, where there is a realization that different applications require a different emphasis on the relative importance of reverse breakdown voltage, V_on, and therefore the associated switching losses.^12,13,50 The results complement the recent realization of 13.5 kV V_B,²⁸ but with a much higher V_on (2.2 V) than achieved here with tungsten anodes.

In Ga₂O₃ rectifiers, there is a trade-off between achieving a high reverse breakdown voltage and minimizing the on-state voltage. A lower on-state voltage is preferable for efficiency reasons. When a rectifier is conducting, there is always some power loss due to this voltage drop. This translates to heat generation, which can reduce efficiency and require additional heat dissipation measures. To achieve a high reverse breakdown voltage, the device needs a wider depletion region within the rectifier, but a wider depletion region also creates a higher resistance to current flow in the forward direction, which leads to a higher on-state voltage. This is the fundamental trade-off between these two characteristics. The choice of which property to prioritize depends on the specific application. If the application prioritizes blocking high voltage spikes, a higher reverse breakdown voltage might be crucial. On the other hand, if efficiency is a prime concern, minimizing the on-state voltage becomes more important.

This study investigated the use of low work function tungsten (W) as a rectifying contact to achieve low on-voltages in the hybrid design of W/Ga₂O₃ Schottky and NiO/Ga₂O₃ heterojunction rectifiers. Devices ranged from pure Schottky to pure heterojunction configurations, with on-voltages from 0.22 V for Schottky rectifiers to 1.77 V for heterojunction diodes and reverse breakdown voltages from 600 to 2400 V. These are the lowest V_on values reported for 600 V-class Ga₂O₃ rectifiers, enhancing their applicability in electric vehicle charging. Reducing switching losses in Ga₂O₃ rectifiers is crucial for efficiency, thermal management, and reliability. Prior studies show that W Schottky contacts with gold overlayers on n-type Ga₂O₃ are stable up to 500 °C,^16,17 despite increased reverse leakage at higher temperatures. Ga₂O₃ diodes have shown comparable switching properties to SiC diodes, with potential applications in EV charging infrastructure across various voltage levels.

The work at UF was performed as part of the Interaction of Ionizing Radiation with Matter University Research Alliance (IIRM-URA), sponsored by the Department of the Defense, Defense Threat Reduction Agency under the Award No. HDTRA1-20-2-0002. The content of the information does not necessarily reflect the position or the policy of the federal government, and no official endorsement should be inferred.

The authors have no conflicts to disclose.

Chao-Ching Chiang: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal); Writing – review & editing (equal). Jian-Sian Li: Data curation (equal); Validation (equal); Writing – review & editing (equal). Hsiao-Hsuan Wan: Data curation (equal); Validation (equal); Writing – review & editing (equal). Fan Ren: Conceptualization (equal); Funding acquisition (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – review & editing (equal). Stephen J. Pearton: Funding acquisition (equal); Resources (equal); Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.