Demonstration of Lightly Doped Gradient Field-Stop Layer for Improved Soft Reverse Recovery in 1200 V SiC MOSFET

The intrinsic body diode of a silicon carbide (SiC) MOSFET is often utilized as a cost- and area-efficient alternative to an external freewheeling diode. In fast-switching applications, the commutation loop stray inductance has a significant impact on the body diode turn-off behavior, which leads to pronounced surge voltages and oscillations. In this work, by introducing a lightly doped gradient field-stop (LDG-FS) structure, both the dynamic loss and breakdown voltage of the device are improved while maintaining the sameon-resistance. Experimental results show that, compared with the conventional step uniformly heavily doped field-stop (HD-FS) structure, the proposed LDG-FS device achieves a 120 V improvement in breakdown voltage. In the same switching circuit, the softness of the body-diode reverse recovery is significantly enhanced, allowing the use of a smaller external gate resistance (${R}_{\text {g,ext}}$) to achieve faster switching speeds. For comparison, under the same${R}_{\text {g,ext}}$, the voltage overshoot is reduced by 19%. Furthermore, when the overshoot voltage is controlled to the same level by adjusting the${R}_{\text {g,ext}}$, the turn-on loss (${E}_{\text {on}}\text {)}$of the LDG-FS device is reduced by 31%. These improvements substantially enhance the overall switching efficiency of the SiC MOSFET.