Almost all semiconductor devices contain metal-semiconductor, insulator-semiconductor, insulator-metal and/or semiconductor-semiconductor interfaces; and their electronic properties determine the device characteristics. This is the first monograph that treats the electronic properties of all different types of semiconductor interfaces. Using the continuum of interface–induced gap states (IFIGS) as a unifying theme, Mönch explains the band-structure lineup at all types of semiconductor interfaces. These intrinsic IFIGS are the wave-function tails of electron states, which overlap a semiconductor band-gap exactly at the interface, so they originate from the quantum-mechanical tunnel effect. He shows that a more chemical view relates the IFIGS to the partial ionic character of the covalent interface-bonds and that the charge transfer across the interface may be modeled by generalizing Pauling’s electronegativity concept. The IFIGS-and-electronegativity theory is used to quantitatively explain the barrier heights and band offsets of well-characterized Schottky contacts and semiconductor heterostructures, respectively.