We have investigated the dependence of the current‐voltage characteristics of AlGaAs/GaAs and AlGaAs/InGaAs resonant tunneling diodes (RTDs) on spacer layer thickness. The measured peak to valley current ratio of the RTDs studied here is shown to improve while the current density through the RTDs decreases with increasing spacer layer thickness below a critical value. We find significant differences in the effect of the spacer layer thickness between AlGaAs/InGaAs and AlGaAs/GaAs RTDs, which we believe to be related to the relative quasi‐bound state energies of the two systems.