A theoretical study of quantum interference phenomena in a T‐shaped semiconductor structure is presented. Transmission and reflection coefficients are computed by use of a tight‐binding Green function technique. As expected, the results resemble the well‐known solutions for the electromagnetic field in waveguides with the main difference that the penetration of the wave function of the electrons can be controlled by external voltages. We conclude that transistor action based on quantum interference should be observable in such structures, and we present general results for the functional dependences of the transmission coefficient which corresponds to a transconductance.