The possibility of achieving maser operation in semiconductors has been considered theoretically by several workers. It appeared likely that all the requirements for a maser could be fulfilled by the recently developed GaAs diffused diodes, which convert electrical energy to narrow-line infrared radiation with extremely high efficiency. In fact, Hall and coworkers have just reported having obtained coherent radiation from such diodes in pulsed operation at 77°K. Working along independent lines, we have also obtained coherent radiation from GaAs diodes at 77°K and greatly improved performance at 4.2°K...