The problem of reliability prediction and assurance is characteristic of wireless devices based on nanoscale multilayer heterostructures because of the sensitivity of heterostructures’ parameters to the degradation processes due to the thinness of layers. In the current work, the degradation of the nanoscale AlAs/GaAs resonant-tunneling heterostructures due to the diffusion of the constituent elements was investigated. Analysis and comparison of data on Al and Si diffusion coefficients in GaAs shows that they strongly vary depending on the conditions of heterostructure fabrication. This happens while the defect density of the grown heterostructures depends on a large number of technological factors such as the substrate temperature during molecular beam epitaxy, chamber pressure, annealing temperature and time, defect density in the initial substrate, and many others. The values of the diffusion coefficients obtained by the authors of this article by IR spectral ellipsometry are consistent with the data of foreign researchers. This allows their use to predict the reliability of resonant tunneling diodes and nonlinear radio transmitters based on them.