This paper describes a newly developed FET Coupled Logic (FCL) circuit that operates at very high frequencies with very low supply voltages below 3.3 V. An FCL circuit consists of NMOS source-coupled transistor pairs for current switches, load resistors, emitter followers and current sources that are controlled by a band-gap reference bias generator. The characteristics and performance are discussed by comparing this circuit with other high-speed circuits. The optimal circuit parameters for FCL circuits are also discussed, and the fact is noted that a larger swing voltage enhances the circuit's performance. The simulated delay of a 0.25 µm FCL circuit is less than 15 ps for a 2.5 V power supply, and the simulated maximum toggle frequencies are over 5 GHz and 10 GHz at 2.5 V and 3.3 V power supply, respectively. The simulation results show that FCL circuits achieve the best performance among the current mode circuits, which include ECL circuits, NMOS source-coupled logic circuits. The delay of the FCL circuit is less than half that of an ECL circuit. The maximum toggle frequency of the FCL circuit is about triple that of NMOS source-coupled logic circuit. Because the FCL circuit uses low-cost CMOS-based BiCMOS technologies, its cost performance is superior to ECL circuits that require expensive base-emitter self-aligned processes and trench isolation processes. Using depletion-mode NMOS transistors for current switches can lower the minimum supply voltage for FCL circuits and it is below 1.5 V. The FCL circuit is a promising logic gate circuit for multi-Gbit/s tele/data communication LSIs.