We have used secondary ion mass spectrometry, local vibrational mode infrared absorption, and electrical characterization to study the incorporation of Si delta‐doped planes in GaAs grown by molecular beam epitaxy at 400 °C, in the concentration range 0.01–0.5 monolayers. A correspondence is observed between the density of SiGadonors, the free electron concentration and the total Si coverage, up to a coverage of ∼1013cm−2; however, above this value, the electron density falls, while [SiGa] remains constant up to a coverage of ∼1014cm−2, and then falls below the detection limit at 0.5 monolayer coverage. These effects have been interpreted in terms of a model which takes account of Si migration and aggregation on the delta‐doped plane during deposition.