Short Circuit Events in Silicon Carbide
Power Devices

Abstract

In recent years, it has become increasingly apparent that Net Zero needs to be achieved. Electrification of transport is vital to completing this goal. The aerospace section of transportation presents unique challenges, with robustness of electronic systems being vital to realising electrification in this sector. Silicon carbide, SiC, power electronic devices are uniquely suitable for use in this extreme environment. However, the reliability of SiC power electronics needs to be fully understood in order to use them in such a safety critical application.

This thesis describes how short circuit events affect SiC power electronic devices, MOSFETs and JFETs, under conditions found in aircraft. The electrical characteristics of the devices were recorded after each short circuit event to track the evolution of damage in the device. The ruggedness of the devices is assessed and the effect of changing the ambient temperature is analysed.

For MOSFET devices, the gate oxide was identified as the key weakness under short circuit conditions, with all devices failing gate–drain short, rendering them unable to turn-on after the failure event. The failure modes of a family of MOSFETs with different values were investigated. The low resistance MOSFET, m, failed after 8~s, with a large spike in gate current. The medium resistance MOSFET failed after 11~s with no change in or to indicate the device was about to fail. The high resistance MOSFETs, =~60~m, 90~m and 140~m, exhibited a large shift in threshold voltage and flatband voltage of 0.32~V and 0.45~V respectively before failing after a 20~s short circuit event. The ambient temperature of the devices was varied within a range of temperatures relevant for aircraft, however, there was no significant change in device failure observed with ambient temperature.

For JFET devices, the increase of junction temperature during the short circuit event was identified as a source of failure. The bias conditions of the JFET were varied, but no significant change in device failure was found under the conditions tested with devices under both conditions surviving up to a 16~s short circuit event. JFETs fail explosively drain–source short. Therefore, if used in safely critical applications, extra care needs to be taken when designing the circuit to avoid catastrophic failure.

In order to further investigate the weakness in the oxide of MOSFETs, radiation from a Co source was used to artificially damage the oxide. Before any short circuit conditions were applied a difference in flatband voltage and threshold voltage of 0.49~V and 0.48~V respectively was found between the 30~krad device and the 10,000~krad device. The total ionising dose did not significantly affect the failure of the device under short circuit conditions. It was found that the high junction temperature in the device during a short circuit event has the ability to partially anneal the device, reducing the effects of radiation. This is an important result because it opens the possibility of repairing radiation damaged devices without the need to remove them from their housing. Further study is needed to fully realise the potential of using short circuits to anneal radiation damaged devices.