The authors propose a novel buried-gate-type MOSFET (BG-MOSFET) with ultrathin gate oxide films formed by rapid thermal oxidation (RTO) technology. The BG-MOSFET fabricated showed good punchthrough characteristics and much smaller hot-carrier degradation. The optimum trench depth is similar to 0.1-0.2 mu m deeper than source/drain junction depth considering current gain, avalanche breakdown voltage and hot-carrier immunity.