In this paper, low temperature and gold (Au)-free Ta/Al-based ohmic contacts fabricated by sputtering on AlGaN/GaN heterostructures are demonstrated on 150 mm GaN-on-Si substrates. The Au-free manufacturing process of Ta/Al-based ohmic contacts implemented by sputtering on large area substrates is shown to give comparable results to conventional Ti/AI-based ohmic contacts and therefore to be suitable for the integration of GaN-based devices in the Si technology production lines. Homogeneous contact resistance (Rc) values with a low mean value of 1.2\ \Omega mm are obtained over the entire substrate using Ta/Al/TaN metal stacks annealed at 550 °C. Low temperature and Au-free Ta/Al/TaN ohmic contacts are also compared to conventional high temperature annealed and Au-containing Ti/AI-based ohmic contacts in terms of sheet resistance (Rsh) stability before and after passivation. The temperature dependence of Rcin both metal schemes is then investigated supporting different transport mechanisms and posing novel reliability challenges related to their integration in metal-insulator-semiconductor (MIS)-high electron mobility transistors (HEMTs). For this reason, the two ohmic contact schemes are integrated into MIS-HEMTs which are compared in terms of on-resistance (RDS, on) at high-temperature operation.