Wide-band gap materials such as gallium nitride (GaN) and silicon carbide (SiC) have long promised that they can lower the system costs of designs while providing better on-resistance, breakdown voltage and smaller transistors than their counterparts made from silicon.
The band gap of a material is related to the strength of the chemical bonds between the atoms in the lattice. These stronger bonds mean that it is harder for an electron to jump from one ring to the next. The consequences are lower intrinsic-leakage currents and higher operating temperatures for the higher-band gap semiconductors. These improvements provide higher power-conversion efficiency and reliability, and cost-effectiveness.
GaN vs. Si Price Parity
The consensus in the semiconductor industry is that SiC will be used for designs mainly above 900 V and GaN for those typically less than 900 V. Recently, some GaN houses have pushed the cost of GaN devices down to parity with similar Si-based devices. With that in mind, let’s look at GaN designs and where they will be used when there is price parity with silicon. For example, the newest families of eGaN FETs from EPC are now lower in price than silicon MOSFETs with the same on-resistance and voltage ratings. This is the first time in 60 years that a non-silicon device has been higher performance and lower price than a silicon counterpart. These eGaN FETs were launched in May 2015 and are starting to replace MOSFETs in traditional strongholds like point-of-load DC/DC converters and isolated DC/DC converters for servers and telecom equipment.
GaN-vantages
Companies producing GaN solutions are picking their niche with some working with 600 V and up (GaN Systems and Transphorm), while others are finding their market in voltages less than 250 V (EPC). For example, Transphorm, the first company with a JEDEC-qualified device prefers to work at >600 V. They see GaN penetrating data centers and telecom power because it provides bridgeless power-factor correction with efficiency ratings greater than the Energy Star’s Titanium+ rating (90 percent to 94 percent efficiency across all loads). Transphorm says the first designs are now moving to production. Photovoltaic inverters made from GaN are 50 percent smaller in size and 50 percent less in weight than silicon. Transphorm says they are working with Yaskawa and Tata on new GaN-based PV inverters.
LED lighting is another up-and-coming design for GaN, according to Transphorm, because it enables a greatly increased density of the driver. For example, a studio light company featured a product at CineGear Expo that used GaN to shrink the ballast design by 70 percent and weight by 50 percent.
EPC says that wireless power, RF-envelope tracking, LiDAR, satellites and several medical applications are using eGaN FETs because of the speed and size advantage. In wireless power, it’s mostly speed advantages as the transmission frequency of 6.78 MHz is too high for silicon. For RF-envelope tracking it’s also speed, as the required switching speeds are too high for even LDMOS transistors. LiDAR systems use the time-of-flight of light to measure the distance to an object. The eGaN FET’s speed translates into measurement accuracy and speed of measurement. That’s why eGaN FETs are dominant in LiDAR-based autonomous vehicles and mapping cars. eGaN FETs are also naturally tolerant of the radiation that exists in outer space. This has motivated many satellite manufacturers to implement eGaN FET-based DC/DC converters and LiDAR systems in space-based products.
Additionally, many medical power applications demand high-frequency operation that conventional Si solutions can’t perform because they have reached their limit. Using GaN enables customers to operate at the desired frequency with high reliability, thanks to the lower power loss and heat dissipation of GaN solutions. Covidien published a paper in the APEC proceedings, showcasing a 1-MHz surgical knife design that uses a GaN design.
Wide-band Gap in Automotive
Dr. Achim Strass, Application Engineering Manager at Infineon, via IDTechEx, says the primary trends in electric-vehicle (EV) traction inverters include wide-band gap semiconductors, double-sided cooling and making conformal shapes for in-wheel motors and for integration into other parts. Reducing cost and improved robustness are important. Inverters are becoming more complex, offsetting cost reduction. Additionally, Dr. Strass says traction inverters will continue to be the third most important item of cost in an EV, increasing as a percentage when there is a move from one motor to more per vehicle. There is already a trend to use more than one electric motor per vehicle, increasing the inverter market faster than the growth-rate of the whole e-vehicle business.
In the IDTechEx report, Power Electronics for Electric Vehicles 2015-2025, Dr. Peter Harrop, Chairman of IDTechEx, says, “Savvy suppliers are already wrestling with what the even later wave of new components can offer such as lithium-ion capacitors and GaN semiconductors. Free market suppliers in China will have to follow. Otherwise their customers will increasingly freeze them out as with the in-wheel motors of the best-selling pure electric bus in the world—the K9 from BYD of China which makes its own key parts.”
Where No Si Can Go
The future for wide-band gap material such as GaN is bright because it will be able to offer design solutions that Si technology simply can’t provide. For example, RF envelope tracking describes a concept where the power supply voltage applied to the power amplifier is continuously adjusted to ensure that the amplifier is operating at peak efficiency for power required at each instant of transmission. According to Alex Lidow, CEO of EPC, it has been around since 1929 but wasn’t possible to implement with tubes or silicon, not even with high-speed LDMOS transistors. It requires too high a frequency and power efficiency. The concept is used to squeeze more digital bandwidth from RF amplifiers for mobile communications. Now, with eGaN FETs, there are transistors powerful enough, high enough in voltage, and fast enough to make this application a reality. The same is true for the colonoscopy pill developed in Israel by Check Cap. This is a sophisticated, high-resolution X-ray machine that has been squeezed into a small pill that can be swallowed. The X-ray information is wirelessly transmitted to a receiver and the pill is low enough in cost (partly thanks to eGaN FET costs) that it does not need to be recovered after it has finished its job. The result is a no-hassle, highly accurate, low-cost colonoscopy for the masses.