The effect of a GaAs smoothing layer on the performance of GaAs/AlxGa1−xAs field‐effect transistors prepared by molecular beam epitaxy was investigated. The GaAs smoothing layer was inserted between the channel layer and the AlxGa1−xAs buffer layer in an attempt to reduce the dependence of interface quality on growth conditions. Current‐voltage characteristics of field‐effect transistors with 1‐μm gate lengths were used to characterize the properties of the heterointerface. Without the GaAs smoothing layer, extremely sharp interfaces, as indicated by the electron velocity, were obtained when the structures were grown at 700 °C. However, the interface sharpness was very sensitive to growth conditions and in particular to the substrate temperature, decreasing from 50 Å at 700 °C to 70 Å at 640 °C and to 260 Å at 580 °C. Incorporation of a 200‐Å‐thick undoped GaAs smoothing layer at the heterointerface decreased this sensitivity to growth conditions. With the smoothing layer, interface sharpnesses of 150, 60, and 50 Å were obtained for substrate temperatures of 580, 630, and 700 °C, respectively.