Abstract Theory, numerical simulations, and experimental measurements of the valley current of a GaAs/AlAs resonant tunneling diode are compared. The effect on the valley current of different interface-roughness correlation models, island sizes, and asymmetric roughness is described. Initially, the valley current increases quadratically with island size. Between 6 and 10 nm there is a crossover and the contribution to the valley current begins to decrease. Asymmetric roughness on normal and inverted interfaces (smooth normal and rough in verted) results in order of magnitude different contributions to the valley current under forward and reverse bias. This asymmetry in the valley current occurs even when the polar optical phonon scattering is taken into account. The polar optical phonon scattering dominates the valley current.