The wave propagation characteristics along the electrodes of traveling-wave field effect transistors (TW-FETs) are described. Due to the fine gate structure of present high-speed FETs, the performance is greatly influenced by the electromagnetic coupling between the gate line and the drain line. No previous study has introduced a design method that can resolve this issue. The main purpose of this article is to clarify the characteristics of TW-FETs, and then, propose new design methods that cope with the coupling. The conditions under which TW-FETs yield ultrafast operation even with coupling are derived and then the results and implications of experimental investigations that measured the impulse response of a TW-FET using electro-optic sampling techniques are discussed.