The embodiments of the invention provide a structure and method for a rad-hard FinFET or mesa. More specifically, a semiconductor structure is provided having at least one fin or mesa comprising a channel region on an isolation region. A doped substrate region is also provided below the fin, wherein the doped substrate region has a first polarity opposite a second polarity of the channel region. The isolation region contacts the doped substrate region. The structure further includes a gate electrode covering the channel region and at least a portion of the isolation region. The gate electrode comprises a lower portion below the channel region of the fin, wherein the lower portion of the gate electrode comprises a height that is at least one-half of a thickness of the fin.