We report for the first time on the suppression of Be diffusion in molecular‐beam epitaxy Al0.1Ga0.9As by incorporating In into the epilayer. The minimum diffusion coefficient of Be‐doped Iny(Al0.1Ga0.9)1−yAs layers with a carrier concentration of 7×1019cm−3and an InAs mole fraction of 0.07 grown at 600 °C was about 2×10−15cm2/s, which is five times smaller than that without In incorporation. The photoluminescence intensity of the layers drastically decreased above a value ofyof 0.05, probably due to crystal degradation resulting from misfit dislocations. A heterojunction bipolar transistor with a 100 nm‐thickp+In0.055(AlxGa1−x)0.945As base layer (Al graded compositionx: 0 to 0.1, emitter area: 4×5 μm2) yielded a dc current gain of 27 at a collector current of 8 mA.