Gold-free Ta/Al-based ohmic contacts fabricated by sputtering on AlGaN/GaN heterostructures and annealed at low temperature were investigated. The presence of a thin AlN spacer layer at the AlGaN/GaN heterojunction is demonstrated to prevent the ohmic contact formation as shown by rectifying behavior after annealing. Ta as an additional capping layer on Al leads to a severe morphology degradation and subsequent deterioration of the metal stack after annealing at 600 °C due to strong Ta-Al alloying verified by transmission electron microscopy. Using the compound metal TiN as capping layer circumvents the alloy formation, thereby making the contacts much more stable under annealing. A low contact resistance of 0.8 Ω mm was obtained using Ta/Al/TiN metal layers annealed at only 500 °C. The nature of the current transport was investigated by analyzing the temperature dependence of the specific contact resistance, which points towards a current path through the AlGaN barrier by thermionic field emission.