Test Setup to Study Threshold Voltage Shift of GaN-HEMTs Under Short- and Long-Term Gate and Drain Bias

GaN-HEMTs offer low on-state resistance and fast switching transitions. Thus, these devices represent attractive candidates to build high efficient power converters with high power density. Unfortunately, charge-trapping effects deteriorate on-state resistance and affect threshold voltage. Undesired threshold voltage shifts impact the switching losses and can cause false turn on due to miller currents. As data sheets lack information regarding threshold voltage instabilities, power electronics engineers need to conduct own measurements. Since charge trapping exhibits time constants in the range of microseconds up to hours, short-as well as long-term studies are required. Furthermore, trapping exhibits several dependencies on operation parameters like temperature as well as drain and gate bias. This calls for a custom test setup capable of acquiring threshold voltage under application-oriented conditions. The one proposed in this paper only requires standard laboratory equipment typically present in every power electronics lab. The test setup alternates between stress or relax phases and short measurement pulses for extracting the threshold voltage under certain conditions. The results show a strong influence of the drain and gate bias on the threshold voltage with long time constants.