Metal‐insulator‐semiconductor capacitors were prepared by doping an Al0.5Ga0.5As layer with oxygen in order to demonstrate a lattice‐matched single‐crystal insulator‐semiconductor heterojunction. Crystal growth was by molecular‐beam epitaxy. Interface trap effects were not observed in capacitance‐voltage measurements, while the current‐voltage measurements show space‐charge‐limited currents for forward and reverse biases.