We have observed a negative differential resistance (NDR) in a single‐barrier tunneling structure in which electrons tunnel from a doped semiconductor emitter layer into a quantum well (QW) layer and subsequently drift laterally to a specially designed contact. Pronounced NDR is seen already at room temperature and at 77 K the peak to valley (PTV) ratio in current is more than 2:1. Our results lend support to a recent hypothesis by Luryi [Appl. Phys. Lett.47, 490 (1985)] that the NDR in double‐barrier tunneling structures is not related to a resonant enhancement of the tunneling probability at selected electron energies, but rather originates from tunneling into a system of electron states of reduced dimensionality. For comparison we have also fabricated a QW structure with two tunneling barriers, in which the parameters of the emitter barrier and the QW are identical to those in the single‐barrier structure. In the double‐barrier structure we have obtained current densities as high as 4×104A/cm2and a NDR with PTV ratios of 3:1 at 300 K and 9:1 at 77 K.