Nobel Lecture: Quasielectric fields and band offsets: teaching electrons new tricks*

Heterostructures, as I use the word here, may be defined as heterogeneous semiconductor structures built from two or more different semiconductors, in such a way that the transition region or interface between the different materials plays an essential role in any device action. Often, it may be said that the interface is the device. The participating semiconductors all involve elements from the central portion of the periodic table of the elements (Table I). In the center is silicon, the backbone of modern electronics. Below Si is germanium. Although Ge is rarely used by itself, Ge-Si alloys with a composition-dependent position play an increasingly important role in today’s heterostructure technology. In fact, historically this was the first heterostructure device system proposed, although it was also the system that took longest to bring to practical maturity, largely because of the 4% mismatch between the lattice constants of Si and Ge. Silicon plays the same central role in electronic metallurgy that steel plays in structural metallurgy. But just as modern structural metallurgy draws on metals other than steel, electronics draws on semiconductors other than silicon, namely, the compound semiconductors. Every element in column III may be combined with every element in column V to form a so-called III-V compound. From the elements shown, twelve different discrete III-V compounds may be formed. The most widely used compound is GaAs—gallium arsenide—but all of them are used in heterostructures, the specific choice depending on the application. In fact, today the III-V compounds are almost always used in heterostructures, rather than in isolation. Two or more discrete compounds may be used to form alloys. A common example is aluminum-gallium arsenide, AlxGa12xAs, where x is the fraction of columnIII sites in the crystal occupied by Al atoms, and 1 2 x is occupied by Ga atoms. Hence we have not just 12 discrete compounds, but a continuous range of materials. As a result, it becomes possible to make compositionally graded heterostructures, in which the composition varies continuously rather than abruptly throughout the device structure.