5. Validating GaN Robustness

In recent years, as GaN power transistors come into widespread use as switches for power converter applications, it is all the more crucial and inevitable to guarantee their reliability. In this chapter, we discuss how to evaluate the robustness of GaN power transistors. In the switching of GaN power transistors, they can be subject to the so-called current collapse that is a specific phenomenon for GaN in which the ON state resistance is increased once the device is exposed to a high voltage. Since the current collapse induces instability of the device in the form of the increase in the temperature, non-uniform internal electric field distribution and so on, it may lead to the reliability issue. Therefore the robustness of GaN transistors should be examined under switching operation besides the conventional reliability tests standardized for Si power transistors. Since current collapse is crucially dependent on the drain current – voltage locus curve during the switching event, the switching reliability of GaN transistor depends on the switching locus. Accordingly a concept of Switching Safe Operating Area (SSOA) is proposed to define the switching conditions wherein the device can be switched safely. As an example, we define the SSOA for our Hybrid-Drain-embedded Gate Injection Transistor (HD-GIT) that is now commercially available. Furthermore we propose the long-time SSOA (lSSOA) in which we guarantee the robustness of HD-GIT under long-time switching operations (e.g., 10 years). The proposed method for confirming the robustness of GaN power transistors can be utilized to estimate the devices’ lifetime when they are employed in a given switching application.