GALLIUM NITRIDE Onsemi’s new vGaN vs legacy GaN

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Onsemi announced the commercialization of its new vertical gallium nitride technology. The company calls it a breakthrough for AI and electrification. But GaN has already been a champion in the power and RF industry; what’s so new about this?

Well, the vGaN - “vertical” GaN is a newly designed power semiconductor device in which current flows vertically from top to bottom or bottom to top. Commercially available legacy GaN is mostly a lateral/planar GaN. vGaN design is vertical rather than lateral.

The new feature in vGaN is its ability to reduce the size of power conversion units and BOM file (Bill of Materials) costs in power electronic and RF applications. The article explains key differences between vGaN and legacy GaN. It does not label either of them superior, but suggests they are equally applicable in the power industry.

vGaN vs Lateral GaN

GaN is already a promising wide band gap semiconductor known for its performance in RF applications and fast-charging solutions driven by high carrier mobility. The legacy GaN is laterally constructed GaN, in which the current flows laterally across the surface. A large fraction of legacy GaN power semiconductors are HEMTs sold in SMD packaging.

Onsemi`s new GaN power semiconductor is a vertical GaN. The term vertical suggests that current flows vertically through the chip. The company lists various applications where vGaN can prove its worth, such as reducing the size of 800 V DC-DC converters in AI data centers, inverters in renewable technologies, energy storage systems, EV chargers, aerospace, defense, and security.

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Manufacturing: Legacy GaN manufacturing process involves depositing GaN thin films on foreign substrates such as silicon, sapphire, and silicon carbide. GaN-on-Si is a common choice for cost-effective solutions, GaN-on-Sapphire for optoelectronic applications, and GaN-on-SiC for power electronics.

Onsemi’s new vGaN is a hexagonal wurtzite structure, manufactured by growing thick and defect-free GaN layers on GaN substrates (GaN-on-GaN). Both vertical nGaN and pGaN are grown by epitaxy.

Construction: In lateral GaN, all three terminals, drain, source, and gate, lie in the same plane. The gate is placed in the same plane between the source and the drain to modulate the channel.

In vertical GaN, all three terminals, source, drain, and gate, lie on different vertical levels, making it 3x smaller than lateral GaN. The drain is usually present at the bottom.

Current flow:Due to the lateral design, current flows horizontally across the surface, from either left to right or right to left. Surface and 2D geometry limit the current density. However, lateral GaN shows good results in many power and RF applications.

Due to vertical design, current flows through the chip - either from top to bottom or bottom to top. The current density is high because the current flows through the thickness of the vertical construction. vGaN performs conduction through the bulk of the transistor body. vGaN can shrink the size of power converters in various power applications.

Voltage capability:Even if the device is compact and used in various applications, lateral GaN voltage solutions struggle to be scaled beyond 900 V.

The proprietary manufacturing process grows vGaN at high temperatures to handle large voltages and currents during operation. vGaN voltage ranges from 1200 V and reaches kilovolts up to 3.3-4 kV.

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Breakdown voltage: Lateral GaN has a lower Baliga figure of merit for the given die size. To increase breakdown voltage, the die size must be increased. Typical range is 650 V for high-reliability devices.

vGaN shows a higher breakdown voltage for the given die area. It has a comparable but higher Baliga figure of merit. Manufacturers can thicken the GaN drift region to support a higher breakdown voltage.

Power density: In lateral GaN, surface conduction limits power density. To increase power density, the die area must be increased. It is important to note that GaN chargers are still compact for the given area.

vGaN shows higher power densities above 1200 V operation. As the current flows vertically through the bulk, the thickness of the drift region can be increased to improve power density.

Defects: In lateral GaN, foreign substrates, such as silicon and sapphire, lead to lattice and thermal expansion mismatch. Defect density becomes higher, leading to reliability issues.

The absence of foreign substrates and GaN homoepitaxial structure limits defect density, epitaxy-related dislocations, and wafer deformations in vGaN. From a manufacturer’s perspective, vGaN improves yields. As no mismatch exists between lattice and thermal expansion, vGaN becomes a highly reliable power electronic device.

Heat distribution: In lateral GaN, heat is distributed laterally. Simply put, heat spreads sideways along the layered structure. It must be directed to silicon, which exhibits low thermal conductivity.

Vertical thermal conduction offers superior thermal stability in vGaN. Heat does not spread laterally but sinks into the chip’s vertical structure. vGaN is based on DSC (double-sided cooling) architectures to improve thermal performance.

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Reliability: Lateral GaN may fail during surges or when the voltage exceeds the breakdown voltage. However, they prove to be reliable under 650 V applications.

vGaN adds avalanche-capable designs, comparable to SiC. It means that vGaN can still function after exceeding the breakdown voltage.

Losses: Lateral GaN incurs high RDs (on) with rising temperature. Conduction losses increase when a large current flows through the device. However, switching losses are very low in lateral GaN, leading to ultra-fast switching speeds.

vGaN shows stable RDs (on) due to the absence of surface traps. Current flows through the bulk transistor layer, leading to 50% lower losses. vGaN also offers higher switching frequencies beyond silicon and SiC.

Conclusion

Researchers experimented with vGaN for years. Onsemi’s vGaN power semiconductor is manufactured through new and proprietary processes. All lateral GaN manufacturing processes are mature and commercialized. Despite offering various new advantages, lateral GaN rules the industry.

vGaN will take time to match the share of lateral devices in the USD5.32 billion GaN market. The construction follows the SiC MOSFETs and other high-power devices. Scalability issues might arise due to incompatibility with standard silicon-based procedures. In addition, manufacturing GaN-on-GaN is expensive.

References

• https://www.power-and-beyond.com/onsemi-unveils-vertical-gan-semiconductors-a-breakthrough-for-ai-and-electrification-a-1298f5b717553e1c85bd28bac6a0a8e5/

• https://www.onsemi.com/solutions/technology/vertical-gan

• https://www.powerelectronicsnews.com/vertical-gan-advantages-in-the-industry/

• https://www.onsemi.com/download/tutorial/pdf/tnd6495-d.pdf

• https://www.precedenceresearch.com/gan-semiconductor-devices-market

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