We review in this paper the electronic properties of semiconductor heterojunctions. We focus on interface-specific phenomena, where the conditions of growth—including controlled contamination—may significantly alter the physical properties of the junction. We start outlining the basic concepts of our first-principles theory of the band offsets. We then discuss in some detail the case of Ge/GaAs, where the band offset significantly depends on interface features, owing to the difference in chemical valence which induces electrostatic effects. For this system, sound experimental evidence of noncommutativity of the band offsets has been reached. We then discuss the effects of ultrathin intralayers at heterojunctions and homojunctions, starting with the case of heterovalent implantations, such as GaAs/Ge/GaAs. In these cases, the intralayers control the band offset: the effect has been predicted by our theory and subsequently experimentally observed in several systems. We review some recent measurements and their current interpretation, particularly in relationship with interdiffusion and atomic ordering at the interface. Finally, we discuss the electronic and optical properties of isovalent intralayers, i.e ultrathin quantum wells, such as AlAs/GaAs/AlAs and Si/Ge/Si. We show how the spatial confinement of carriers in an isovalent intralayer strongly enhances interband optical properties and can be used for the optical characterization of semiconductor interfaces at the atomic scale
