DC stress tests on high power semiconductor devices at nominal device ratings yielded unexpected device failures. Without prior indication the devices were destroyed spontaneously anywhere in the bulk. The failure rate depends exponentially on the applied voltage. By transferring the test setup into a salt mine 130 m below ground we were able to prove that cosmic radiation is the cause for these failures. So far the only means to reduce the failure rate is to reduce the maximum field within the device by appropriate design.
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