On the short-circuit and avalanche ruggedness reliability assessment of SiC MOSFET modules

MOSFETs, GTOs, IGBTs, and IGCTs are power semiconductors that provide building blocks for a number of converter circuits in higher-power applications. Silicon carbide has however appeared recently as a wide bandgap semiconductor. This material has received particular attention considering that is has been known to replace silicon counterparts in a number of applications. This is reference to the excellent physical properties of silicon carbide. Above all, state of the art silicon carbide gadgets have been reported in literature attaining blocking voltage of about 10-15kV for MOSFETs.

However, scarce information is available in literature about the reliability of silicon carbide devices. This is particularly when several gadgets are connected in parallel inside the package for higher current ratings. A few researchers have focused on reliability problems associated to gate oxide interface quality and others have focused on the reliability potential of single die or discrete packages. Although chip level reliability originates from the manufacturer where several tests including thermal cycling, high reverse bias, high humidity, and high temperature are done, operational reliability parameters remain closer to the end user to judge the merit of a module.

In their strategic research dealing with high power electronics for energy transmission and distribution networks researchers led by Dr. Muhammad Nawaz at ABB corporate research in Sweden proposed to address the short circuit capability and unclamped inductive switching avalanche ruggedness of various voltage/current rating silicon carbide MOSFET power modules from different vendors. They selected a set of five different modules with varying current ratings in order to evaluate the device and package technological maturity in view of operational reliability. Their research work is published in Microelectronics Reliability.

The authors performed short circuit tests under hard-switching fault conditions. When a short circuit occurred, the current began to rise with a slope dictated by dc-link voltage, how fast the G-S capacitance changed, and stray inductances. At one point, the current peaked at a value that depended on the gate pulse duration implemented to initiate the gate terminal of the device under test.

A short circuit pulse duration of the modules was consistently increased until failure occurred. Different failure mechanisms were observed where all the three device terminals became shorted at the same time and when only the D-S terminals were shorted.

The authors performed short circuit experiments at room temperature for all modules and selected one module to be tested at varying temperatures in order to study the effects of temperature on the short circuit survivability. A set of tests was also performed where the pulse duration was varied gradually from 0µs to 10µs. All modules successfully passed the short circuit test at 500V without failure until 10µs.

From the point of view of the user, 67% of the rated voltage was used here, as nominal supply voltage considering that most field applications demand this range of supply voltage. Therefore, the authors selected 800V as a nominal voltage for 1200V class module.

A module rated at 1.7kV survived short circuit tests at voltages of up to 1000V for a 4µs pulse duration. However, the module failed when the authors increased the supply voltage to 1100V. Before failure, a gate-source voltage drop was observed, and was associated with a high G-S leakage current. The main failure mode was however observed to be the thermal runaway, which led the gadgets into avalanche breakdown mode. In the course of the Unclamped Inductive switching tests, a good number of samples from the three vendors failed. The failure of these modules was caused by external diode connected in parallel with the MOSFETs. One module from the same vendor, which did not have external diode and another from a different vendor with an external diode, survived the unclamped inductive switching tests.

The study by Claudiu Ionita and colleagues successfully provided a benchmark for the usage of SiC modules in various power applications.