The paper describes the results of optimizing rapid thermal annealing (RTA) of ohmic contacts to AlGaN:Si layers with a high aluminum content (70 mol%) and various electron concentration. The contact characteristics were  measured using the transmission line method (TLM). It has been found that for highly doped Al0.7Ga0.3N:Si layers (>1018cm−3), the RTA annealing of Ti(25nm)/Al(80nm)/Ti/Au contact at a temperature 900 °C for 60 s makes it possible to obtain the minimum contact resistance of 8 Ω×mm and specific contact resistivity of 9×10−4 Ω·cm2 with high uniformity over the surface of a 2-inch substrate. For lightly doped Al0.7Ga0.3N:Si layers (