The electron mobility of single modulation‐doped GaAs‐(AlGa)As heterojunctions is strongly dependent on electron density and Al concentration. A low‐temperature persistent photoconductive effect is employed to vary the areal electron densitycontinuouslywithin asinglesample by nearly a factor of 3. Over this density range the mobility increases monotonically by as much as a factor of 4, quasilinear with density. At equivalent carrier concentrations heterojunctions with lower Al concentration show higher mobilities. At low temperatures a peak mobility of 365 000 cm2/Vs is found at an areal density of 7.0×1011cm−2with an interparticle spacing equivalent to a three‐dimensional density of 5.9×1017cm−3.