Silicon metal-oxide semiconductor field-effect transistors (MOSFETs) act as simple electrical switches that allow us to build large-scale integrated circuits (LSIs) with very simple architectures. Owing to progress in fabrication technologies, we can now build LSIs that include tremendous numbers of transistors. However, this increase of transistors causes a large power dissipation in a small Si chip, which will limit the further growth of functionality of the LSIs in the not-too-distant future. To overcome the limitation, we need new functional devices that operate under different principles and dissipate as little power as possible.