We have fabricated the sub-30 nm asymmetric n-channel metal–oxide–semiconductor field effect transistors (NMOSFETs) and investigated its operation and characteristics. In this work, two key ideas are newly introduced in order to improve the device performance. One is the introduction of silicon-on-insulator (SOI) substrate to remove junction leakage paths. The other is the modification of mask layout for performance optimization and mass production. By using SOI substrate and modifying mask layout, the ON/OFF current ratio of the fabricated device is quite increased when we compared with the previous work because the source/drain junction leakage and parasitic current are suppressed. Moreover, the fabricated device has excellent scaling properties in terms of short channel effect.