The subthreshold characteristics are degraded by tunneling current with reduction of channel length. The optimum design rules for flat-band voltages and oxide thicknesses are investigated for subthreshold characteristics. The conditions of Vfbt>VfbbVfbt>Vfbb and tox1>tox2tox1>tox2 for 5-nm DGMOSFET can simultaneously reduce the threshold voltage and ΔVon−offΔVon−off.