Aluminum nitride, diamond semiconductor, gallium oxide and vertical GaN are all ready, each with its own advantages and disadvantages.

After years of research and development, several suppliers are close to shipping power semiconductors and other products based on next-generation wideband gap technology.

These devices take advantage of the properties of new materials, such as aluminum nitride, diamond, and gallium oxide, and they are also used in different structures, such as vertical gallium nitride power devices. However, although many of these technologies have characteristics that exceed those of today's power semiconductor devices, they will also face challenges in the process of transferring from the laboratory to the fab.

Power semiconductors are usually specialized transistors used as switches in high-voltage applications such as automobiles, power supplies, solar power, and trains. These devices allow current to flow in the "on" state and stop in the "off" state. They increase efficiency and minimize energy loss in the system.

For many years, the power semiconductor market has been dominated by devices using traditional silicon materials. Silicon-based power devices are mature and inexpensive, but they have also reached their theoretical limits.

This is why people are interested in devices that use wide band gap materials, which can surpass the performance of today's silicon-based devices. For many years, suppliers have been shipping power semiconductor devices based on two wide band gap technologies-gallium nitride (GaN) and silicon carbide (SiC). Power devices using GaN and SiC materials are faster and more efficient than silicon-based devices.

Several suppliers have been committed to developing equipment using next-generation wide band gap technology. These materials, such as aluminum nitride, diamond, and gallium oxide, have a greater band gap energy than GaN and SiC, which means they can withstand higher voltages in the system.

Today, some suppliers are shipping special LEDs that use aluminum nitride. Others plan to launch the first wave of power devices built around new materials in 2022, but there are also some challenges. All these technologies have various disadvantages and manufacturing problems. Even if they go into production, these devices will not replace today's power semiconductors, whether it is silicon, GaN or SiC.

"They provide incredible high performance, but very limited in wafer size," said David Haynes, managing director of strategic marketing for Lam Research. "They are largely more academic rather than commercial interest, but as technology advances, this situation is changing. But the small substrate size and lack of compatibility with mainstream semiconductor manufacturing technology means they may only be used For the small-batch production of extremely high-performance equipment, especially for demanding applications such as smart grid infrastructure, renewable energy, and railways."

Nevertheless, there is a wave of activities here, including:

Figure 1: Different materials and gaps. Source: company report/semiconductor engineering

What is a power semiconductor? Power semiconductors are used in power electronic equipment to control and convert the power in the system. They can be found in almost every system, such as cars, mobile phones, power supplies, solar inverters, trains, wind turbines, etc.

There are many types of power semiconductors, each of which is represented by a number with a "V" or voltage. "V" is the maximum operating voltage allowed in the device.

Today's power semiconductor market is dominated by silicon-based devices, including power MOSFETs, super junction power MOSFETs, and insulated gate bipolar transistors (IGBTs).

Power MOSFETs are used in low voltage, 10 to 500 volt applications, such as adapters and power supplies. Super junction power MOSFETs are used in 500 to 900 volt applications. At the same time, IGBT is a leading mid-range power semiconductor device for 1.2 kV to 6.6 kV applications, especially automotive applications. Infineon’s senior vice president of sales, marketing and distribution Shawn Slusser said: “IGBT power models are basically replacing fuel injectors in cars.” They supply power from batteries to motors. "

IGBTs and MOSFETs are widely used, but they have also reached their limits. This is where wide band gap technology comes in. "The band gap refers to the energy difference between the top of the valence band and the bottom of the conduction band in a semiconductor," Infineon said. "The greater distance allows wide band gap semiconductor power devices to operate at higher voltages, temperatures and frequencies."

The band gap of silicon-based devices is 1.1 eV. In contrast, the band gap of SiC is 3.2 eV, while the band gap of GaN is 3.4 eV. Compared with silicon, these two materials enable devices to have higher efficiency and smaller form factors, but they are also more expensive.

Each device type is different. For example, there are two types of SiC devices-SiC MOSFET and diode. Silicon carbide MOSFETs are power switching transistors. Silicon carbide diodes pass current in one direction and block current in the opposite direction.

For 600V to 10kV applications, silicon carbide power devices use a vertical structure. The source and gate are on the top of the device, while the drain is on the bottom. When a positive gate voltage is applied, current flows between the source and drain.

Silicon carbide is manufactured in a 150mm wafer fab. In the past few years, silicon carbide power semiconductors have been put into mass production. Paul Knutrud, Marketing Director of Onto Innovation, said: "Silicon carbide has high breakdown field strength, thermal conductivity and efficiency, making it an ideal choice for power conversion chips for electric vehicles.

Figure 2: How today's power switches are classified. Source: Infineon

Developing vertical GaN Several suppliers have been developing products based on next-generation materials and structures, such as aluminum nitride, diamond, gallium oxide, and vertical GaN.

In years of research and development, vertical GaN devices are promising. GaN is a binary III-V group material used in the production of LEDs, power switching transistors and radio frequency devices. The breakdown field of GaN is 10 times that of silicon. "High power and high switching speed are the main advantages of GaN," said Onto's Knutrud.

Today's GaN power switching devices are manufactured in a 150mm fab and are based on high electron mobility transistors (HEMT). GaN devices are lateral structures. The source, gate, and drain are located on the top of the structure. Horizontal GaN devices have been put into mass production.

Some companies are putting GaN devices into production in 200mm wafer fabs. "For GaN, it is the improved performance of GaN-on-silicon technology at 200mm and even 300mm in the future. This is the basis for technological development," Lam's Haynes said.

Today's GaN devices use silicon or SiC substrates. On top of the substrate is a thin layer of aluminum nitride (AlN), then an AIGaN buffer layer, and then a GaN layer. Then, a thin AlGaN barrier layer is deposited on top of the GaN to form a strained layer.

Today, several companies are participating in the GaN power semiconductor market. Today's lateral GaN power semiconductor devices operate at 15 to 900 volts, but there are several technical challenges in operating these devices beyond these voltages.

On the one hand, there is a mismatch between different layers. "It's really just because when you grow GaN on different substrates, you end up with a lot of defects due to the mismatch between the two crystal lattices. Many defects per square centimeter can lead to premature breakdown and reliability. Sexual issues,” said Rick Brown, CTO of Odyssey Semiconductor.

Work to solve these problems is ongoing, but lateral GaN is currently staying below 1,000 volts. This is where vertical GaN fits. It promises to operate at 1,200 volts and above.

Like other power semiconductor devices, vertical GaN devices have a source and gate on the top of the device, and a drain on the bottom. In addition, vertical GaN devices use bulk GaN substrates or GaN-on-GaN. According to Odyssey, the GaN substrate allows vertical conduction GaN transistors with fewer defects.

"If you look at silicon-based high-voltage devices and silicon carbide high-voltage devices, they are both vertical topologies. For many reasons, it is the preferred topology for high-voltage equipment. It occupies a smaller area, which reduces capacitance and is There is an inherent safety factor in having high-voltage terminals on the other side instead of the gate terminals," Brown said.

Figure 3: Horizontal GaN device. Source: Odyssey Semiconductor

Figure 4: Vertical GaN device. Source: Odyssey Semiconductor

Today, Kyma, NexGen, Odyssey, Sandia and other companies are studying vertical GaN devices. Kyma and Odyssey are adding 100 mm (4 inches) bulk GaN substrates.

"Vertical GaN is emerging, and we are selling products to researchers and laboratories," said Jacob Leach, Kyma's chief technology officer. "The industry has encountered some challenges in making epitaxial wafers. We have different technologies. We can manufacture the films required for vertical GaN at low cost."

The GaN substrate is ready, but the development of the vertical GaN device itself is more difficult. For example, manufacturing these devices requires an ion implantation step to implant dopants into the device. "The only reason people didn't use a vertical conductive topology for GaN was that there was no good way to do impurity doping. Odyssey has found a solution," said Brown of Odyssey.

Odyssey is developing vertical GaN power switching devices in its own 4-inch wafer fab. The plan is to ship in early 2022. Other people's goals are in the same period.

"We have vertically conductive GaN devices. We have demonstrated pn junctions," said Odyssey CEO Alex Behfar. "Our first product is 1,200 volts, maybe 1,200 to 1,500 volts. But our roadmap takes us all the way to 10,000 volts. Due to capacitance and some other issues, we hope to be in the frequency and voltage range that silicon carbide cannot access In the near future, we hope to be able to provide equipment for industrial motors and solar energy. We hope to give electric vehicle manufacturers opportunities to further increase the range of vehicles. That is by reducing the weight of the system and having better performance equipment. From In the long run, we hope to realize functions such as mobile charging."

If or when vertical GaN devices emerge, these products will not replace today's lateral GaN or SiC power semiconductors, nor will they replace silicon-based power devices. But if the technology can overcome some challenges, vertical GaN devices will have a place.

Seanchy Chiu, senior director of technology development at UMC, said: "GaN vertical devices on bulk GaN substrates have brought some excitement to the possible next-generation power electronic equipment, but there are still some key issues that need to be resolved." "Based on physics, vertical Power devices can always drive higher power output than lateral devices. But GaN bulk substrates are still expensive, and the wafer size is limited to 4 inches. Pure foundries are using 6-inch and 8-inch processes to manufacture competitive power Device. Because of its vertical carrier transport, it is necessary to control the quality of the substrate crystal and minimize defects."

There are other questions. "GaN substrates are more expensive than SiC substrates, and the electron conduction in the vertical direction in GaN is only roughly the same as SiC," said Alex Lidow, CEO of EPC, a supplier of horizontal GaN power semiconductors. "Compared with SiC, the lateral mobility of electrons in GaN is 3 times higher, but the mobility in the vertical direction is the same. In addition, the heat conduction efficiency of silicon carbide is three times higher. This leaves little motivation for vertical GaN devices. "

Gallium Oxide Semiconductor At the same time, several companies, government agencies, R&D organizations and universities are studying β-gallium oxide (β-Ga2O3), which is a promising ultra-wide band gap technology that has been developed for several years.

Kyma said that gallium oxide is an inorganic compound with a band gap of 4.8 to 4.9 eV, which is 3,000 times larger than silicon, 8 times larger than silicon carbide, and 4 times larger than gallium nitride. Kyma said that gallium oxide also has a high breakdown field of 8MV/cm and good electron mobility.

Gallium oxide also has some disadvantages. This is why gallium oxide-based equipment is still in the research and development stage and has not yet been commercialized.

Nevertheless, some suppliers have been selling wafers based on this technology for research and development purposes for some time. In addition, the industry is studying gallium oxide-based semiconductor power devices, such as Schottky barrier diodes and transistors. Other applications include deep ultraviolet photodetectors.

Flosfia, Kyma, Northrop Grumman Synoptics, NCT and other companies are studying gallium oxide. The U.S. Air Force and the Department of Energy and several universities are pursuing it.

Kyma has developed a 1-inch diameter gallium oxide wafer, while NCT is shipping 2-inch wafers. NCT recently developed a 4-inch gallium oxide epitaxial wafer using the melt growth method.

"Gallium oxide has made progress in the past few years, mainly because you can produce high-quality substrates. Therefore, you can grow gallium oxide ingots by standard Czochralski or other types of liquid phase growth methods," Kyma Said Leach.

This is a crystal growth method widely used in the semiconductor industry. The biggest challenge is to manufacture power devices based on this technology.

"The challenge of gallium oxide is twofold. First of all, I have not seen a real p-type doping method. You may be able to make p-type thin films, but you will not get any hole conductivity. Therefore, making bipolar devices has been Not under consideration. You can still make unipolar devices. People are studying HEMT-type structures in diodes and gallium oxide. Some opponents say, “If you don’t have a p-type, then forget it. This just means it’s in place. There are not so many applications in the field," Leach said. "The second biggest is thermal conductivity. Gallium oxide is quite low. For high-power type applications, this can be a problem. In the conversion, I don’t know if this will become a killer. People are doing engineering work to oxidize Gallium is combined with silicon carbide or diamond to improve thermal performance."

Nevertheless, the industry is still studying equipment. "The first power device using gallium oxide will be a Schottky barrier diode (SBD). We are developing SBD and aim to start sales in 2022," said Takekazu Masui, an official and senior sales manager at NCT.

NCT is also developing high-voltage vertical transistors based on this technology. In the NCT process, the company developed a gallium oxide substrate. Then, it forms a thin epitaxial layer on the wafer. The thickness of this layer can range from 5 μm to 10 μm.

By using a low donor concentration and a 40μm thick epitaxial layer as the drift layer, NCT achieved a breakdown voltage of 4.2 kV. The company plans to produce 600 to 1,200 volt gallium oxide transistors by 2025.

NCT has overcome some of the challenges of gallium oxide. "Regarding thermal conductivity, we have confirmed that we can obtain thermal resistance that can be put into practical use by making the components thinner like other semiconductors. So we don't think this will be a big problem," Masui said. "NCT is developing two p-type methods. One is to make gallium oxide p-type, and the other is to use other oxide semiconductors such as nickel oxide and copper oxide as p-type materials."

Looking to the future, the company hopes to develop equipment that uses larger substrates to reduce costs. Reducing defects is another goal.

Diamond and aluminum nitride technologies For many years, the industry has been looking for diamonds that may be the ultimate power device. Diamond has a wide band gap (5.5 eV), high breakdown field (20MV/cm) and high thermal conductivity (24W/cm.K).

Diamond is a metastable allotrope of carbon. For electronic applications, the industry uses synthetic diamonds grown through a deposition process.

Diamond is used in industrial applications. In the field of research and development, companies and universities have been working on diamond field effect transistors for many years, but it is not clear whether they will move out of the laboratory.

AKHAN Semiconductor has developed diamond substrates and coated glass. The equipment-level development is in the research and development stage. "AKHAN has realized 300mm diamond wafers to support more advanced chip requirements," said Adam Khan, founder of AKHAN Semiconductor. "In high-power applications, the performance of diamond FETs is better than other wide band gap materials. Although AKHAN's doping achievements are huge, manufacturing equipment around customer expectations requires a lot of research and development, technical skills and time."

There are many variations of this technique. For example, Osaka City University has demonstrated the ability to bond GaN on a diamond substrate, creating a GaN semiconductor technology on diamond.

Aluminum nitride (AlN) is also of interest. AlN is a compound semiconductor with a band gap of 6.1 eV. According to AlN substrate supplier HexaTech, the field strength of AlN is close to 15MV/cm, the highest of any known semiconductor material.

Gregory Mills, vice president of business development at HexaTech, a subsidiary of Stanley Electric, said: “AlN is suitable for ultra-short wavelength, deep ultraviolet optoelectronic devices at the edge of the band as low as about 205nm. “In addition to diamond, AlN has the highest thermal conductivity of these materials. It can achieve excellent high-power and high-frequency equipment performance. AlN also has unique piezoelectric capabilities and can be used in a variety of sensors and radio frequency applications. "

Several suppliers can provide AlN wafers with diameters of 1 inch and 2 inches. AlN has begun to receive attention. Stanley Electric and other companies are using AlN wafers to produce ultraviolet LEDs (UV LEDs). These dedicated LEDs are used for disinfection and decontamination applications. HexaTech said that when microorganisms are exposed to wavelengths between 200 nanometers and 280 nanometers, UV-C energy can destroy pathogens.

"As we said, devices based on single crystal AlN substrates are transitioning from R&D to commercial products, depending on the application area," Mills said. "The first of these is deep ultraviolet optoelectronics, especially UV-C LEDs. Because of their ability to sterilize and inactivate pathogens (including the SARS-CoV-2 virus), there is a surge in demand."

Many years ago, HexaTech received an award from the U.S. Department of Energy for developing aluminum nitride power semiconductors. There are several challenges here. First, the substrate is expensive. "I don't know how significant aluminum nitride is here, because it has problems with both n-type and p-type doping," Kyma's Leach said.

Conclusion Despite this, devices based on various next-generation materials and structures are making progress. They have some impressive properties. But they must overcome many problems.

EPC’s Lidow said: “This means that a large amount of capital investment will be required to put them into mass production.” “The additional benefits and the size of the available market need to justify the large capital investment.”

Related stories The silicon carbide competition begins as SiC develops towards higher voltages, BEV users get faster charging, longer cruising range, and lower system costs. Improve the reliability of GaN and SiC power semi-wars

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