During the last twenty years the majority of solid-state device work has been concerned with the utilisation of semiconductors with a band gap (EG ) > 0.7 eV, i.e., large with respect to the thermal-activation energy, kT, at room temperature. The p-n diode, transistor, and, more lately, the integrated circuit have used germanium (0.72 eV) and silicon (I.I eV) as the basic material, while Gunn diodes and semiconducting lasers have depended upon the properties of GaAs with a band gap of I.45 eV. A survey of the literature shows that in recent years a much greater research effort has been put into small-band-gap materials, say, with EG < 0.2 eV and, indeed, into semimetals in which the valence band and conduction band can be considered to overlap at some point in E/k space. The interest in these materials has arisen in two ways. The advent of the laser has made it important to be able to detect radiation in the infrared and the far infrared; for example, the CO2 laser is one of the most powerful available and its emission peak lies at 10.6 μ or 0.12 eV. A small-band-gap semiconductor using intrinsic generation of electron/hole pairs could, in principle, be used in the photoconductive mode to detect such radiation. Another totally different usage has arisen from the work on low-temperature coolers. The Nernst-Ettingshausen effect has been known for over a century but it was not until recently that an effect which was a scientific curiosity was used to produce significant cooling at temperatures below 150 K.