n-Type organic field-effect transistors are operated with high transconductance at very low gate voltage using ionic-liquid electrolyte for the gating layers. Tetracyanoquinodimethane single crystals and C60 thin films are respectively interfaced with ionic liquid of 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide known for its low viscosity and high ionic conductivity, so that high-density electrons are rapidly accumulated in the semiconductor surfaces. The transistors are gated by high electric field confined to a molecular scale Helmholtz layer with the application of minimum gate voltages. The high-transconductance single-crystal device exhibits excellent air stability and the C60 thin-film transistor has realized the highest normalized transconductance among reported n-type organic transistors, together with remarkable improvement in threshold voltage as compared with that in conventional SiO2 devices.