IEDM2024|Development of GaN Power Device Technology

Introduction

Gallium nitride (GaN) power devices are transforming the power electronics industry by significantly improving the efficiency of many consumer applications. GaN technology has already seen widespread adoption in low-power applications ranging from 65 W to 3.2 kW, such as fast chargers and power supplies. Recent technological breakthroughs are now propelling GaN into high-power applications, which is expected to have profound economic, ecological, and societal impacts [1].

High-Voltage GaN Technology

The development of 1200 V GaN technology marks a major advancement in the field of power electronics. GaN high electron mobility transistors (HEMTs) can span a commercial voltage range from 100 V to 1200 V, making them strong competitors against silicon-based IGBTs, silicon CoolMOS, and silicon carbide (SiC) transistors.

Figure 1: Schematic of 1200 V GaN HEMT in cascode configuration on low-cost, large-size insulating sapphire substrate. Include device structure, comparison of SiC and GaN performance metrics, and performance characteristics.

Using sapphire as the substrate material represents a key innovation. Its excellent electrical insulation eliminates breakdown between the drain and substrate. During front-end manufacturing, the III-nitride buffer layer thickness on sapphire can be reduced by over 60%, significantly lowering epitaxy cost while maintaining good crystal quality and high insulation on both 150 mm and 200 mm wafers. In the back-end process, sapphire can be thinned to 150–200μm to match the thermal conductivity of silicon. As the preferred substrate for GaN LEDs, sapphire benefits from a mature processing knowledge base and an industrial-scale manufacturing ecosystem.

Experimental results show that a 70 mΩ normally-off GaN FET based on sapphire, in a TO-247 package and dual-die configuration, achieves over 99% efficiency in a 900 V-to-450 V buck converter operating at 50 kHz. This device demonstrates excellent switching performance with Ron∙Qg = 0.9 Ω∙nC and Ron∙Qrr = 11 Ω∙nC. These results indicate that optimized sapphire-based GaN technology can be highly competitive in the 1200 V power device market.

High-Current Capability

Figure 2: Switching waveform and efficiency curve of a single 10 mΩ GaN die, showing record 99.3% efficiency and 14 kW output power at junction temperature of only 120°C.

Progress in high-current GaN technology is exemplified by prototype devices with an on-resistance of 10 mΩ and rated DC current exceeding 170 A. The chip area spans several tens of square millimeters and can fit into standard TO-247-3L packaging. Hard switching waveforms demonstrate switching speeds of 50 V/ns and 4 A/ns, supporting high power and high-frequency switching. A 240 V-to-400 V boost converter operating in hard switching mode at 50 kHz achieves peak efficiency of 99.3% at 4 kW and maintains stable performance up to 14 kW. Notably, the junction temperature remains just 120°C at 14 kW, indicating room for further power scaling.

This superior performance is attributed to fast switching speeds minimizing switching losses, depletion-mode GaN in cascode configuration with low-voltage Si-MOSFETs achieving dynamic on-resistance <10%, and low resistance temperature coefficient (less than 1.8× between 25°C and 150°C, comparable to SiC trench MOSFETs). These characteristics collectively contribute to low conduction losses during operation.

Enhanced Robustness

Figure 3: Illustration of patented GaN technology achieving 5 μs short-circuit withstand time (SCWT) in motor drive inverters.

In motor drive applications, GaN devices must pass stringent JEDEC or AEC-Q0101 qualifications and tolerate short-circuit events caused by overload, breakdown, firmware errors, current surges, or external faults. In 2021, Transphorm demonstrated patented GaN technology achieving up to 3μs SCWT on 50 mΩ devices. This year, technology achieved 5 μs SCWT on 15 mΩ devices operating at 12 kW. These TO-247 packaged devices are rated at 650 V and 145 A DC current. At a drain bias of 400 V, they exhibit 5 μs SCWT and passed 1000-hour high-temperature reverse bias (HTRB) stress tests at 175°C. These results challenge the conventional belief that GaN lacks short-circuit tolerance. Importantly, modern gate drivers can respond to fault conditions within ~1 μs, allowing enough time to safely shut down the system and prevent device failure.

Bidirectional Switching Innovation

Figure 4: Monolithic GaN bidirectional switch with common drain and shared drift region for compact area, improved performance, and reduced component count.

Figure 5: GaN BDS implementation using depletion-mode monolithic GaN and low-voltage Si-FET in cascode configuration, enhancing threshold voltage, gate margin, reliability, and noise immunity.

Figure 6: Circuit topologies using GaN BDS, showing isolated matrix bidirectional active bridge and non-isolated T-type neutral-point clamped structure for efficient power conversion.

Thanks to its lateral structure, GaN is well-suited for monolithic integration. By integrating two transistors in a reverse series configuration, a “bidirectional switch” (BDS) is created. This device has two opposing source terminals and is controlled by two opposing gates, enabling bidirectional current conduction and bipolar voltage blocking. This simple and unique GaN device structure is ideal for AC front-end applications requiring handling of both positive and negative AC waveforms.

In this work, a GaN BDS is demonstrated using a monolithically integrated depletion-mode GaN HEMT cascoded with two low-voltage silicon MOSFETs for normally off operation. The monolithic integration allows a shared high-voltage drift region, reducing die size by 40% compared to two discrete GaN switches. The low-voltage silicon MOSFETs provide high threshold voltage (4 V), large gate margin (+20 V), high reliability, and strong immunity to noise and parasitic turn-on. The bidirectional cascode device employs stacked-die integration to minimize footprint and interconnect resistance and inductance. The solution is assembled in a single TO-247 package with an isolated tab.

Figure 7: Switching waveform of GaN BDS in single-stage AC/DC front-end using matrix active bridge, showing three-phase sinusoidal AC input and DC output characteristics.

The GaN BDS has been tested in a single-stage AC/DC front-end with a matrix active bridge configuration. Results demonstrate successful voltage blocking and system operation under both AC polarities. These technological advancements are paving the way for more efficient, compact, and reliable power conversion systems across data centers, AI, transportation, and renewable energy applications.

Reference

[1] U. K. Mishra, D. Bisi, G. Gupta, C. J. Neufeld, and P. Parikh, "The Future of GaN is also High Voltage, High Current, and Bidirectional," in 2024 IEEE International Electron Devices Meeting (IEDM), San Francisco, CA, USA, 2024