Special Report: Gallium nitride for satellites
SweGaN is a small agile start-up company producing cutting-edge gallium nitride (GaN) material for space applications. Linköping, a small town in southeast Sweden, is the centre of development for the revolutionary material gallium nitride (GaN) on silicon carbide (SiC) substrates, which promises enormous savings on telecom infrastructure and huge reductions in CO2 emissions. The development was made by researchers at Linköping University (LiU), who eventually spun out a company called SweGaN.
SweGaN, which is now commercialising the technology, uses a hot-wall chemical vapour deposition (hot-wall CVD) technique initially developed for SiC epitaxy. The fledgling company has now finalised two years of commercial activity, making steady headway with increasing revenues and small profits. SweGaN maintains its close ties with LiU, but now performs a significant amount of the development.
SweGaN’s competitive edge
Growing hetero-epitaxial material is often difficult. Where one normally always has to deal with lattice mismatch and, more importantly, the mismatch in coefficients of thermal expansion (CTE), GaN on SiC is particularly difficult as you need to grow a thin layer of aluminium nitride (AlN), normally called the ‘nucleation layer’, in order for the GaN to grow in a two-dimensional fashion. Though the AlN layer is very thin (typically 100 to 200nm, but in SweGaN’s case it can be reduced to only about 30nm) the interface creates a thermal boundary to the SiC. It acts almost like a thermal blanket locking the generated heat losses in the GaN material. SiC is the preferred substrate because of its high thermal conductivity and the best performing devices are obtained using GaN-on-SiC material. Much of the point in having a high thermal conductivity substrate, however, is lost when much of the heat is imprisoned in the top GaN layer, where the device is formed.
SweGaN, together with LiU, has developed a patented process that greatly reduces the thermal barrier, improving heat dissipation through the barrier by around five times compared to the closest competitor. This will significantly lower the channel temperature, improving efficiency, and reducing the risk of device failure. Heat is the number one enemy for a device. If you reduce the channel temperature by only 25°C, you can increase the mean time to failure of the device by a factor of ten. Thanks to the high quality nucleation layer, SweGaN’s material also sports a significantly lower dislocation density that enables much more advanced high-electron-mobility transistor structures.
Another major competitive advantage of SweGaN is the very high mobility of the structures. This has been achieved with another patented process that sharpens the interface between the channel layer and the AlGaN barrier layer. The electrons pass through a narrow channel created by a barrier. The mobility is a material property indicating how well the electrons can move in this channel and it is limited by the roughness of the interface and so-called ‘random alloy scattering’ of the barrier. With the sharpening process SweGaN can offer material with mobilities around 2,200cm2/Vs, which is about 30% higher than the material from the closest competitor. The improved mobility results in a higher efficiency of the devices.
SweGaN’s material for space applications
SweGaN looks at space as a very small but highly prestigious market to be in, where its material advantages can be fully exploited. Thermal management is an issue high on the list for satellite manufacturers. Anything that runs hot must be cooled down through radiation. Thermal management, however, is both expensive and heavy. By increasing the heat dissipation inside the material, heat can be more easily removed by the cooling equipment of the satellite, or, to put it a different way, less cooling equipment is required to achieve a lower channel temperature of the device, which saves both weight and substantial amounts of money.
The high efficiency is also a very important issue for satellite manufacturers. Less power is required to run the devices, resulting in a reduction of the solar panels needed for a communication satellite.
SME Instrument phase 1
SweGaN has recently won an SME Instrument phase 1 grant and will use this opportunity to look closer at the space market, and to highlight its competitive advantage by working through some numbers and determining milestones together with manufacturers of communication modules and satellites. The company will also generate some test devices with their partners that process the material to demonstrate how devices made on SweGaN’s material performs. This will help in generating milestones and putting numbers to SweGaN’s competitive advantage.
SweGaN will also look at how well they can scale the process and what the production costs will be in full-scale operation.
SweGaN only makes material today. However, it is considering the expansion of its activities to start manufacturing devices through ‘fab-less’ device manufacturing, where the company can tailor the device design, taking the best advantage of their material quality. During phase 1, SweGaN will examine the commercial and technological advantage/risk and opportunity of moving in this direction. The company welcomes new partners to collaborate or discuss with. At the end of phase 1 SweGaN hopes that the verification will show that it should apply for the SME Instrument phase 2.
Olof Kordina
SweGaN AB
+46 70 495 0594
