The theory for a new form of transistor is presented. This transistor is of the "field-effect" type in which the conductivity of a layer of semiconductor is modulated by a transverse electric field. Since the amplifying action involves currents carried pre-dominantly by one kind of carrier, the name "unipolar" is proposed to distinguish these transistors from point-contact and junction types, which are "bipolar" in this sense. Regarded as an analog for a vacuum-tube triode, the unipolar field-effect transistor may have a mµ of 10 or more, high output resistance, and a frequency response higher than bipolar transistors of comparable dimensions.