The bipolar transistor and FET axe compared, considering both today's most advanced implementations and “ultimate” scaled-down devices. The differences between die devices are quantified in terms of transconductance and intrinsic speed. Old limits are re-examined and ways of extending the devices toward their ultimate in performance are proposed. While the major emphasis is on silicon, a comparison is made with the GaAs MESFET. Other semiconductor devices are discussed in the context of “bipolar-like” and “FET-like” devices, and the HEMT is considered a very promising candidate for high-speed logic. While Josephson junction logic is not discussed at length, it is a continual point of reference. In addition to comparing devices, the on-chip wiring environment is discussed, especially concerning its impact on power-delay products. Copyright © 1982 by The Institute of Electrical and Electronics Engineers, Inc.