Three-dimensional organic field-effect transistors are developed with multiple vertical channels of organic semiconductors to gain high output current and high on-off ratio. High-mobility and air-stable dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene thin films deposited on horizontally elongated vertical sidewalls have realized unprecedented high output current per area of 2.6 A/cm2 with the application of drain voltage -10 V and gate voltage -20 V. The on-off ratio is as high as 2.7×106. Carrier mobility of the organic semiconductor deposited on the vertical sidewalls is typically 0.30 cm2/Vs. The structure is built also on plastic substrates, where still considerable current modulation is preserved with high output current per area of 70 mA/cm2 and with high on-off ratio of 8.7×106. The performance exceeds practical requirements for applications in driving organic light-emitting diodes in active-matrix displays. The technique of gating with electric double layers of ionic liquid is also introduced to the three-dimensional transistor structure.