A semiconductor functional element is composed of at least one bifurcated branch conductive path coplanar with a heterojunction in a semiconductor with a band discontinuity that produces a potential well, and at least two gate electrodes designed for digital operations and a common electrode facing each other. The gate electrodes cross both paths and the two gate electrodes are located outside of one path and the common electrode is located outside the other path so that electron wave conditions at the heterojunction are locally influenced by an electric field which can be changed by selecting a gate electrode to apply a voltage thereby forming a logic or a functional circuit.