Author: Richard Stevenson
In our field, if everyone has a common desire, it may be desirable to obtain a large-size GaN single crystal substrate that is acceptable in cost. This is because the current GaN substrate is small in size and high in cost, which limits the nitride. Device development. The difficulty in producing GaN single crystal substrates has prompted many companies to find alternative ways to find other substrate materials to grow nitrides.
In the past ten years, a company is exploring along the above technical route, which is the French epitaxial film expert Picogiga (a subsidiary of Soitec). Jean-Luc Ledys, the company's chief operating officer, said, "We are the only company in the world that offers GaN HEMT epitaxial wafers based on three different substrates."
Aonex's wafer development process involves moving a 1" GaN layer onto a 2" polycrystalline AlN substrate. Image source: Aonex
Picogiga's first wide bandgap product is a SiC-based GaN epitaxial wafer. Its performance is very good, because it uses a SiC single crystal substrate, which is expensive. In order to supply low-cost solutions, Picogiga has subsequently introduced Si-based GaN epitaxial wafers, but the thermal properties of Si substrates are not good, these products are only suitable for secondary applications. Recently, Picogiga has opened up a third technical route, polycrystalline SiC-based GaN (SopSiC); its moderate price and thermal properties are similar to SiC crystals.
Ledys explains: “SopSiC is a thin layer of silicon that is bonded to polycrystalline SiC by bond bonding.†There are three reasons for choosing polycrystalline SiC, one that is readily available on the market, and the other is only costly. 10-20% of single crystal SiC, the final polycrystalline SiC process is compatible with Picogiga's MBE process and can upgrade wafer growth to 12".
Ledys believes that this substrate is suitable for the production of high voltage Schottky diodes for rectification, but the voltage of Si-based devices is limited to 250V. At the moment, "either Si-based bipolar devices, but not efficient; or SiC-based devices, but expensive."
The key to producing SopSiC substrates is the Smart Cut technology, which has been used to make insulator SiC (SiCOI) materials. A few years ago the company made the first 2" SiCOI substrate, but it was later shelved. "For us or our customers, SiCOI has not been a good one, no matter which side." The reason is that the supply of SiCOI is not good, and we rely too much on SiC suppliers, which makes no sense to expand the source material supply chain. â€
Picogiga is also developing GMOO materials for microelectronics applications, but the company notes that GaN is a major potential choice for homoepitaxial growth of high-brightness white LEDs . Ledys admits, “There are serious bottlenecks in GaN substrates like SiC, that is, there are very few people who can grow GaN single crystal substrates. However, we are prepared to go all out and even consider ourselves to grow GaN substrates.â€
At Aonex Technologies, the core of the prototype substrate is also wafer bonding, which is the bonding of sapphire or GaN films to polycrystalline AlN. The cost of this method is relatively low, and the power handling capability of GaN is considerable. James Zahler, one of the founders of the California startup, believes that "we are now focusing on AlN-based GaN, considering both market and technology." The substrate grown from this synthetic material meets the requirements of epitaxial wafers. Its mass is equivalent to that grown on single crystal GaN, but at a lower cost, making it an ideal material for lasers and high performance LEDs.
For vertical structure LEDs, Zehler believes that the sapphire substrate on AlN is the second, lower cost solution. He believes that the substrate can be produced by flip chip bonding for LED production, which is simpler than laser stripping, which is commonly used to strip sapphire on nitride epitaxial wafers. In addition, the thermal expansion coefficients of the nitride epitaxial layer and the underlying AlN are relatively matched, so that the epitaxial wafer is less prone to warpage during the growth process.
Another option is a metal substrate, which is being developed at the Naval Research Laboratory in Washington. James Freitas, a member of the project team, explained, “The metal substrate has ohmic contact and very good thermal conductivity, which is very attractive to us.â€
The surface morphology and electrical properties of nitride films grown on TiC substrates are somewhat similar compared to materials grown on SiC and sapphire substrates. Excellent carrier injection ensures the electrical properties of TiC, which is more conducive to the production of LEDs and lasers. However, the current problem is that the defect density in the nitride epitaxial layer is still too high. At the same time, although this substrate material is a promising candidate, to achieve commercial production, it is still necessary to expand the size to the standard of industrial production - currently the size of Freitas is only 5 × 5mm. However, Freitas expressed regret that "the lack of interest in single crystal TiC materials". If you continue to follow up on this technology, it may point to another way out for tricky nitride growth.
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