Great improvement in the electrical and optical properties of molecular‐beam‐epitaxy‐grown InAlAs was obtained by using InP buffer layers and high substrate temperatures of 555 °C. Undoped InAlAs layers arentype and the highest room temperature electron mobility is as high as 5100 cm2/V s with a carrier concentration of 1×1015cm−3. The room‐temperature photoluminescence spectrum has a sharp edge emission with a spectral half‐width as narrow as 42 meV.