We show that the current‐voltage characteristics of resonant tunneling structures are drastically influenced by the difference of electronic effective masses between the electrodes and the quantum well. In particular, if the mass in the well is larger than that in the emitter, the current peak is shifted to lower voltages, relative to the more conventional case of equal masses. This situation is illustrated experimentally with GaAs/AlAs/GaAs heterostructures, in which &Ggr; electrons emitted from GaAs tunnel resonantly through the AlAsXpoint, where the mass is considerably heavier.