By Ali Husain, Corporate Marketing & Strategy, ON Semiconductor
Power electronic devices are vital to the operation of nearly every device that runs off electricity, yet they are an underappreciated segment of the semiconductor space.
Generally, they are found in power converter circuits, to convert AC from the grid to DC voltage, or between DC voltages, or run electric motors (DC to AC conversion), and more. Their continuing improvement has increased the efficiency of every electronic device, and are today considered crucial to enabling energy savings and carbon-reducing technologies such as LED lighting, solar power generation and electric vehicles.
Regularly found in power electronics today are semiconductor materials are the so-called wide-bandgap (WBG) materials, Silicon Carbide (SiC) and Gallium Nitride (GaN). They gained their name due to their electron energy bandgap is wider than silicon’s, which gives them very beneficial characteristics such as lower electrical resistance and higher frequency switching than the current power-workhorse devices IGBTs and MOSFETs.
Although often grouped together, in reality, SiC and GaN have important differences between them, with separate “sweet spot” where the material is most suitable.
Wafer sizes are the first big difference: SiC ingots are grown from a single-crystal seed wafer by chemical vapour deposition (CVD) or physical vapour transport (PVT). Both methods require high temperature and are slow compared to creating silicon ingots. SiC wafers also require slicing the ingot into disks. Since this is a very hard material and difficult to cut – even with a diamond saw, it proves challenging. There are several methods to do ingot separation, but they can introduce defects into the single crystal.
In contrast, GaN substrates are not cut from ingots, but are grown by CVD on top of silicon wafers, which in itself is a challenge since there’s a lattice constant mismatch between the two materials. There are methods to engineer this strain, but there is a risk of defects which will affect reliability. Because GaN sits on top of the silicon, GaN power devices are considered “lateral”, i.e. their source and drain are on the same side of the wafer. – in contrast to silicon and SiC power switches, where the main current path is vertical, through the chip.
SiC and GaN also have different voltage levels where they are optimal. GaN devices rated for breakdown voltage of about 100V will find uses in mid-voltage power conversion from 48V down, with applications in cloud computing and telecom infrastructure. Additionally, power supplies and wall warts will contain 650V GaN power switches, which is the right voltage rating for AC to DC conversion with the wide input voltage range of 90-240VAC. The high frequency of GaN allows the passive components of the power supply to be significantly smaller, resulting in overall much more compact solution.
In contrast, SiC devices are designed for 650V and above. It is at 1200V and higher that SiC becomes the best solution for a many applications, such as solar inverters, electric vehicle chargers and industrial AC to DC conversion. Another long-term application is the solid-state transformer, where the current copper and magnet transformers are replaced with semiconductors.
The next revolution in power electronics is already upon us. The up-and-coming wide-bandgap materials of silicon carbide and gallium nitride will help make the future of power electronics more efficient and more compact for a wide variety of applications.