Valence- and conduction-band offsets can be induced at GaAs(100) polar homojunctions by means of ultrathin Si intralayers. The microscopic interface dipole responsible for the creation of such offsets can be varied by changing the Si intralayer thickness; the consequently variable band discontinuities represent a tunable parameter with potential applications in band-gap engineering. Even though the existence and the direction of these discontinuities are in agreement with theoretical predictions, the existing models overestimate the amount of the offset and cannot explain its dependence on the intralayer thickness. The presence of Si outdiffusion can be a possible explanation for this partial disagreement.