Empowering oscilloscopes to measure GaN

One of the hottest segments in electronics design and test these days is, believe it or not, power. That's Tektronix technical marketing engineer Randy White's takeaway from the Applied Power Electronics Conference and Exposition (APEC 2012) last month in Lake Buena Vista, FL. Conference participants, White said in a recent phone interview, were looking boost efficiencies and reduce power losses by moving to new semiconductor materials. Silicon, White said, is reaching its performance limits, and it's no longer cost-effective to try to squeeze out 3% or 4% better performance through fab and process improvements. Consequently, just as scope makers like Tektronix have moved to technologies like SiGe for their high-performance instruments, power electronics companies are moving to technologies including silicon carbide (SiC) and gallium nitride (GaN).

GaN is particularly promising, said White. It offers a wide bandgap that translates to low on-resistance, and it's served in demanding aerospace applications for nearly 20 years. The technology has traditionally been expensive, but, White said, International Rectifier has modified its fab process to grow GaN devices on a silicon substrate using a standard, low-cost CMOS process. Such innovations are opening up commercial, industrial, and automotive applications areas.

Such devices—operating at hundreds of volts and tens or hundreds of megahertz—are posing serious test and measurement challenges. High bandwidth and high voltage are usually mutually exclusive, White said, but the combination represents the sweet spot for GaN.

Not only will scopes require more precision to measure these devices—they will need high resolution as well in order to measure 600-V source-drain voltages along with millivolt-level gate voltages. Averaging techniques, White said, can reduce noise and improve resolution for repetitive waveforms, but users often need to capture single-shot transient waveforms to debug a circuit and understand why it might shut down unexpectedly. One approach would be to employ boxcar averaging, White said.

White cited an APEC presentation titled “Putting GaN to Work in your Power Supply” by Robert V. White, consultant to Efficient Power Conversion, that provided a good overview of GaN technology while highlighting measurement challenges. High voltage single ended probes capable of operating at 2.5 kV and 8000 MHz might suffice for 600-V devices, the EPC consultant said, but noted that the best high-voltage differential probes only operate to about 100 MHz. The consultant also questioned whether a 5-GS/s scope with 1-GHz bandwidth and 350-ps rise time would be sufficient for measuring GaN circuits, and he added that directly measuring current in high-power devices is pretty much impossible.

In general, Randy White said, Tektronix's approach to high-voltage, high bandwidth measurements is to educate the market about probing and optimizing scope setup for high-power applications.

White doesn't expect IR to mass-produce GaN parts using its silicon substrate process for another 12 to 18 months, giving companies like Tektronix to develop strategies to handle the devices. Nevertheless, he said, “The pressure is on the test-and-measurement vendors.” As to how Tektronix and other vendors will respond, stay tuned.