The effects of undoped spacer layers on the electrical properties of single barrier heterostructures (both alloy and superlattice) have been investigated by measuring incremental slope resistance over the bias range -400 to +400 mV and at convenient temperature intervals between 70 and 290 K. The zero bias slope resistance, Rs(0), and the effective barrier heights increase with spacer thickness. Also, the low-temperature slope resistances, R s(V), decrease exponentially with the magnitude of the bias, V, while the effective barrier heights, deduced from high-temperature measurements, decrease approximately linearly. This suggests that the decrease in R s(V) with bias is due simply to the voltage-induced decrease in effective barrier height. Rs(0) varies exponentially with zero bias effective barrier height for both alloy and superlattice barriers and this is consistent with the Γ electrons dominating the current transport through the barriers. All of our Rs(V) curves are asymmetric and, using Airy function calculations, we have modeled curves similar to the experimental ones by assuming different doping levels for the two doped GaAs layers on either side of the barriers. This is possibly due to Si migration into the "undoped" barrier or the spacer layer closest to the substrate.