Cosmic Ray Failures in Power Semiconductors

Cosmic rays are particles radiating from the galaxy to the earth. From these particles, high-energy neutrons are seen to cause failures when they collide with the base nuclei in power semiconductors. These collisions can trigger different failure mechanisms depending on the component type. Moreover, the failures are random and occur within nanoseconds without any prior sign. This thesis introduces cosmic ray failures as a phenomenon in power semiconductors, the affecting factors, the estimation of failure probabilities, and existing testing procedures. Also, a test setup with natural terrestrial radiation was developed to compare test results from artificial radiation sources. Lastly, methods to improve the cosmic ray robustness of power semiconductors were discussed. The affecting factors are the blocking voltage, junction temperature, altitude, and radiation flux. The voltage was seen to be the most effective factor because when a certain voltage threshold is achieved, the failure rates increase exponentially. The junction temperature and altitude can be included in the estimation of failure probability with formulas composed of experimental data. Lastly, radiation flux was seen to be dependent on solar activity, latitude, and altitude. For quantifying cosmic ray failures, three different empirical models were presented. Comparison between these models showed differences and indicated the need for testing with real hardware to get accurate failure rates. However, these models as well as Monte Carlo techniques and TCAD simulations were noted to help in the early phase of development to adjust the radiation robustness. The test setup developed in this thesis was the first step for cosmic ray testing at Danfoss Drives. The setup included multiple IGBT power modules with the gates short-circuited to ensure a blocking state. During the test, a high collector-emitter voltage was applied constantly. Six failures occurred during the test and showed a good match to the estimation from accelerated tests. Regarding the methods for reducing cosmic ray failures, adjusting the width of the drift region was seen to be effective. Also, surrounding the components with neutron-attenuating materials seemed to decrease the probability of failures. Lastly, some previous research results were combined in the comparison of silicon and silicon carbide. It seemed that the latter should be more robust supporting the trend of using silicon carbide in high-voltage applications