Abstract The P-N junctions are part of the construction of semiconductor devices. They are formed by the combination of P-type and N-type diffusions. This paper discusses a soft (small early conduction) P-N junction breakdown. The P-N junction in this work is a part of a differential amplifier, which is widely used in analog/mixed signal devices. The paper outlines the test techniques to detect the differential amplifier failure, the circuit analysis (design and simulation), the fault isolation, and the root cause analysis with data from the wafer fabrication process to support plasma charging on the emitter. The real physical defect was not observable. However, with the help of lab data, the failure can be explained as plasma charging.