The effect of changing the length of the spacer layer between two vertically integrated resonant tunneling diodes (RTDs) is experimentally determined. Three different wafers, each containing two RTDs, were grown by molecular beam epitaxy, with spacer layers of 1200, 700, and 200 Å, respectively. A fourth wafer with a single such RTD was grown as a control sample. Two of the control samples wired in series show two current peaks, the temperature dependence of the current‐voltage (I‐V) curves being correctly predicted by a nonlinear load model. The I‐V characteristics of the stacked devices with 1200 and 700 Å quantum wells between the RTDs also show two current peaks, confirming that the bulk of electrons lose longitudinal wave function coherence between the two double‐barrier structures. The first derivative of the I‐V curve for the samples with 700 and 1200 Å spacers displays evidence of quantum interference between the cathode well and the central spacer as a second‐order effect. The first major peak in the structure with a 200 Å spacer between the quantum wells differs from the first peak in the other structures, and the difference is attributed to quantum interference effects.