Why Test & Measurement Matter to Wide Bandgap Semiconductors?

Article By : Maurizio Di Paolo Emilio

Keysight's Alan Wadsworth and Mike Hawes review the critical aspects of Test and Measurement for analyzing wide-bandgap solutions.

Assessing the quality and durability of electronic devices is the domain of the test and measurement (T&M) function. T&M is particularly important for gallium nitride, both because of the wide-bandgap material’s immense potential to deliver the higher-efficiency power conversion need for emerging applications and because GaN lacks silicon’s long history in the field. The characteristics of switches based on wide-bandgap semiconductors, such as high electron mobility, high breakdown field, and much lower on-resistance, have encouraged power design engineers to start considering the possibilities for improved power. WBG power modules offer features and capabilities that are orders of magnitude greater than those of their silicon counterparts, including 10× the voltage-blocking capability, 10× to 100× the switching speed, and one-tenth the energy losses. They are also intrinsically radiation-hardened (rad-hard) and offer a theoretical junction temperature operation of up to 600°C. The technology could provide power systems with power densities up to 10× higher than current silicon-based devices, in addition to lower cooling requirements. SiC promises lighter-weight components for lower fuel consumption and lower emissions for the aerospace industry. This material facilitates higher switching and higher power density for a given voltage and current rating in a smaller, lighter device. GaN, like silicon, can be used to create semiconductor devices such as diodes and transistors. A power supply designer could choose a GaN transistor instead of silicon for its small form factor and high efficiency. GaN transistors also dissipate less power and offer higher thermal conductivity than silicon devices with higher thermal management requirements. GaN transistors are increasingly used in various fields — in the automotive sector, for the supply of electrical power, and for the conversion and use of current — and are expected to replace their silicon predecessors in those applications. EE Times Europe asked Alan Wadsworth, business development manager for Keysight Technologies’ precision and power products, and Mike Hawes, a power solutions consultant at Keysight, to review the critical aspects of T&M for analyzing wide-bandgap solutions. EE Times Europe: Power semiconductor manufacturers believe that GaN-based devices hold the key to addressing a primary hurdle for the expansion of renewable energy. What are the challenges in terms of T&M to guarantee a good GaN-based product? Alan Wadsworth: GaN has some fundamental advantages over silicon when used in a switch mode power converter — for example, higher voltage, reduced losses, more compact [form factor], and faster switching. However, it is a newer technology with more risk than “tried and true” Si-based power semiconductors. Therefore, there are several challenges for GaN-based product designers.

Characterization of GaN devices

GaN device characterization presents some challenges that are common to all wide-bandgap devices and some that are specific to GaN. One of the common challenges is that data sheets are often insufficient to accurately represent the needed operating performance of the device. There is also a need to characterize very small on-resistance accurately under normal operating conditions (which are at hundreds of amps and thousands of volts). The device must be accurately characterized under thousands of volts of drain-to-source bias. Gate-charge characteristics must be measured under a range of operating conditions (which, again, are at hundreds of amps and thousands of volts). And there is a need for dynamic characterization, including conduction, drive, and switching losses. GaN devices also have some characteristics that are unique to them and are not shared, for example, with SiC devices. The unique feature of GaN devices is that they can experience a phenomenon known as current collapse. This behavior causes the on-resistance of the transistor to change dynamically after turning on, with the initial value of the on-resistance dependent on both the magnitude of the drain voltage and the time spent in the off state. For obvious reasons, unless this phenomenon is suppressed or understood, then it not possible to design reliable circuits using GaN devices.
Dynamic power device analyzer
Figure 1: Dynamic power device analyzer with double-pulse tester (PD1500A) (Image: Keysight)

Modeling of GaN devices

Because of the fast-switching capabilities for GaN devices — for example, rise and fall times of less than 10 nanoseconds — accurate device models are rare. Simulators that are incapable of distributed circuit analysis will not accurately reflect the ringing and instabilities in the design. Many traditional power device models and simulators will not reflect the true operation of the design, causing a trial-and-error approach to design with multiple prototype iterations.

Reliability of GaN devices

[Because it lacks] the decades of use in power supply/converter designs [that silicon can claim], GaN requires significant testing to determine the reliability of the devices in mission-critical applications like renewable energy. A significant amount of investment is being made by the industry to better evaluate the capability of GaN power devices to meet the needs of multiple power applications. Within the last few years, JEDEC created the JC-70 working group to develop appropriate standards for WBG power semiconductors, in particular GaN and SiC. The areas of focus for the subgroups within JC-70 are reliability, datasheets, and test methods/characterization. EETE: How does T&M for wide-bandgap semiconductors compare with the process for silicon? Can you provide an example? Mike Hawes: One key difference between Si and WBG devices, especially GaN, is switching speed. A GaN device typically switches 10 to 100 times faster than a comparable silicon transistor. The switching frequency of a GaN device operating in a power conversion circuit is, itself, not fast enough to create issues, but higher-frequency components in the switching waveform can create electromagnetic interference [EMI]. EMI is, of course, an issue for silicon transistors [as well], but it is more difficult to mitigate in GaN devices. The reason is that faster devices produce faster voltage changes, which can potentially cause the false turn-on of field-effect transistors. If this were to occur, then the resulting surge current creates tremendous heat that could cause catastrophic circuit failure. Device-modeling software can help prevent these sorts of issues in switching circuits, if it can accurately predict their behavior. However, accurately measuring the switching behavior of GaN devices is not an easy task. EETE: What are the requests of your clients in the GaN Market, in particular for the energy sector? Hawes: Based on the issues discussed in item #2, many customers want an off-the-shelf, standardized, and supported solution to accurately characterize the switching behavior of GaN devices. This is typically accomplished using what is known as the double-pulsed test method. In 2019, Keysight introduced a commercially available double-pulse test (DPT) solution (PD1500A) for silicon and SiC devices, that has been installed at several customer locations worldwide. Keysight enabled the PD1500A to accept a customized GaN DUT board, to allow DPT for GaN HEMT and GaN GIT devices. GaN cascode devices should be supported shortly. Although a customized GaN DUT board is required, the PD1500A is now capable of being configured to test discrete Si MOSFETs, IGBTs, SiC MOSFETs and GaN MOSFETs EETE: GaN is designed to provide solutions for some of the major societal challenges in the fields of digitalization, energy efficiency, and mobility of the future. What are the technologies that can offer innovation for leadership in the T&M market? Wadsworth: Characterization of GaN devices is critical for designers in the markets you’ve mentioned. The industry is struggling to measure the dynamic characteristics of GaN devices with such high voltage and current requirements, combined with the high switching speed (bandwidth) requirements. Parasitics in the characterization equipment and setup often mask the real dynamic performance of the GaN devices being tested [requiring]  new, low-inductance approaches to the DPT fixture design, as well as current measurement and even connection of the device to the fixture. Models need to be improved to consider the high-frequency characteristics of the GaN devices coupled with simulators that accurately consider a distributed circuit analysis, which is capable of simulating high-frequency effects. Because of the temperatures of power converters used in alternative energy and e-mobility, modeling/simulation needs to correctly represent the thermal effects of the design. It is an exciting time for power conversion designers. However, as is typical with new technology changes, additional modeling, characterizing and reliability testing are required to enable the GaN power semiconductor to achieve its high expectations in these markets.

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