| Summary: | In this work, an anomalous thick oxide-degradation phenomenon in n-type 650-V class super-junction VDMOS transistors is investigated. An unexpected threshold voltage (V<sub>t</sub>) decrease was observed with high positive bias temperature instability stress, and the saturation current (I<sub>Dsat</sub>) increases with the stress time. Repeatable and reproducible behaviors have been achieved from multiple devices under test. Based on simulation and experimental results, it is found that the high-energy electrons (caused by high positive gate voltage) in the n-type region at the bottom of the gate oxide layer (top of the N-pillar) are injected into the gate oxide. The high-energy electrons generate electron-hole pairs during the transport to the anode, leaving holes in the gate oxide layer, and thus decreased V<sub>t</sub> and increased I<sub>Dsat</sub>. Finally, C-V measurement is also carried out which further confirms the above analysis.
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