Although a resonant tunneling diode (RTD) has been expected as high-speed novel devices, there are only a few reports on RTDs based on a Si/SiO/sub 2/ system. Recently, we have fabricated RTDs by a new and simple method based on ultrathin silicon-on-insulator (SOI) structures and found that a RTD with a Si-well width of 2 nm shows NDC in its current-voltage (I-V) curve. However, in our experiments, devices with the NDC characteristics are not the majority and different characteristics are observed in most devices. In the present paper, we investigated I-V characteristics for RTDs with a various BOX thickness and discussed an electron confinement effect. It was found that RTDs with a thicker BOX layer show better electron confinement, due to structural perfection.