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A new normally-on buried-oxide (BOX) trench-gate bipolar-mode JFET (BTB-JFET) is reported for high-frequency low-voltage low-power-loss dc/dc converter applications. The BOX structure of the device is realized by localized thermal oxidation at the bottom of the gate trench. The fabricated BTB-JFET has a breakdown voltage of 21 V at VGS = -3 V. Due to the BOX under the gate region, the gate-drain capacitance CGD of the device is decreased by up to 30% at zero source-drain bias compared with that of the conventional trench-gate bipolar-mode JFET. The lower CGD reduces the switching times and the voltage dips during turn-on and turn-off. The resistive turn-on and turn-off times of the device are decreased from 31.5 to 30 ns and 12 to 10.5 ns, respectively. This approximately provides a 5% reduction in ton and 12% in toff, which is in agreement with the simulation results.