Role of Threshold Voltage Shift in Highly Accelerated Power Cycling Tests for SiC MOSFET Modules

In silicon carbide (SiC) power MOSFETs, threshold voltage instability under high-temperature conditions has potential reliability threats to long-term operation. In this paper, the threshold voltage shifts caused by the instability mechanisms in accelerated power cycling tests for SiC MOSFETs are investigated. In conventional power cycling tests, the positive threshold voltage shift can cause successive ON-state resistance increases, which can sequentially increase junction temperature variations gradually under fixed test conditions. In order to distinguish the increased die voltage drop from the bond wire resistance degradation, an independent measurement method is used during the power cycling tests. As the number of cycle increases, SiC die degradation can be observed independently of bond wire increases during the tests. It is studied that the SiC die degradation is associated with the threshold voltage instability mechanisms. Unlike the bond wire lift-off failure, the die degradation and the related die resistance increase can stop the power cycling test earlier than expected. In addition, a new test protocol considering the die degradation is proposed for the power cycling test. By means of power device analyzer, the failure mechanism and the degradation performance of SiC MOSFETs before and after the power cycling test are compared and discussed. Finally, experimental results confirm the role of threshold voltage shifting and identify different failure mechanisms.