An indirect band-gap semiconductor may be converted to a semimetal, or vice versa, by application of pressure. At low temperature, an excitonic phase or some other anomaly must occur in the neighborhood of the transition pressure. We also discuss the direct-band-gap case and the case where a band gap is zero by symmetry, as in gray tin.

DOI:https://doi.org/10.1103/RevModPhys.40.755