We examined the role of substrate quality on the epitaxial microstructure and performance of pseudomorphic InGaAs/AlGaAs/GaAs high electron mobility transistors (HEMTs). High resolution x-ray diffraction, high resolution x-ray topography, and transmission electron microscopy indicate that, for a given channel layer thickness, the misfit dislocation density is always lower for HEMT structures grown on substrates having lower threading dislocation densities. Furthermore, the onset of misfit dislocation formation occurs at higher channel layer thicknesses for HEMTs grown on substrates having a lower threading dislocation density when compared to those grown on substrates with a higher threading dislocation density. However, the ratio of the density of misfit dislocations to threading dislocations is greater than one on the low dislocation density substrates, which demonstrates that other misfit dislocation nucleation sources (i.e., surface particulates) are significant when there is an insufficient density of threading dislocations. The Hall conductivity measurements show that the performance of HEMT structures improves with higher substrate quality, showing that the range of epitaxial layer metastability increases with improved substrate quality. ©1997 American Institute of Physics.