Convective and absolute instabilities in semiconductors exhibiting negative differential mobility

It is shown that, in a semiconductor exhibiting negative differential mobility, the coupling between diffusion effects and the tendency for space-charge accumulation can lead to temporal growth rather than spatial amplification if the dielectric relaxation frequency exceeds a certain threshold value. This conclusion results from an analysis of the dispersion relation for longitudinal waves in the semiconductor. The criterion for temporal growth is a condition for absolute instability, as opposed to convective instability, which would indicate rather spatial amplification. © 1971 The American Physical Society.