Mitigation of Single Event Burnout for Future Aerospace Applications

Abstract

With the global challenge to achieve Net Zero, the balance between greenhouse gasses produced and removed from the atmosphere, by 2050, aviation, a major contributor to the generation of greenhouse gas emissions, producing over 900 million tonnes of CO2 emissions annually, is committed to a step change in propulsion technology. The most realistic strategy to achieve these drastic CO2 reductions is through the electrification of aircraft, which will utilise the all-electric and hybrid power systems. The realisation of these MW scale power systems will inevitably rely on the incorporation of wide bandgap semiconductors, such as SiC - a semiconductor with excellent material properties, including critical electric field strength, high thermal conductivity and high electron saturation drift velocity. To make this goal a reality, the superior properties of SiC are required - through offering the ability to operate at power levels significantly beyond those of traditional Si.However, at the current time, knowledge of the interaction of radiation from cosmic rays with SiC power electronic devices is unknown, limiting their adoption in flight critical aerospace applications.

The increasing supply voltages required to make the future of flight a reality will generate extremely high electric fields within devices. Coupled with the elevated concentration of cosmic rays at flight altitudes, these conditions create the perfect storm for the Single Event Effect (SEE) - the instantaneous alteration of device response to radiation interaction. The destructive form of the SEE is the Single Event Burnout (SEB), which results in the catastrophic failure of a device, with often explosive consequences. SiC is rapidly becoming the semiconductor of choice to enable circuits to operate with high supply voltages. However, the response of SiC power devices during operation to radiation is unknown. In this research it is shown that SiC, in comparison to Si, offers a 60% reduction in cosmic ray sensitivity when equivalent voltage ratings are considered. It has been found that Si fails when a
deposited charge equivalent to 0.2% that of a silver ion commonly used in SEB testing. In contrast the radiation response of SiC is superior, with no failures occurring for
any deposited charge up to three times greater than those used in testing for any bias derating up to 99% of the breakdown voltage. Here the data show that SiC is robust
against aerospace specific operating conditions and has the potential to replace Si as the material of choice for high reliability aerospace applications. The suitability of 2D SiC structures for aerospace, however, remains unclear.

For future aerospace applications the SiC JFET is the device of choice for use in power systems and flight control surfaces. With strong electrical performance and minimal response to total dose effects it seems to be the ideal candidate, however, questions still exist with regards to its single event response. For the first time the SEB sensitivity of a JFET designed for real world aerospace specific scenarios has been examined. The 2D nature of the device results in increased SEB sensitivity with elevated electric fields at the gate and source - leading to catastrophic failure and device melting at drain - source bias deratings as low as 40%. To mitigate this the separation between the gate and source has been increased - leading to radiation hardened designs which reduce the SEB sensitivity. By increasing the gate - source separation of the original JFET by 4.0 µm a peak electric field reduction of 64% is observed which had resulted in a 99% reduction in peak drain - source current density of the radiation hardened structure in comparison to that of the original. Despite these modifications, high peak temperatures existed for the SiC JFETs studied, with even the modified device reaching over 1500 K after heavy ion impact. A bespoke SiC
JFET with an additional channel was investigated as a potential device to mitigate the SEB induced currents.

The SiC Lateral JFET (LJFET), consisting of both a vertical and a lateral channel provides enhanced current control over the standard JFET studied. Through simulation of an array of real world aerospace specific scenarios this bespoke device showed no regions of heightened SEB sensitivity with no failures at a 40% drain - source bias derating with a deposited charge equivalent to 300% that of a silver ion commonly used in SEB testing. Like the standard JFET SEB did occur, however, at elevated biases. At an industry standard drain - source bias derating of 70% the LJFET failed with device melting after impact. Through using the same technique employed to radiation harden the standard JFET a reduction in the SEB sensitivity of the LJFET was observed. Through including an additional 4.2 µm of regrowth between the gate and source of the device it could withstand harsher radiation conditions, due to the creation of a low electric field region below a magnitude of 0.5 MV/cm, 30% that of
the original. The modified LJFET at an industry standard drain - source derating is robust against all heavy ion deposited charge conditions studied, up to and including
300% that of a silver ion commonly used in SEB testing. As future aerospace applications will require the use of high voltages in the presence of real world cosmic ray environments, the use of the LJFET and modifications are key towards the mitigation of Single Event Burnout.