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A power vertical double-diffusion metal-oxide-semiconductor (VDMOS) transistor is designed with the step oxide trench based on the oxide-bypassed concept proposed by Liang and co-workers. It is suitable for breakdown voltage below 300 V to obtain ultra-low specific on-resistance. The electric field of the drift region is modulated because of the new electric field peak introduced by the various thicknesses of sidewall oxide. As a result, the breakdown voltage was increased no less than 20 because of more uniform electric field distribution in the drift region, while the specific on-resistance was reduced by 40 60 resulting from the step oxide trench compared with the oxide-bypassed structure. The limit line of the silicon has been broken because the trade-off is improved between the breakdown voltage and the specific on-resistance. The low Ron,sp of 15.0 m mm2 with the breakdown voltage (BV) of 150 V and 28.0 m mm2 with the BV of 230 V in the proposed step oxide trench MOS are improved greatly compared to the Ron,sp of 163.4 m cm2 with the BV of 150 V and 475.75 m mm2 with the BV of 230 V in the conventional VDMOS.