Abstract
We demonstrate superb large-area vertical β-Ga2O3 SBDs with a Schottky contact area of 1 × 1 mm2 and obtain a high-efficiency DC–DC converter based on the device. The β-Ga2O3 SBD can obtain a forward current of 8 A with a forward voltage of 5 V, and has a reverse breakdown voltage of 612 V. The forward turn-on voltage (VF) and the on-resistance (Ron) are 1.17 V and 0.46 Ω, respectively. The conversion efficiency of the β-Ga2O3 SBD-based DC–DC converter is 95.81%. This work indicates the great potential of Ga2O3 SBDs and relevant circuits in power electronic applications.1. Introduction
Power devices and circuits are the most important parts of the electrical energy conversion system. Meanwhile, power devices and circuits based on ultra-wide bandgap semiconductors can contribute to reducing the power consumption in the conversion[1].
β-Ga2O3 is considered to have great potential in power electronic applications due to its wide bandgap of approximately 4.8 eV, high critical electric field of 8 MV/cm and high Baliga’s figure of merit of 3444[2-4]. These properties make β-Ga2O3 power devices promising for high voltage, high power and other applications[5, 6].
In the past decade, β-Ga2O3 devices, especially Schottky barrier diodes (SBDs), have developed rapidly, whose performances have been improved significantly and currently approach those of SiC and GaN[7-12]. At present, the works of large-area devices mainly focus on the combination with edge termination[13-16], while the baseline devices or named termination-free SBDs are rarely investigated for large-current applications. Our recent work demonstrated that the performance of small-area SBDs can be greatly improved by interface engineering[11], thus it is a chance for large-area devices. The high-performance SBDs with free termination may better reflect the application potential of Ga2O3 SBD. In a word, the Ga2O3 SBD is more mature for applications and needs to be further demonstrated for its application potential.
In this work, we achieved a high-performance large-area vertical β-Ga2O3 SBD with a Schottky contact area of 1 × 1 mm2, and then realized its application in a DC–DC converter with high efficiency. The β-Ga2O3 SBD obtained good forward characteristics of 8 A@5 V, a low Ron of 0.46 Ω and a high breakdown voltage (Vbr) of 612 V. A prototype of the DC–DC converter is demonstrated using the β-Ga2O3 SBD, then a conversion efficiency of 95.81% is obtained.
2. Device fabrication and characterization
The schematic cross section and optical image of the β-Ga2O3 SBD are shown in Fig. 1. The Ga2O3 substrate has a doping concentration about 7.0 × 1018 cm−3 with a thickness of 610 μm, and the 8.5 μm-thick Ga2O3 epitaxial layer grown by halide vapor phase epitaxy (HVPE) has a doping concentration of approximately 1.9 × 1016 cm−3. After organic and acid cleaning, the upper surface of the epitaxial layer is removed by ICP180 to remove the unreliable surface[11]. Following the piranha solution, the backside of the Ga2O3 substrate is coated with Ti/Al/Ni/Au (20/200/50/50 nm) metal stacks by electron beam evaporation (E-beam), and then undergoes rapid thermal annealing at 470 °C in N2 for 1 min to improve ohmic contact. The Schottky electrode with Ni/Au (50/100 nm) is deposited by the E-beam system. The Schottky contact area of the β-Ga2O3 SBD is 1 × 1 mm2.
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Figure 1.(Color online) (a) Schematic cross section of the β-Ga2O3 SBD. (b) Optical image.
Fig. 2(a) shows the forward conduction characteristics of the β-Ga2O3 SBD. The forward turn-on voltage (VF) and the on-resistance (Ron) are 1.17 V and 0.46 Ω, respectively. A forward current of 8 A can be obtained at a forward voltage of 5 V in pulse mode (50-μs pulse width and 1% duty cycle). Meanwhile, the Vbr of the β-Ga2O3 SBD is 612 V as shown in Fig. 2(b).
Figure 2.(Color online) (a) Forward conduction characteristics and (b) reverse breakdown characteristics of the 1×1 mm2.
The performance of the β-Ga2O3 SBD is benchmarked against some reported state-of-the-art large-area β-Ga2O3 SBDs with electrode areas above 0.2 mm2 in the plot of Ron,sp versus Vbr in Fig. 3[9, 16-19]. The specific on-resistance (Ron,sp) is 4.6 mΩ·cm2. Associated with the Vbr of 612 V, the β-Ga2O3 SBD presents a FOM of 81.4 MW/cm2. Compared with the reported work, the fabricated β-Ga2O3 SBD in this work exhibits superior performance.
Figure 3.(Color online) Ron, sp versus Vbr benchmarks of reported state-of-the-art large-area β-Ga2O3 SBDs with electrode areas above 0.2 mm2.
In order to judge the relative performance of the device with the commercial SBDs based on Si and SiC, and to quantify the remaining gap to be closed in the future, we compared our β-Ga2O3 SBD with the commercial Si FRD (STTH1L06, 600 V/1 A) and SiC SBD (CSD01060A, 600 V/1 A) as shown in Table 1. From the results, we can obtain that our β-Ga2O3 SBD shows a comparable performance with commercial Si FRD and SiC SBD, while the β-Ga2O3 device is just in its infancy. Reducing the on-resistance and increasing the breakdown voltage are still the key points of our work in future development.
A double-pulse test (DPT) circuit was designed to evaluate the switching performance of β-Ga2O3 SBD[16], and the reverse recovery characteristic of β-Ga2O3 SBD was measured when the device switched from a forward current of 1 A to a reverse bias voltage of 100 V with a di/dt of 500 A/μs. The reverse recovery characteristics of the Si FRD, SiC SBD and β-Ga2O3 SBD are contrasted in Fig. 4, and the properties of the β-Ga2O3 with commercial Si and SiC devices are shown in Table 1. We can obtain from the experimental results that the reverse recovery characteristic of the β-Ga2O3 SBD has an apparent advantage over Si FRD and approaches to SiC SBD.
Figure 4.(Color online) The reverse recovery characteristics of the Si FRD, SiC SBD and β-Ga2O3 SBD.
3. Application in the DC–DC converter
In order to demonstrate the application potential, the β-Ga2O3 SBD is encapsulated in the TO-220 package, and then implemented in a DC–DC converter circuit. The circuit configuration of the converter is shown in Fig. 5, and the specifications of the converter are summarized in Table 2.
Figure 5.(Color online) Schematic of the DC-DC converter based on the β-Ga2O3 SBD.
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A 650 V/180 mΩ discrete GaN FET with part number TPH3206PSB (Transphorm) is used for switching control. The gate driver of Si8261 (Skyworks) is used to drive the GaN FET, and the gate-source voltage (VGS) is +9 V during the on-state and 0 V during the off-state. The input voltage (VIN) is selected to be 200 V, and the converter is operated at a switching frequency (f) of 100 kHz and a duty cycle (D) of 40%.
Fig. 6 shows the β-Ga2O3 SBD-based DC–DC converter and the testing platform. The square signal for the gate driver was generated by an arbitrary function waveform generator (Keysight, 33600A), and the auxiliary voltage for the gate driver (VAUX) was provided by a DC power supply (ITECH, IT6333C). The input voltage (VIN) was generated by an auto range DC power supply (ITECH, IT6526C), and the output signal (VOUT) was tested through a DC electronic load (ITECH, IT8902E). The voltage and current waveforms were monitored by an oscilloscope (Keysight, MSOX6004A).
Figure 6.(Color online) Photograph of the β-Ga2O3 SBD-based DC-DC converter and the testing platform.
The experimental waveforms of the gate-source voltage (VGS), the output voltage (VOUT), the inductor current (IL), the diode voltage (VD) and the diode current (ID) in the β-Ga2O3 SBD-based DC–DC converter are shown in Figs. 7 and 8. The spike in the waveform of the diode current (ID) is due to the reverse recovery characteristics of the SBD. The experimental results are shown in Table 3, the output voltage of the converter is approximately 329.7 V, and the output voltage ripple is less than 0.5%. The conversion efficiency of the β-Ga2O3 SBD-based DC–DC converter is 95.81%.
Figure 7.(Color online) Experimental waveforms of the VGS, VOUT and IL in the β-Ga2O3 SBD-based DC-DC converter.
Figure 8.(Color online) Experimental waveforms of the VGS, VD and ID in the β-Ga2O3 SBD-based DC-DC converter.
4. Conclusion
In conclusion, we have achieved a high-performance large-area vertical β-Ga2O3 SBD with a Schottky contact area of 1 × 1 mm2 and obtained a high-efficiency DC–DC converter based on the device. The β-Ga2O3 SBD can obtain a forward current of 8 A at a forward voltage of 5 V, and has a Vbr of 612 V. The conversion efficiency of the β-Ga2O3 SBD-based DC–DC converter is 95.81%. The decent performance of Ga2O3 SBDs and their circuits shows great potential in power electronic applications. Future works will introduce the edge termination technique to this baseline device.
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