This article describes thermally stable Schottky contacts using low N content double layered WSiN for InGaP/(In)GaAs metal–semiconductor field‐effect transistors with self‐aligned ion‐implantedn+layers. Transmission electron microscopy observations show that the microscopic interfacial reaction between WSiN and GaAs is completely suppressed when N content in the WSiN is 10% or less. As a result, a WSiN/GaAs‐cap/InGaP Schottky contact shows high thermal stability after annealing at 800 °C for 100 min, even though the GaAs cap is as thin as 25 Å. Moreover, the double‐layer WSiN structure suppresses reduction in the carrier concentration in the channel during activation annealing. The carrier concentration of the GaAs‐cap (40 Å)/InGaP (100 Å)/ InGaAs‐channel (100 Å) film, 6.5×1018cm−3, decreases to less then 2×1018cm−3with low N content WSiN (N=10%, 4000 Å) after annealing at 900 °C for 0.1 s, but it remains 3.8×1018cm−3with the double layered WSiN (N=37%, 3800 Å/N=10%, 200 Å).