The 1.2 kV vertical GaN fin-channel junction field-effect transistor (JFET) is an emerging industrial device with low specific on-resistance (R_ON), normally-off operation, and avalanche capability. This article reports the first comprehensive evaluation of 1.2 kV, 75 mΩ gallium nitride (GaN) JFET in converter applications. The RC-interface gate driver is optimized through the double-pulse test (DPT). To quantify the device's conduction loss, an in-situ measurement of dynamic R_ON is performed in a continuous DPT at steady-state. The vertical GaN JFET shows no dynamic R_ON issue. Subsequently, a GaN JFET-based half bridge is evaluated in a zero-voltage-switching (ZVS) buck converter under various frequencies, duty cycles, and load conditions. For comparison, similarly-rated SiC mosfet and Si IGBT are tested in the same converter. Benefitting from the lowest output capacitance and output charge, the GaN JFET requires a short deadtime and enables MHz operation in the 800-V ZVS buck converter. The GaN converter achieves a maximum efficiency of 97.7% at 1 MHz and 98.0% at 500 kHz and shows a general advantage in frequency and efficiency compared to the same converters based on SiC mosfet and Si IGBT. In addition, under the operational driver condition, the 1.2 kV GaN JFET shows a long short-circuit withstanding time of over 40 μs at 800 V. These results provide key reference for the application of 1.2 kV GaN JFETs and suggest that a GaN device with proper designs can achieve excellent stability and robustness.

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