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We investigated the nucleation sites of expanded single Shockley-type stacking faults (1SSFs) in a silicon carbide (SiC) metal–oxide–semiconductor field effect transistor (MOSFET) and demonstrated epitaxial layers designed for bipolar-degradation-free SiC MOSFETs. Since the sufficient hole density just below the basal plane dislocation (BPD)-threading edge dislocation (TED) conversion points induces 1SSF expansion, we derived the dependence of the nucleation depth on the applied current condition from the BPD-TED conversion points of 1SSFs. We first simulated and determined the three-step current conditions applied to a body diode in a SiC MOSFET so that a sufficient amount of holes would be supplied to the drift layer, to the buffer layer, and inside the substrate in the SiC MOSFET. An in operando x-ray topography analysis was conducted with the determined conditions for dynamically visualizing 1SSF expansion motions, and 1SSFs expanded at different forward current densities were successfully extracted. The depths of the BPD-TED conversion points of the extracted 1SSFs were analyzed, and it was experimentally clarified that these depths, i.e., the nucleation sites of expanded 1SSFs, became deeper with forward current densities. The bipolar degradation characteristics of SiC MOSFETs were evaluated as a function of the forward current density, and the validity of the simulation model was verified by experimental results. We also confirmed that bipolar degradation can be suppressed to some extent by using a substrate with a low BPD density, and SiC MOSFETs with a high-nitrogen-concentration epitaxial layer showed high reliability under bipolar operation. Depending on the application of SiC MOSFETs, the epitaxial layers should be designed to prevent the hole density inside the substrate from exceeding the threshold for 1SSF expansion. [ABSTRACT FROM AUTHOR]