First-layer Si (FL-Si) Si/Ti/Al/Mo/Au contact metallizations are demonstrated to form low resistance and high temperature thermally stable Ohmic contacts on AlGaN/GaN high electron mobility transistor (HEMT) structures. Electrical and surface morphology characterizations have indicated that contact behavior significantly depends on the thickness of FL-Si used, where FL-Si (5 nm) scheme showed the most optimal behavior. The contact resistances of FL-Si schemes are stable for up to 300 h during thermal aging at 500 and 600 °C. Aging at 700 °C resulted in gradual degradation of contact resistance with values less than 1 Ω mm Ohmic behavior still maintained after 50 h of thermal treatment. No deterioration in the sheet resistance of the heterostructure has been detected upon thermal aging. Atomic force microscopy, Auger electron spectroscopy, and transmission electron microscopy characterizations have been utilized to identify the effect of FL-Si incorporation on the nature of intermetallic and interfacial reactions. Results from this study demonstrate that FL-Si-based metallizations have the potential to meet the critical requirements of low resistance, high temperature thermal stability and smooth surface morphology for the fabrication of AlGaN/GaN HEMTs.