A quaternary InAlGaN Schottky barrier layer is used in GaN high electron mobility transistors (HEMTs) in this study. For potential application to high speed electronic devices, GaN HEMTs with a quaternary InAlGaN Schottky barrier layer are fabricated and characterized. The fabricated device shows a maximum drain current density of 1.8 A mm⁻¹, a maximum trans‐conductance of 557 mS mm⁻¹, on/off current ratio higher than 10⁵, a subthreshold swing (S.S.) of 140 mV/decade, and a gate‐diode leakage current of 1.2 mA mm⁻¹. For the device with a 100‐nm‐long gate footprint, a current gain cut‐off frequency (f_T) of 102 GHz and a maximum oscillation frequency (f_max) of 130 GHz are extracted at the same bias condition (V_DS = 8 V and V_GS = −2.2 V) whose L_g · f_T product is evaluated to be 10.2 GHz · µm.