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The behavior and the failure mechanism of a monolithic bidirectional ESD protection device under a system-level ESD pulse are investigated. The device is realized by two diffused vertical back-to-back connected diodes. When the bottom p-n junction is stressed with an 18-kV system-level pulse, the device exhibited the typical signature of second thermal breakdown. Surprisingly, no ESD damage could initially be observed by measuring the poststress leakage current, even when the ESD level was further increased to 30 kV. A degrading IV behavior was later found to be associated with the second thermal breakdown. Based on simulations, local crystal defects are proposed to explain the observed degradation of the IV behavior. It is shown that, in this case, leakage-current measurements alone fail to detect such ESD damage. Failure analysis of the device confirms the existence of the crystal damage.