Meeting SiC and GaN measurement challenges

Keithley, a Tektronix company, designs and produces a variety of advanced electrical test instruments and systems for electronics manufacturers in high-performance production testing, process monitoring, product development, and research. Keithley and Tektronix together keep an eye on the pulse of the industry, according to Lee Stauffer, senior staff technologist at Keithley. “One of the areas really growing fast is power electronics,” he said. That growth is being driven by the push for higher efficiency and the green-energy movement with the capability to obtain power from a variety of sources—including wind and solar—and convert it into a form that can be used in applications ranging from lighting to industrial control.

“This whole concept of power management is something that Keithley and Tektronix are really focusing on now … the end-to-end power management cycle.” That cycle, he added, extends from the component level to power-electronics system compliance.

Keithley and Tektronix make a variety of instruments that can be used throughout that cycle, as described in a recent article on power-device test.1 As an example, Stauffer cited a power-supply design, “Tektronix oscilloscopes and high-power probes and Keithley power supplies and DMMs are used a lot in the design cycle of a power supply,” he said, adding that within the last two years Tektronix and Keithley also have introduced a new line of power analyzers that can measure efficiency and determine EMI compliance.

Of particular use in power-device test, Stauffer said, are Keithley’s SourceMeter source-measure units, including a recently introduced 3,000-V model useful for measuring SiC and GaN devices, which have very low leakage currents and very low capacitances. Silicon devices are lossy, he said, but SiC and GaN can inherently switch a lot faster. Now, he said, you can build power devices and turn up the frequency in your converter, adding, “We’ve actually seen some GaN power converters that are running at megahertz speeds.” Keithley instrumentation, he said, enables measurement of picoamps of leakage current in contrast with microamps or tens of microamps of leakage in silicon devices.

In addition to the instruments, Keithley has developed cabling and interconnect as well as new test fixtures and probe-station interfaces. “We really had to engineer the complete component test solution,” he said. “And this is something that Keithley has been doing in semiconductor tests for more than 30 years. This core technology of being able to measure semiconductors both at a package level and at a wafer level is really what sets Keithley apart from others in measuring power components.”

In addition, Stauffer said, Keithley is adding modern user interfaces to its traditional instruments. Keithley’s “Touch, Test, Invent” maxim, he said, takes a page from consumer electronics and adds a touch-screen graphing interface to the SourceMeter. “So you can actually get curves, graphical curves, right on the SourceMeter itself,” he said. “That’s really handy and convenient for a quick characterization of components.” And Keithley has gone a step further and lets you control SourceMeter directly from your Android smartphone—enabling you to email measurement results. “Keeping up with modern user interfaces is very important to us,” he said.

Another area on which Keithley focuses is data conversion. “Keithley has always been known for our very high-precision A-to-D converters,” Stauffer said. “We measure things out to 7½ digits with more than 24 bits of precision.” More recently, the company has added high-speed digitizing capability. “For example, our most recent generation of SourceMeters and our newest digital multimeter have both the high-precision A-to-D and the high-speed digitizing capability,” he said. “That’s a particularly interesting combination in a precision instrument—you now can do high-speed, transient-type measurements, or you can do the high-precision measurements that we’re noted for.”

Many vendors offer instruments in open-architecture modular form factors that can be integrated into systems. Stauffer said Keithley designs many of its instruments for system use, but instead of employing a standard like PXI, Keithley has implemented what it calls the Test Script Processor (TSP) platform, which includes a high-speed virtual backplane.

“The advantage of having this virtual backplane is that we can add as many instruments as we want,” Stauffer said. The approach also supports very high-power instruments. For example, he said, one of the company’s SourceMeters is 2-kW-capable, a level of power that calls out for a standalone instrument configuration. But the TSP link virtual backplane, he said, allows multiple SourceMeters to be connected together and respond to each other as if they were one instrument.

And TSP extends from the lab to the production floor. “The TSP architecture allows us to create a test system, whether it’s the simplest little curve tracer on the bench for doing just initial device evaluation or a full-blown production parametric process-monitor system,” Stauffer said. “We have a lot of flexibility there, and we also support that entire life cycle”—from initial concept through obsolescence and the search for a new and better device.

Reference
Nelson, R., “Pursuing efficiency from wafer to system,” EE-Evaluation Engineering, August 2015, p. 16.