Abstract: A previously ignored property of a degenerate 2-dimensional gas of charge carriers in a quantum well (to be termed the quantum-capacitance effect) makes possible a novel class of transistors. In these devices the collector (a quantum well having high transverse conductance) is located between gate and emitter, with a barrier layer between emitter and collector, and a relatively thin barrier layer between collector and gate, and the chemical compositions and/or thicknesses of the various layers are chosen such that application of a voltage to the gate results, as a manifestation of the quantum-capacitance effect, in an induced charge in the emitter, whereby a current between emitter and collector can be controlled by means of a voltage applied to the gate. Transistors according to the invention potentially are very fast. Exemplarily the invention is embodied in a GaAs/AlGaAs heterostructure.