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Semiconductor alloys provide a natural means of tuning the magnitude of the forbidden gap and other material parameters so as to optimise and widen the application of semiconductor devices. With the advent of small-structure systems, such as quantum wells and superlattices, the effects of alloy composition, size, device geometry, doping and controlled lattice strain can be combined to achieve maximum tunability. In this article, the concepts and ideas peculiar to the description and modelling of semiconductor alloy systems are reviewed with a view to providing a link between electronic structure and optical and transport properties.The material parameters of intrinsic and extrinsic alloys are normally derived from those of the endpoint compounds in terms of a simple interpolation procedure. This model assumes that the material is a nearly perfect random alloy and that the electronic structure can be generated from suitably averaged parameters of the constituent compounds. Any deviation from this virtual crystal approximation (VCA) is treated separately as a perturbation. Consequently, the concepts and methods developed for compound semiconductors can be employed in a straightforward manner to describe alloys. However, recent experimental and theoretical studies on several semiconductor alloys (e.g. GaInAs, HgCdTe) indicate that the VCA breaks down whenever the mismatch between the electronic properties of the constituent atoms exceeds a certain critical value. The breakdown of the VCA is manifested by (i) the formation of ordered structures of lower symmetry, and (ii) the weakening of sp3 bonds on one sublattice which leads to selective localisation and charge transfer. In the case of defects, the position of deep levels and resonances reflects strong mixing by the short-range part of the potential of Bloch states spanning a range of many tens of volts. The relationship between the level depth and localisation as a function of alloy composition is non-linear and unrelated to the bowing of the alloy band gap. In the assessment of the electronic properties of submicron structures containing an alloy semiconductor, the bulk alloy properties and the effects of confinement, doping and strain must be treated on an equal footing. The bulk alloy states are mixed and the corresponding optical and transport parameters are changed. A certain degree of such mixing normally occurs in lattice-matched undoped systems as a result of the interaction between the states belonging to adjacent layers. This leads to the formation of well localised states which 0034-4885/85/081091+ 64$09.00 @ 1985 The Institute of Physics 10911092 M Jaros lie outside the confining barriers and which cannot be properly accounted for by the simple envelope function model. In narrow and medium-thick wells (< 100 A) the confinement effect is a sensitive function of the short-wavelength Fourier components of the potential. A breakdown of the vcA-which alters the description of alloy properties in a qualitative manner-has significant consequences for confine...