NiGeW has been successfully implemented as a contact material bn GaAs MESFETs in a production environment. However, when identical contacts were used on heterostructure field effect transistors (HFETs), a strong interaction was observed between NiGeW and GaAs/AlGaAs epitaxial layers after annealing. Although NiGeW was considered a refractory contact and was not expected to react with the substrate, we detected defects at the GaAs/AlGaAs interface by cross-sectional transmission electron microscopy. These electrically active defects degrade the HFET performance significantly. As a result, the HFET epitaxial structure had to be re-optimized in order to minimize the observed adverse effect.