Take on GaN measurement challenges

& -December 10, 2012

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 support space-sensitive or heat-sensitive applications. High power semiconductor test applications are becoming increasingly demanding, requiring test instrumentation capable of characterizing significantly higher rated voltages and peak currents than ever before. Even more significant, breakdown and leakage test are typically performed at 2–3 times the level of the rated or operating voltage, making the need for instrumentation capable of sourcing and measuring ever-high voltages increasingly obvious. When the devices are in the ON state, they have to pass through tens or hundreds of amps with minimal loss; when they are OFF, they have to block thousands of volts with minimal leakage currents.

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 given that it is still young in its life cycle, it will see significant improvement in the years to come.

GaN materials typically have much higher power density, smaller size, better high temperature performance, higher frequency response, lower leakage, and lower ON resistance than silicon, all of which add up to greater operating efficiency. In turn, devices based on GaN offer faster switching speeds and 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. GaN-based devices also have far lower leakage than their silicon equivalents, so at the same time as there is a need for sourcing higher voltages in testing, there is a need for greater current measurement sensitivity. It can be quite challenging to characterize these new devices at very low levels of current.

From a test perspective, these materials require far more capable test equipment than silicon-based technologies did. For example, for an oscilloscope, that means that not only higher bandwidth but greater sensitivity is necessary. Gone are the days of using a voltage probe out of the box as-is and expecting little signal distortion and loading.

For I-V characterization and test applications, solutions such as custom systems that attempt to integrate power sources with current measurement instruments simply can’t provide the low current accuracy required to characterize next-generation devices and materials. As single-quadrant devices, power supplies cannot sink power; therefore, they require several seconds for the capacitance charge to bleed off after testing, which slows the test process, which is particularly problematic in production applications. In addition, in many cases, they lack the necessary power to support today’s operating or characterization levels. Such custom-designed systems also typically require large test engineering teams to develop and maintain them. Although commercial ATE systems have always been used for power semi production test applications, their cost, size, and lack of characterization and low current measurement capabilities make them equally impractical for R&D and QA/FA applications.

In this article, we’ll provide an overview of GaN and then focus on the test challenges.

Power breakthroughs
The market for GaN power semiconductors is forecast to grow from almost zero in 2011 to over $1 billion in 2021, according to a 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 300V. 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 reports 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.; 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 could be located right where most of the power conversion needs to happen, reducing long wiring runs and IR losses. GaN devices can stand 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.

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