Ohmic contacts to AlGaN/GaN with different metal stacks on Si or Sapphire substrate are fabricated and compared in this paper. For Au-capped ohmic contacts, the lowest contact resistances of 0.7 Ω·mm and 1.3 Ω·mm are achieved by Ti/Al/Ti/Au (20/110/40/50 nm) and Ti/Al/Ni/Au (20/110/40/50 nm) stacks, respectively. It also shows that the substrate material and epitaxial structure play an important role in ohmic contact engineering. For CMOS compatible Au-free structures, the Ti/Al/W (20/100/30 nm), Ti/Al/Ni/W (20/100/20/10 nm) and (20/100/10/20 nm) are demonstrated with the minimum contact resistance values of 0.45, 1.3, and 1.6 Ω·mm, respectively. The three metal stacks of Au-free ohmic contact are compared and obtained results are explained.@en