Schottky characteristics of undoped AlInAs grown by metal‐organic chemical‐vapor deposition have been investigated.I‐Vcharacteristics and their dependence on donor concentrations in AlInAs layers were explained well by assuming the existence of an interfacial layer. The presence of oxygen atoms at the metal‐AlInAs interface was revealed by Auger electron spectroscopy measurements. The oxide formation on AlInAs surface may be an inevitable result because of the high composition of aluminum atoms in AlInAs. Reduction in donor concentration in AlInAs layers is effective in minimizing the influence of the interfacial layer on barrier height lowering. Reduction in gate leakage current was successfully demonstrated for fabricated GaInAs/AlInAs high‐electron‐mobility transistors by reducing donor concentration in AlInAs Schottky layers.