By Llew Vaughan-Edmunds
 

As technology evolves, so does the demand for more power. Wide bandgap (WBG) materials such as gallium nitride (GaN) are demonstrating their potential as the backbone of next-generation power semiconductors. They use less power and deliver performance superior to that of well-established silicon devices. Such applications as consumer chargers, data centers, 5G, and electric vehicles are key growth markets with the same requirements: smaller size, more power, less loss.

The compound semiconductor GaN fulfills all of these, which will be the key to its adoption in coming years. Compared to silicon, GaN offers superior switching performance, loses less heat energy during switching, and can function with increased stability at higher temperatures, enabling engineers to build more compact, faster, and more reliable devices while reducing the need for cooling mechanisms.

THE DEMAND FOR POWER

Smartphones

As smartphones become more power hungry in order to run multiple apps at faster speeds, battery life barely lasts the day. Moreover, a standard 5W charger charges extremely slowly. Smartphone manufacturers are starting to realize the consumer need for fast charging and are readying a new generation of larger chargers that deliver up to 65W to significantly reduce charge time. Using GaN-based high-electron-mobility transistors (HEMTs) can halve the size of the charger while tripling its power and enabling it to run up to 20 times faster than silicon-based superjunction MOSFET (SJMOSFET) designs.

Data Centers

The growth of cloud computing, mobility, IoT, machine learning, and streaming services has created a huge need for more data to be stored and computed. As a result, more than 7 million data centers exist today that consume more than 200TW of power. This is equivalent to approximately 2% of the global electricity consumed in 2019 and equates to the same carbon dioxide emissions as the global aviation industry. Approximately 30% of the electricity is used to cool these facilities. By improving server efficiency and reducing power and heat losses, a significant amount of energy could be saved, reducing both electricity costs and the facility’s carbon dioxide footprint.

Server power supplies consist of a power-factor correction (PFC) stage (e.g., totem pole) and a resonant DC-DC stage (LLC resonant converter). The output voltage is usually 12V; however, the trend is towards 48V, as power-hungry CPUs and specialized GPUs demand more power. Also, the higher voltage reduces power losses up to 16 times in distribution rails. GaN can benefit each stage of the converter (Figure 1). For the PFC stage, its low capacitance and zero reverse recovery allow for a simple totem-pole configuration; for the LLC converter stage its faster switching and smaller losses make possible smaller magnetics and capacitors. Minimal dead times in synchronous rectification using GaN also reduces losses.

Figure 1. GaN transistors can substantially increase the power density of server boards over that of existing MOSFET designs. (Source: GaN Systems, 2020)

Electric Vehicle On-Board Chargers

The proliferation of electric vehicles is contributing to the demand for faster charging and higher efficiencies. In 1996, General Motors released the EV1, using a 16.5kW lead acid battery. Its range was 70–90 miles and it took 7.5 hours to charge. Today, the Tesla Model 3 has a 80kW lithium-ion battery with a range of 310 miles and a charging time of 35 minutes using Tesla’s V3 Super Charger.

The on-board charger (OBC) is located within the car and converts and charges the battery from a power source. It must be highly efficient, lightweight, and reliable. Today’s solution typically consists of using silicon SJMOSFETs to condition, convert, and transfer the power into the battery. It’s roughly 18 inches by 25 inches, weighs close to 13 pounds, and has an efficiency of approximately 94%.

Next-generation OBCs are replacing SJMOSFETs with GaN high-electron-mobility transitors (HEMTs) whose higher switching frequency reduces the size of the magnetics, capacitors, and heatsinks in the system. This reduces the overall system size and weight by 30–40%, while achieving efficiencies close to 97%.

THE GROWING MARKET FOR GaN

Before 2019, the GaN power market was primarily in niche applications. In the last year, however, GaN has appeared in fast chargers (>28W) sold with smartphones. Its smaller form factor, high efficiency, and cost-competitiveness for this application make it highly desirable for phones and also for laptop computers. Key applications for GaN are switch-mode power supplies (SMPS) because they allow fast switching and demand high efficiency. The main growth segments are expected to be travel adaptors (<100W), server power, OBCs, and wireless charging. We are now seeing the initial ramp of GaN in travel adaptors, and once the technology becomes trusted in the field, we expect it will be adopted into higher-power, more critical applications such as the automotive and data center markets (Figure 2).

Figure 2. GaN market adoption in electric vehicles awaits reliability confidence; this will evolve from consumer chargers and process improvements in high-volume manufacturing. (Source: © 2019 IHS Markit)

Silicon is not, however, on its way out. SJMOSFETs dominate the market and are the choice technology in the segments cited above. Silicon technology is well established and trusted, and it will evolve still further. Additionally, designers have years of experience working with these devices; therefore, segmentation of technologies can be expected, depending on the sophistication of the system.

Today, GaN is competing against SJMOSFETs for SMPS, high-speed insulated-gate bipolar transistors (IGBTs) for uninterrupted power supplies, medium-voltage MOSFETs for telecommunications, and low-voltage MOSFETs for server point-of-load voltage regulators and synchronous rectification. As these markets are extremely price sensitive, GaN is expected to be introduced first in the high-end segments (Figure 3).

Figure 3. GaN is suited for high-frequency power supplies, while SiC is appropriate for applications requiring higher power and robustness, such as motor drives and industrial power. As WBG devices become better established in the market, technology adoption will become more defined. (Source: Yole Développement)

GaN DEVICE MANUFACTURING CONSIDERATIONS

Each process involved in manufacturing GaN HEMTs must be very precise to achieve the best device performance and reliability. The fast switching, high power density, and high voltage breakdown of WBG devices demand extremely high-quality epi layers and dielectric deposition with precise etching and metal deposition.

MOCVD

MOCVD is critical in creating the GaN device as it grows the various epi layers above the substrate. Defect density, within-wafer uniformity, and wafer-to-wafer repeatability are key considerations in MOCVD development, especially when transitioning to 200mm. Given the different lattice constants and thermal coefficients during expansion of GaN and silicon, growing epitaxial GaN on silicon to create stable and reliable HEMTs is a challenging process in super lattice structures and stress management.

Etch

Etching is also a critical process in manufacturing GaN devices. Two notable challenges are the high selectivity of GaN/AlGaN and the potential for an over-etch on the AlGaN from the p-GaN etch to cause roughness on the surface, which reduces sheet resistance. Additionally, HEMTs with recessed gates require a certain AlGaN thickness, which must be well controlled and highly repeatable. Atomic-layer precision and advanced endpoint monitoring are essential.

CVD

The GaN HEMT structure typically has multi-layer field plates to minimize voltage peak stresses at gate-drain contact and dynamic RDS(on). Films such as SiO2 and SiN are used as dielectrics. These must be of the highest quality to minimize film contamination and reduce thermal degradation at elevated temperatures while improving film stoichiometry. In addition, film stress must be controlled to avoid wafer bowing. This can be achieved through RF power among other process parameters.

Surface passivation of SiN has been proven to improve two-dimensional electron gas conductivity by creating higher carrier concentration, which in turn improves device performance. Alternative materials, such as Al203, use atomic layer deposition to improve device performance.

PVD and Plating

GaN HEMTs are lateral devices with a very high current density, so most of the losses happen at the top of the die. In a normal discrete package, the bottom of the die would be attached to the copper lead frame. However, the silicon substrate has a relatively low thermal conductivity, which results in a higher operating junction temperature of the device. Operating too close to the maximum junction temperature adversely affects reliability and temperature-dependent characteristics such as RDS(ON). Therefore, it is crucial to promote heat transfer away from the die. Implant techniques that reduce the resistance of the ohmic contact help improve heat dissipation. Also, depositing thick copper on the top of the die increases thermal capacity and conductivity, while allowing for the possibility of sintering copper lead frames and clips. This improves reliability in power cycling and significantly reduces mechanical stress resulting from mismatched coefficients of thermal expansion.

CONCLUSION

As energy demands rise, the search for greater power efficiency is spurring growing interest in GaN as a successor to silicon semiconductors in high-power, high-efficiency applications such as adaptors, 5G, data centers, and electric vehicle chargers. However, manufacturing GaN devices demands exceptional film quality and a high degree of process precision in epitaxy, dielectric and metal deposition, and etch. While its market was very limited before 2019, GaN is now taking hold in travel adaptors, with automotive and data center applications expected to follow once the reliability of the technology has been firmly established.

For additional information, contact llew_vaughan-edmunds@amat.com