Si planar‐doped AlInAs layers grown by metalorganic chemical vapor deposition (MOCVD) showed an excellent uniformity in doping concentration. It was caused by the saturation of the doping concentration in the high doping region. Applying this planar doping to the AlInAs/GaInAs heterointerface field‐effect transistor (HIFET), the variation of the threshold voltage (Vth) due to unevenness of distribution of doping concentration was reduced to values below 50 mV over a 2‐in.‐diam wafer.