A comparative study of Ta/Al/Mo/Au and Ti/Al/Mo/Au metallizations for AlGaN/GaN high electron mobility transistors is presented. By the optimization of surface treatment schemes and annealing temperature, contact resistances of 0.172 and 0.228 Ω mm, and specific contact resistivities of 2.96×10-7 and 1.09×10-6 Ω cm2 were obtained for Ti/Al/Mo/Au and Ta/Al/Mo/Au, respectively. Auger electron spectroscopy (AES), x-ray diffraction (XRD), and transmission electron microscopy (TEM) were utilized to study microstructural changes occurring in the metallization layers as a result of heat treatment. Results indicate dynamic systems of severe intermixing between atoms from the metallization layers and epilayers resulting in changes in surface morphology, as determined by atomic force microscopy (AFM). The formation of intermetallics and interfacial compounds was observed. The combined effect of surface treatment and annealing-induced reaction is proposed as the mechanisms for low-resistance Ohmic contact formation.