Microwave radiation is observed from electrically biased semiconducting BiSb alloys. The radiation emitted from ${\mathrm{Bi}}_{92}$${\mathrm{Sb}}_8$ and its harmonics to the fourth order are very narrow band and coherent. The mechanism of emission is postulated to be a super-radiant maser effect due to transitions between paramagneticlike levels pumped by the electric current.

• Received 9 April 1969

DOI:https://doi.org/10.1103/PhysRevLett.22.1062