Abstract
This paper reinvestigates the possibility of coherent plasma-wave oscillations in direct-gap semiconductors when the plasma frequency is near the band gap. A theory of plasma instability is developed, based on the collective-coordinate method. Threshold, the nonlinear region above threshold, and the electric field are investigated from the perspective of the collective-coordinate method, and the results are applied to specific narrow-gap semiconductors. When the plasma-wave emission is coherent, the cross section for light-plasmon scattering can be enhanced many orders of magnitude. Experiments are proposed.