The next generation of GaN for electrification

Gallium nitride is a key component of future energy-efficient electric vehicles and 5G networks. Hexagema startup from Lund University, is developing a new solution at the Swedish Research Institute ProNano tests. This solution should promote greater electrification and a sustainable future. In an interview with Power Electronics News, Michael Bjork, CEO of Hexagem, and Michael Salter, senior project manager at Rise, noted the importance of this collaboration, which will lead to Hexagem technology being implemented in future energy applications.

“They’ve demonstrated their technology on a small scale in research labs, but now they’re moving the process to larger plate sizes in larger volumes in our lab to try to expand the technology,” Salter said.

“The advantage that ProNano gives us is that Hexagem gets access to modern equipment and tools, in particular to the MOCVD [metal-organic chemical-vapor deposition] capable of 6-inch plates, ”Bjork said. “As a rule, such a startup tool is difficult to finance on your own. In addition, Rise staff has extensive experience with GaN materials, processing and characterization. ”

Industry

Industries, from the automotive industry to telecommunications, are pushing to invest in more efficient electricity conversion and electrification, as social pressure and regulation reduce CO₂ emissions are rising. Traditional Si-based semiconductor power technologies, such as insulated gate bipolar transistors, have major limitations in terms of operating frequency and speed, as well as low performance at high temperatures and low currents. Similarly, the frequency and high-temperature performance of high-voltage Si FETs are limited. As a result, wideband semiconductors are becoming increasingly popular in many applications.

GaN power semiconductors are gaining weight as a key component in the next generation of high-performance EVs, helping to reduce size and weight while increasing efficiency. These considerations solve problems with range. Engineers can use GaN to create energy electronic systems that are 4 times smaller and lighter and have 4 times less energy loss than Si-based systems. Among the benefits is zero reverse recovery, which reduces switching losses in chargers and traction inverters, as well as higher switching frequency and speed. In addition, reducing on and off losses can help reduce the weight and volume of capacitors, inductors and transformers for applications such as chargers and EV inverters.

GaN plate

New generation energy efficient semiconductors will help create new solutions for a sustainable future. All this means less CO₂ emissions and thus improving global warming in the long run. Forecasts by many agencies show that by 2050, electricity consumption will increase to more than 200 TWh, a large amount that requires attention at the design level.

Semiconductor substrates, or plates, aim to control the electric current and thus the performance of the entire end device. Round plates are cut into stamp-sized pieces encapsulated in microchips containing millions of transistors.

Most semiconductors are made of silicon, but various options are being developed to meet the need for greater efficiency in power applications. For example, a car requires more than 1,000 chips, and the number is likely to grow. Future developments in semiconductors will contain more transistors, helping integration.

Hexagem is developing semiconductors with GaN-on-Si plates. Hexagem’s activities to develop high-quality GaN-on-Si are aimed at reducing costs and benefits on the scale of future applications. They look at higher demands in terms of stress. According to Hexagem, the proposed technology will keep costs low through the use of existing infrastructure.

GaN-on-Si technology does not have a good reputation in terms of development. This has its problems, and the cultivation of GaN-on-Si is difficult due to the mismatch of material properties at the interface at the atomic scale between GaN and Si.

“A key advantage and challenge for this technology will be to provide a lower defect density and increase the thickness of the GaN to turn on 900 to 1200 V power supplies,” Bjork said.

Manufacturers are increasingly relying on their ability to develop both a product and a supply chain at the same time. One of the main advantages of GaN-on-Si is that it is produced on Si substrates, so now the width is 150 mm, and is planned to increase to 200 mm, and most reactors can accommodate both.

Defects

In the manufacture of semiconductor epitaxial materials dislocations are formed – i.e. defects in the material. The more defects in the semiconductor, the fewer devices on the plate can be produced, which increases the cost. In addition, poor material interfaces cause more resistance to the device’s channels, resulting in more energy being consumed during operation, making the chip less energy efficient.

“We have installed and are operating a MOCVD reactor to grow GaN on a silicon wafer,” Salter said. “We are investing in this equipment and infrastructure, which requires additional equipment and specialized technological means, such as gas and water for cooling. Growing GaN with low defect density is one of the biggest challenges for the application of gallium nitride as well as quality control of the material during application. Controlling the concentration of the base material and dopants during application allows you to achieve low-resistance device contacts at the device level. It also affects the types of breakdown voltages and current levels that devices can withstand. In addition, the presence of a good interface between the semiconductor material and the contacts of the device helps to optimize the electrical characteristics of the manufactured device.

The Rise ProNano test and demonstration facility offers a variety of infrastructures to help you speed up startup testing without having to spend a lot of money on expensive equipment. Nanowires are fabricated using MOCVD ProNano technology to produce thin layers of GaN on the Si plate. The plates are processed and validated at each level to create samples in which GaN nanowires can “grow” and then merge together to obtain high-quality thin epitaxial layers. The plate is then examined under an electron microscope to check its quality.

According to Hexagem, there are currently 100 million defects per square centimeter in GaN-on-Si semiconductors. “It would be a density of dislocations of the order of 10⁸ see^–2; The current state of Si plates today is from medium to high 10⁸ see^–2“Bjork said. Hexagem believes that we can significantly improve the quality of the material and soon approach 10 million defects per square centimeter and thus surpass competitors.

“Defects are usually dislocations that cause leaks in the opposite state,” Bjork said. “With Hexagem technology, we can reduce dislocations in two stages: first, by filtering GaN buffer dislocations by growing GaN nanowires, and second, by carefully merging wires into a flat GaN layer that controls the formation of new dislocations.”

Vertical waffles

The goal is to have larger plates or produce thicker vertical GaN semiconductors. According to Hexagem, today the most common layers are 2 to 4 microns thick. Their goal is to produce solutions with a thickness of 10 microns in 2022.

“Larger plates mean improved economies of scale by increasing the number of devices per plate,” said Bjork. which will be important for the market of devices from 900 to 1200 V. It will also allow vertical design of the device, which will bring benefits such as decoupling voltage scaling from the device location.Hexagem aims to develop high-quality GaN-on-silicon for 1200- B and licensing technology to industrial partners. ”

Hexagem technology allows current to flow vertically rather than horizontally across the plane of the plate, thus reducing the size of the components, but most importantly, they can handle even higher currents and voltages.

Semiconductor processing

As noted above, MOCVD is the main procedure for fabricating transistors from semiconductor materials with a large band gap. Specific gases (or vapors) flow at specified temperatures and pressures on the substrate surface in the MOCVD system. The upper layers “develop” into a crystalline structure thus, one atomic layer at a time, across several layers. The technique is known as epitaxy, or crystal growth. To create 3D nanostructures, such as nanowires, layers can be grown over the entire surface of the substrate or on individual portions of the substrate.

Epitaxial layers can be grown using the MOCVD process on substrates of various materials such as silicon, silicon carbide, GaN, diamond or sapphire. Materials have varying degrees of difficulty to obtain multiple crystal defects.

Next, plate transistors and other electronic components are created by additional processing steps common to semiconductor manufacturing, such as lithography, etching, or metal or insulator deposition. Finally, the finished waffle is cut into parts the size of a postage stamp, which are enclosed in an electronic package, tested and shipped along supply chains, such as Samsung, Volvo, Apple and Ericsson, and finally get into the microprocessor. in a mobile phone or car.

The main steps in creating GaN plates on silicon are washing the substrate followed by image growth in a clean medium and further cleaning the substrate during image growth. The next step is to create GaN semiconductors using MOCVD epitaxy. Based on the specific technological need, the substrate is placed in a MOCVD chamber to produce crystal layers in various combinations of GaN, AlGaN or InGaN. An electron microscope is then used to study the material and tests are performed to confirm the electrical conductivity and other properties of the material.

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Maurizio Di Paolo Emilio holds a PhD. in physics and is a telecommunications engineer. He has worked on various international projects in the field of gravitational wave research, developing a thermal compensation system, X-rays, space technology for communication and engine control. Since 2007, he has collaborated with several Italian and English blogs and magazines as a technical writer specializing in electronics and technology. From 2015 to 2018 he was the Editor-in-Chief of Firmware and Elettronica Open Source. Maurizio loves to write and tell stories about power electronics, wide range semiconductors, automotive, IoT, digital, energy and quantum. Maurizio is currently the Editor-in-Chief of Power Electronics News and EEWeb, as well as a European correspondent for the EE Times. He is the host of PowerUP, a podcast about power electronics. He has contributed to a number of technical and scientific articles, as well as several Springer books on energy collection and data acquisition and management systems.