GaN-based transistors: excellent power-conversion efficiency, high switching speeds, compact circuit designs
In recent years, the power electronics industry’s interest in gallium nitride (GaN)-based transistors has grown tremendously.
In power electronics applications, electrical energy is converted from one form to another – for example, from alternating current (AC) into direct current (DC) (or vice versa), or DC from low voltage to high voltage (or vice versa). GaN has all the features to convert power in a highly efficient way across a wide range of output power and/or at high frequencies.
First, GaN is a wide bandgap material (3.2eV) with a high critical electric field for breakdown. Second, the GaN/AlGaN materials system, central to GaN transistor architectures, exhibits high electron mobility. This is due to the formation of a two-dimensional electron gas at the GaN/AlGaN interface, which creates a low-resistance conductive channel.
The combination of these material properties enables fast switching devices with low device parasitics and minimal energy losses during switching. This translates into a high power-conversion efficiency, which is the ratio of the output power to the input power. In this metric, GaN-based devices surpass conventional Si-based power transistors (including mature Si MOSFETs and insulated-gate bipolar transistors (IGBTs)), especially when a wide range of output power and/or high switching frequencies are required.
The high switching speeds come with an extra bonus: passive components, such as transformers, inductors, and capacitors – vital components in power delivery and conversion systems – can be made small and lightweight. This benefits the (ecological) footprint, cost, and reliability of the overall power electronics system.
GaN-based fast chargers entering mass production
GaN’s breakthrough in mass production came around 2023, when GaN-based fast battery chargers were introduced into the market. For these applications, ‘Powered by GaN’ has become a product brand.
The market adoption of GaN for these applications is believed to be a stepping stone for further market expansion. The technology is expected to enable a new generation of products that can enter the market with reduced form factors, smaller weight, and improved energy efficiency compared to Si-based systems.
Examples are on-board chargers and DC/DC converters in automotive applications. GaN-based transistors also have a role to play in energy storage systems and residential solar inverters, capturing their share of the still-growing PV market. They can potentially be used in power supplies for telecom and data servers, as well as for AI data centers.
GaN may also tap into the robotics market, where it can deliver compact motor drives at high operating frequencies. Or consider home appliances such as refrigerators and washing machines, where favorable energy labels are more often the deciding factor for consumers.
GaN is the go-to material for high-power applications because the critical voltage that induces breakdown of the transistor lies about 10 times higher than in Si. Also, for low-power applications (20V to 100V), GaN has an edge over Si due to its superior switching speed and compact form factor, which benefits multiple applications.
It opens the way towards small and efficient 48V DC/DC converters and 12V point-of-load (PoL) converters that we find in many electronic systems. Smartphones, tablets, or laptops, for example, contain multiple devices that work on different voltages. The mobile systems require not only AC/DC and DC/DC converters for charging the battery and distributing 12V over the entire PCB board. They also contain PoL converters that further down-convert the voltage to below 1V and deliver it to each device individually, close to its load.
One of the driving forces behind GaN technology development is the need for decarbonization and energy efficiency across industries. Mass production is currently ramping up, with multiple semiconductor companies breaking through.
But success also hinges on the ability to establish a robust ecosystem and jointly drive innovation from GaN growth to packaging solutions. Just as in CMOS, developing GaN power electronics requires close coupling between design, epitaxy, process integration, and applications.
Lateral HEMT devices, bi-directional switches, vertical GaN FETs, and more: a wide collection of GaN-based devices
Today, the bulk of GaN R&D efforts focuses on three types of devices: lateral high-electron mobility transistors (HEMTs), bi-directional switches, and vertical GaN field-effect transistors (FETs).
Lateral HEMTs: industry’s workhorse for power converters
Virtually every GaN power transistor on the market today is composed of a lateral AlGaN/GaN-based HEMT structure. In this lateral configuration, the three transistor terminals (source, gate, and drain) reside at the surface, in the same plane. By a gift of nature, the two-dimensional electron channel at the AlGaN/GaN interface forms spontaneously, with no need for external gate bias.
This means the device is normally on and requires a negative gate bias to deplete the channel and turn it off: it is a depletion mode or d-mode device. D-mode devices are the preferred choice for applications operating at high power and voltage levels. Think about on-board chargers for automotive, for which the low on-resistance of the conduction channel is a critical metric.
Most of the embedded power electronics systems, however, require normally-off (enhancement mode or e-mode) devices for fail-safe operations. The HEMT device architecture, therefore, must be modified to allow e-mode operation. The most widely used approach is to add a p-type doped GaN layer under the gate electrode.
At imec, these lateral p-GaN gate e-mode devices form the basis of a mature p-GaN HEMT platform for a variety of power electronics applications. By engineering the material stack and using adequate substrates, different p-GaN HEMT devices with operation voltages ranging between 40 and 650V have been demonstrated.
E-mode power devices can alternatively be made in so-called cascode d-mode HEMT configurations, where a d-mode HEMT is combined with a Si-based lower-voltage power device. While the HEMT enables operation at high voltages, the Si-based power device ensures fail-safe operation.
Figure 1 - Process cross-section of an imec lateral p-GaN gate HEMT device
Do you want regular updates on imec’s semiconductor research?
The great potential of lateral p-GaN HEMTs has inspired researchers to push their performance and capabilities further. Researchers at imec, for example, succeeded in aggressively scaling e-mode p-GaN HEMTs to operate at 1,200V [1].
The fabricated p-GaN HEMTs exhibit an on/off ratio of 108, a low specific on-resistance of 5.8mΩ-cm2, and a hard breakdown voltage as high as 1,800V. These features make the 1,200V p-GaN HEMTs attractive building blocks for in-rack power distribution systems in data centers, which tend to move to 800V and beyond.
Bi-directional switches: potential for battery protection and AC/AC converters
Lateral p-GaN HEMTs are traditionally used to serve AC/DC or DC/DC converter operations. Their device architecture can be modified to enable bi-directional switches, which conduct current and block voltage in both polarities. Such switches are desired, for example, in battery protection circuits and matrix converters – efficient AC/AC converters that do not require a DC link.
In traditional Si implementations, a bi-directional switch is formed by connecting two discrete unidirectional MOSFETs or IGBTs in anti-parallel or anti-series configuration. GaN HEMT technology offers a much simpler design approach by adding a second gate to a conventional HEMT structure.
By using this dual-gate architecture, imec demonstrated 650V and 1200V p-GaN bi-directional switches, providing symmetrical electrical operation in positive and negative bias conditions while achieving comparable performance to traditional HEMTs [2].
Figure 2 – SEM cross-section of imec’s 650V dual-gate p-GaN bi-directional switch [2].
Vertical GaN devices: an alternative for beyond 650V operation
Scaling 650V p-GaN HEMTs above 650V comes with a double penalty for the footprint: the gate-to-drain distance needs to increase for higher breakdown voltage and robustness, which in turn increases the on-state resistance. Hence, the footprint of the HEMT increases quadratically with the voltage for the same specific on-state resistance.
A vertical GaN-based MOSFET device is being explored as an alternative device architecture for beyond 650V operation. In a vertical GaN device, the source and gate are at the device surface, but the drain is fabricated at the bottom of the epi stack. As such, the electrical field is spread vertically throughout the whole material stack. Different vertical architectures are being investigated, including a trench-gate MOSFET architecture as shown by imec [3].
Figure 3 – Vertical GaN device with trench-gate MOSFET architecture [3].
Further on the horizon: multi-channel devices and more
Among the notable developments of the past years are multi-channel GaN devices, offering similar advantages as gate-all-around devices for traditional logic CMOS applications: larger current density at a reduced footprint, while preserving a low off current and series resistance. GaN is a relatively young technology with much room for further device exploration.
In the years to come, we expect many innovations serving next-generation power electronics devices.
The lack of p-channel GaN HEMTs: industry’s workarounds
One major concern has long occupied GaN researchers: there are no p-channel GaN devices that can operate at an acceptable level of performance.
CMOS designs use a combination of n- and p-channel devices to ensure the most efficient performance of logic operations. But in GaN, the mobility of holes is about 60 times worse than that of electrons. That means a p-channel device – where holes are the principal carriers – would be 60 times larger than the n-channel counterpart.
This ‘asymmetric’ configuration of n and p would be highly inefficient. Industry gets around this issue in several ways. In resistor-transistor logic, the p-channel device is replaced with a resistor. Although this comes with a trade-off between switching time and power consumption, it works well enough for many power electronics circuits.
An alternative is direct-coupled FET logic, resulting from the co-integration of d-mode and e-mode HEMTs. Both solutions are pursued by imec. A more disruptive approach uses n-channel GaN HEMTs with Si pMOS devices, using 3D integration techniques to stack the two devices.
Substrates for GaN epitaxial growth: upscaling to 200mm and 300mm wafer sizes
The performance of GaN devices is not only determined by the material stack and device architecture but is also strongly linked to the substrate on which the GaN material stack is grown. What is more, the choice of the substrate is of paramount importance in determining whether GaN technology is economically viable.
From a technical point of view, native GaN substrates offer the best material quality. But they are expensive and, so far, only available on small wafer diameters (up to 150mm (6-inch)). As such, they can hardly follow the trend that marks the power electronics industry today: 200mm (8-inch) substrates have become an industry standard, and upscaling to 300mm (12-inch) is considered whenever feasible.
Larger wafer diameters can lower device manufacturing costs, making them the preferred choice for cost-sensitive markets, such as fast chargers, industrial and consumer power systems, and power delivery systems for mobile devices, GPUs, and CPUs.
Si is one of the substrates that allows scaling up to larger wafers, while also allowing CMOS-compatible fabrication methods for making GaN-on-Si devices. In recent years, industry made significant progress in overcoming the challenges related to the epitaxial growth of the AlGaN/GaN heterostructure on Si. The mismatch in thermal expansion coefficient and crystal structure of the materials can be accommodated by engineering the buffer layer between AlGaN/GaN and Si, and by optimizing the epitaxial growth conditions.
Figure 4 – Lateral p-GaN gate HEMTs on a 200mm Si wafer.
Today, GaN-on-Si is on the eve of a new era: the transition from 200mm to 300mm Si wafers. The benefits of transitioning to 300mm Si wafers go beyond reducing manufacturing costs. The CMOS-compatible GaN-on-Si technology will gain access to 300mm state-of-the-art equipment that exceeds the capabilities of a 200mm facility. Expanding the processing tool and metrology ‘box’ with advanced capabilities will allow the development of more advanced GaN-based power devices
Imec is one of the pioneers that, in close cooperation with the GaN ecosystem, is supporting this development with both 300mm epi growth and GaN-on-300mm Si HEMT fabrication and characterization [4]. Prime concerns are the control over the bow of the 300mm wafers and their mechanical strength. As these issues worsen for thicker material stacks, which are needed for high-voltage applications, the first use cases for 300mm GaN-on-Si will be in the lower voltage range.
Examples are PoL converters for CPUs and GPUs. These processors typically operate at low output voltages (around 0.5V) but require extremely high currents, often exceeding 1,000A. Advanced GaN PoL converters promise to efficiently manage this power delivery without excessive losses.
Figure 5 – Mask set for 300mm GaN-on-Si module work, developed in the frame of imec’s 300mm GaN program [4].
For higher voltage applications, industry looks at alternatives such as QST® (Qromis substrate technology), a material with a polycrystalline AlN core. Compared to Si, QST® is better matched with the GaN/AlGaN buffers in terms of thermal expansion. The mechanical strength of QST® and its capability to more easily control the wafer bow have already been demonstrated on 200mm wafers, and 300mm semi-spec and CMOS-compatible QST® engineered substrates are under development. 200mm QST® was the substrate of choice for imec’s 1,200V p-GaN HEMT devices and vertical GaN FETs [1,3].
Other substrates are being looked at for specific power electronics applications, but today they are mainly in the research phase: gallium oxide (Ga2O3) and SiC-on-Si. A new and promising substrate on the horizon is enabled by Soitec’s Smart CutTM of GaN (SmartGaN technology), where a monocrystalline GaN seed layer is fabricated on top of a poly-crystalline SiC core wafer [5].
Trends in circuit and system development: various levels of monolithic and hybrid integration; increased system-level optimization
Initially, most GaN-based power systems have been formed from multiple chips. The GaN switches are assembled as discrete components and connected to other, non-GaN-based components through a printed circuit board (PCB). The downside of such an approach is the presence of parasitic inductances that affect the performance of the devices. Take a gate driver, for example, a component that supplies current to the GaN transistor’s gate to turn it on and off. Discrete GaN transistors with drivers on a separate chip suffer from parasitic inductances between the output stages of the driver and the input of the transistor. The best way to reduce the parasitics and exploit the superior switching speed of GaN is to make both driver and HEMT in GaN technology and integrate them on the same chip. Such monolithic integration has long proven difficult, mainly due to the lack of complementary GaN-based devices and difficulties in obtaining power domains on a single die for isolating the different components.
But in recent years, tremendous progress has been made in monolithically integrating GaN power switches with other building blocks. Different R&D approaches are being pursued. On the one hand, limited functionality is monolithically co-integrated to make the GaN power device more intelligent and robust. Examples of such ‘smart discretes’ arise from the co-integration with current sensors, temperature sensors, the output stage of a gate driver, and over-voltage protection circuitry. On the other hand, we see power electronics systems emerge with maximal monolithic integration, including half-bridges, full gate drivers, floating supply voltages, isolated level shifters, dead-time control, and protection circuitry, etc.
Imec has demonstrated the monolithic integration of a variety of GaN components using its GaN-on-Si, GaN-on-QST®, and GaN-on-SOI (Si-on-insulator) technology platforms. An example is a monolithic 200V GaN half-bridge IC with integrated gate drivers and level shifters. A half-bridge circuit is a fundamental topology used in power electronics, and consists of two semiconductor switches (here, p-GaN HEMTs) connected in series. In DC/DC converters, for example, a half-bridge converter efficiently steps down or steps up DC voltages. A monolithic half-bridge IC design with integrated drivers and level shifters, manufactured on imec’s GaN-IC platform, resulted in highly efficient conversion capabilities over a range of realistic operating conditions, with 98.3 percent peak power stage efficiency [6]. Other examples are imec’s half-bridges, which are monolithically integrated with drivers, current and temperature sensing circuits, both for 100V and for 650V applications. Additionally, high-performance GaN Schottky barrier diodes were added to the portfolio, enabling further enhancement of the power efficiency of GaN power ICs [7].
Figure 6 – Monolithic half-bridge with drivers, current, and temperature sensors for (top) 650V and (bottom) 100V applications.
Besides an increased level of monolithic integration, we also see examples of hybrid integration emerging for tailored performance. An example is an 800V inverter made from a combination of GaN-based 650V bi-directional switches and 1,200V SiC-based switches. Or Si-based BCD chips (chips that combine Si-based bipolar, CMOS, and DMOS technologies) with a GaN device on top, integrated with die-to-wafer or die-to-die 3D integration techniques.
The power electronics industry is not a ‘winner-takes-it-all’ market. We will see various combinations coexist, trading off power conversion efficiency, design simplicity, speed, robustness, size, and cost to the benefit of the application. With this goal in mind, increasing attention is also being paid to system-level optimization rather than component-level optimization.
Reliability, robustness, and packaging: crucial factors for technology adoption
Reliability, robustness, and packaging are also critical factors for the successful adoption and integration of GaN devices into various applications.
At the device level, reliability is affected by the management of peak electrical fields through field plate designs, charge trapping in the dielectric close to the interface with the AlGaN polarization layer, and the quality and defectivity of the epitaxial layers.
Advancements in the metal-organic chemical vapor deposition (MOCVD) growth technique and process control have played a pivotal role in mitigating these challenges.
One peculiarity of p-GaN HEMTs is their shorter lifetime at lower temperatures (e.g., -25°C) when operating under forward gate voltage bias. This can be explained by a negative activation energy of the failure mechanism. This can be of importance for certain applications and operating conditions (e.g., deploying GaN-based devices in northern countries) and necessitates extending (gate) reliability tests in the low temperature range.
GaN reliability should, however, not only be considered at the device level. Reliability and robustness can be enhanced at the system level by adding components and circuits that help control the GaN transistors’ gate and suppress transients – i.e., temporary changes in voltage or current that occur during switching operations.
Under-voltage lockouts, overcurrent protection, and temperature sensors that feed their data back to the gate drivers are all crucial to unleashing the full performance and reliability of GaN switching devices.
And finally, the high-voltage and high-power capabilities of the GaN-based devices and systems necessitate effective packaging solutions that can handle high voltages and temperatures without causing electrical breakdown.
Conclusion
GaN-based power electronics systems have tremendous potential in various industries, where they can support the overall decarbonization and contribute to the growing electrification and digitalization of society.
Advancements in GaN wafer technology, device architectures, reliability and robustness, increased experience in system-level use, and massive industry investment in the technology have recently enabled its market breakthrough. GaN-based fast chargers are now being offered commercially, but the technology is expected to tap into new markets with a completely new generation of compact and energy-efficient products.
GaN is a relatively young technology, with much room for innovation and optimization. But its success will also depend on the ecosystem's ability to collaborate intensely at all levels - from GaN IC design and GaN epi growth to final applications.
This article was originally published in PCIM Magazine.
Want to know more?
[1] ‘1.2kV enhancement-mode p-GaN gate HEMTs on 200mm engineered substrates,’ S. Kumar et al., IEEE Electron Device Letters, Vol. 45, n° 4, April 2024;
[2] ‘Monolithic 650-V dual-gate p-GaN bidirectional switch,’ G. Baratella et al, IEEE Transactions on Electron Devices, Vol. 71, n° 11, November 2024;
[3] ‘Route toward commercially manufacturable vertical GaN devices,’ K. Geens et al., IEEE Transactions on Electron Devices, Vol. 71, n°3, March 2024;
[4] ‘Imec launches 300mm GaN program to develop advanced power devices and reduce manufacturing costs,’ imec press release 2025;
[5] ‘Advanced substrates for power electronics,’ T. Boudet, 2023 International VLSI Symposium on Technology, Systems and Applications (VLSI-TSA/VLSI-DAT), HsinChu, Taiwan, 2023, pp. 1-2, doi: 10.1109/VLSI-TSA/VLSI-DAT57221.2023.10134040;
[6] ‘A monolithic 200V GaN half bridge IC with integrated gate drivers and level-shifters achieving 98.3% peak efficiency,’ D. Aygun et al., 2022 IEEE Applied Power Electronics Conference and Exposition (APEC), DOI: 10.1109/APEC43599.2022.9773755;
[7] ‘Imec demonstrates successful monolithic integration of Schottky diodes and depletion-mode HEMTs with 200 V GaN-IC,’ imec press release 2021.
