One of the hottest and most competitive segments in electronics design and therefore test these days is power. That’s because of the strong demand for energy efficiency to make the most of batteries, help lower energy bills, or to support space-sensitive or heat-sensitive applications.
After 30 years, silicon MOSFET development has approached its theoretical limits. Progress in silicon has slowed to the point where small gains involve significant development cost. Alternative semiconductor materials such as Silicon Carbide (SiC) and Gallium Nitride (GaN) are emerging as the materials of choice. GaN in particular is gaining favor in many areas due to its ability to utilize silicon as a substrate, therefore bringing prices in line with silicon MOSFET. And since it is still young in its life cycle, it will see significant improvement in the years to come.
These new materials provide efficiency not only through faster switching speeds but also lower turn on voltages (Rds On). Of course, any new technology introduces not only a unique set of design challenges, but test and measurement challenges as well. From a test perspective, these materials require test equipment that not only has higher bandwidth, but is also more sensitive. Gone are the days of using a voltage probe out of the box as-is and expecting little signal distortion and loading. In this article, we’ll provide an overview of GaN and then focus on the test challenges.
The market for GaN power semiconductors is forecast to grow from almost zero in 2011 to over $1 billion in 2021, according to a new report from IMS Research(1). The research firm analyzed all of the key end markets for the products and found that power supplies, solar inverters and industrial motor drives would be three main drivers of growth.
The report notes that the speed of GaN transistor developments has accelerated in the last two years. The launch of International Rectifier's "GaNpowIR" and EPC's "eGaN FET" devices started the low voltage market in 2010. And Transphorm's 600V GaN transistors opened the possibility of GaN competing with high voltage MOSFETs and IGBTs.
A key reason for the bullish forecast for GaN is new processes that leverage existing production infrastructure. These fab processes bring down the cost of GaN semiconductors from around 10X that of traditional silicon to a competitive level, especially for applications that require the performance boost. The basic approach is to grow GaN on top of a silicon substrate with an aluminum nitride buffer layer.
EPC’s process (2), for example, begins with inexpensive silicon wafers. A thin layer of aluminum nitride (AlN) is grown on the silicon to isolate the device structure from the substrate. The isolation layer for 200 V and below devices is 300 V. On top of this, a thick layer of resistive GaN is grown. This provides a foundation on which to build the GaN transistor. An electron-generating material is applied to the GaN. This layer creates a quantum strain field with an abundance of free electrons. Further processing forms a depletion region under the gate. To enhance the transistor, a positive voltage is applied to the gate in the same manner as turning on an n-channel, enhancement-mode power MOSFET as shown in Figure 1. This structure is repeated many times to form a power device. The end result is an elegant, cost-effective solution for power switching.
Figure 1. EPC’s GaN leverages existing production infrastructure to bring cost efficiencies.
In terms of applications, the IMS report predicts that GaN will gain traction at first in power supplies where the total system cost savings outweigh the unit price penalty of the device. These include PC and notebook adaptors, servers, etc., and domestic appliances like room air-conditioners, PV microinverters, electric vehicle battery charging and other new applications are likely to adopt GaN power devices in the future.
With their wide bandgap, GaN devices are very attractive for high-temperature applications. For instance, automobile manufacturers are interested in GaN devices for power conversion in hybrid vehicles. In the past, engine designers have used silicon power MOSFETs in these applications, but typically needed to locate electronics far from the engine block due to temperature concerns. Ideally, the power semiconductors would be located nearby for shorter wiring runs, less weight and lower IR losses. GaN devices reportedly can withstand temperatures of up to 300°C and continue to operate efficiently.
In information processing and storage systems, the whole power architecture can be reevaluated to take advantage of the outstanding switching capabilities of GaN materials. As output voltage increases for AC/DC converters, efficiency goes up. As bus voltage increases, transmission efficiency goes up. As frequency increases, size goes down. EPC claims that GaN enables the last stage which enables the first two while increasing AC/DC efficiency when used as synchronous rectifiers. They also allow for intermediate stage converters to be removed for single step conversion, saving the size and cost of the intermediate stage converter.