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Incorporating nonlinear resistive field grading materials (FGMs) onto metal-brazed substrates has been widely investigated as an efficient electric field reduction strategy at triple points (TPs) within ultrawide bandgap [(U)WBG] power modules. However, most investigations have been carried out using either dc or sinusoidal ac voltages despite actual (U)WBG power modules operating with high-frequency square voltages featuring high-slew rate ( dv/dt ). Thus, this study introduces a field-dependent conductivity (FDC) layer to analyze electric field reduction under high-frequency, high-slew-rate square voltages. Using COMSOL Multiphysics, both coated and uncoated structures were modeled to evaluate electric field reduction. When employing nonlinear FDC coating, the findings demonstrate a notable decrease in field stress, even under square voltages with rapid rise times and high frequencies. However, relying solely on the nonlinear FDC layer may not adequately address the electric field concerns, particularly when factoring in protrusions on metallization layers and reducing layer coverage. In response to this challenge, protrusions at the metal ends are incorporated into a protruding substrate configuration. This entire structure is then coated with a nonlinear FDC layer. The combined impact of the protruding substrate and nonlinear FDC layer effectively reduces the electric field. However, when the rise time is shortened to 75 ns and the frequency is raised to 500 kHz, the electric field stress around TPs exceeds the insulation’s withstand strength. This finding underscores the need for further research into alternative strategies as the prevalent strategies are unable to effectively mitigate electric fields in real-world operating conditions of (U)WBG power modules.