【World Express】High-Voltage Gallium Oxide Devices Show Promise

      Ultra-wide-bandgap (UWBG) semiconductors have superior intrinsic material properties compared with silicon and WBG materials like silicon carbide and gallium nitride. Amongst the different UWBG materials, gallium oxide is showing increasing promise for future use in high-voltage power electronics. This article summarizes some of the intrinsic properties of this material and showcases some recent high-voltage device advances. It is based on a PSMA webinar given by Uttam Singisetti, a professor of electrical engineering at the University of Buffalo.

Intrinsic material properties of gallium oxide

      The beta phase of gallium oxide (β-Ga₂O₃) has emerged as a key candidate for evaluation as the choice of UWBG material. Several factors play into this. Table 1 lists some of the basic material properties of silicon, SiC, GaN and β-Ga₂O₃.

Table 1: Some basic properties of silicon, SiC, GaN and β-Ga₂O₃ (Source: Singisetti, U., 2024)

      The higher bandgap and electric field strength are two advantages of β-Ga₂O_3. This allows for more efficient device scaling in high-voltage power devices and can consequently result in improved metrics for both conduction and switching losses. The ideal Baliga figure of merit (BFOM) shown in Table 1 is commonly used to describe the tradeoff between the on-state resistive loss and the breakdown field. This, however, does not include two important factors:

1、Incomplete ionization from dopants

2、Background impurities in the substrates

      The availability of shallow dopants is a key enabler in achieving performance close to the theoretical BFOM limits. UWBG materials, however, can suffer from a lack of such dopants. In the case of aluminum nitride (AlN) and diamond, high-dopant–ionization energies make it difficult to effectively achieve high levels of activated dopants, especially at room temperature. This leads to low electrical conductivity. Fortunately, β-Ga₂O₃has shallow, n-type donor dopants (tin, silicon).

      The impurity states within the bandgap can further compensate for dopant densities and degrade device performance. Impurity levels can be orders of magnitude higher in UWBG materials compared with silicon. AlN and diamond both suffer from this effect, with background concentrations that can range into the 1e16/cm³ range. In WBG materials, GaN also suffers from relatively poor background concentrations of above 1e15/cm³. β-Ga₂O₃again has an advantage here, with progress in epitaxy and substrate growth resulting in background charge concentrations of below 1e15/cm³. A good example of the importance of low background doping and a high bandgap is a silicon IGBT rated to 6.5 kV that would need a minimum blocking-voltage thickness of 220 µm and a background impurity below 4e13/cm³, which is hard to achieve and would in any way result in a very high R_DS(on), while β-Ga₂O₃would need only an 8-µm-thick blocking layer with a net doping concentration of 3e16/cm³.

      The modified BFOM comparisons taking these above effects into account are shown in Figure 1. These plots show the advantage of β-Ga₂O₃over other materials, especially at voltage ratings above 1 kV.

Figure 1: A comparison of the modified BFOM metrics, considering dopant activation and background impurities: (a) shows the conduction loss metric, while (b) shows the switching loss metric. (Source: Zhang, Y., & Speck, J., 2020)

Substrate growth

      High-quality substrates of β-Ga₂O₃at relatively low costs are possible with melt growth, similar to silicon. This is a key advantage compared with WBG materials like SiC, which need to use costly sublimation methods. An example of a company that is manufacturing 100-mm β-Ga₂O₃substrates is Novel Crystal Technology, based in Japan.

β-Ga₂O₃ high-voltage devices

      Let’s now look at some of the work done by Singisetti’s group and others in the creation of β-Ga₂O₃high-voltage power devices. Several atomic-layer-deposited (ALD) dielectrics have been used as a gate dielectric to create lateral n-channel MOSFET devices. Silicon dioxide (SiO₂) is a promising candidate, with a large conduction band offset and low interface states at room temperature. Initial MOSFETs created exhibited a breakdown outside the channel region. Failure analysis pointed to high fields in the air above the gate-field-plate (GFP) region. The use of a composite field-plate dielectric (comprising PECVD and ALD SiO₂), a recessed MBE-grown channel and a high-field-strength epoxy polymer (SU-8) passivating film above the GFP resulted in MOSFETs with the highest reported breakdown voltage (BV) of over 8 kV in lateral MOSFETs. This work showed that field management techniques are critical. A cross-section schematic and BV curves of this device are shown in Figure 2.

Figure 2: Cross-section view and BV curves of a ß-Ga₂O₃ MOSFET (Source: Sharma, S., & Singisetti, U., 2020)

      These devices still suffered from relatively poor R_DS(on). It was found that vacuum annealing post-RIE etch created damage recovery and improved R_DS(on), without affecting the BV.

      MESFET devices with a Schottky gate have shown promise. The 4.4-kV MOSFETs with a power FOM (PFOM = BV² ÷ R_DS(on)) exceeding 100 MV/cm² and specific R_DS(on) ≈ 20 Ω-mm² have been demonstrated using a silicon nitride passivation dielectric.3 While this PFOM was much better than the theoretically achievable number with silicon, it’s still well short of ß-Ga₂O₃’s theoretical limits. The use of improved epitaxial growth techniques and a FINFET MESFET structure have demonstrated electron mobilities of 184 cm²/V-s. This 4.4-kV device, which uses 25 fin widths of 1.2 to 1.5 µm, achieved a record PFOM of 0.95 GW/cm².

High-temperature operation

      As shown in Table 1, β-Ga₂O₃suffers from poor thermal conductivity. This can put a burden on cooling requirements in high-power applications. Some intrinsic advantages, however, help β-Ga₂O₃perform well at high temperatures. The extremely low intrinsic carrier density and a combination of other factors enable a low thermal degradation coefficient to be achieved. In comparison with GaN, where the R_DS(on) at 125°C can be over 2× that at 25°C, the R_DS(on) for β-Ga₂O₃moves little with temperature.

      The SBD was operated to 600 K and exhibited a much smaller, tenfold increase in the reverse leakage current from 300 K to 500 K at 500 V, compared with at least a hundredfold increase of the same in vertical GaN and SiC SBDs at similar ratings. The MESFETs were operational to the measured 500°C. These examples demonstrate the potential use for β-Ga₂O₃devices in high-temperature, high-voltage operation.

      Work has also been done to improve the packaging for thermal resistance reduction. A double-side-packaged large area (4.6 × 4.6 mm) β-Ga₂O₃vertical SBD device rated for 15 A was used with silver sintering on both sides of the die. The top anode junction-ambient thermal resistance was measured to be 0.5 K/W, which is lower than that of similarly rated SiC SBDs. This and other work that involves thinning down the substrate below 100 µm highlights that the low thermal conductivity need not be a showstopper in high-voltage, high-power applications.

Use of heterojunctions/superjunctions to create bipolar devices

      The p-doping of β-Ga₂O₃is difficult due to the lack of shallow acceptors and strong self-trapping of holes. Heterojunction and superjunction devices using n-doped β-Ga₂O₃with p-doped nickel oxide have been successfully demonstrated as diodes and MOSFETs. High-quality interfaces have been successfully created between these two materials and good device performance that creates the advantages of the p-n junction, including avalanche and surge capability, which can be a vital robustness criterion in many power system applications.