Instabilities in semiconductors with negative differential mobility

A small-signal analysis for an infinitely extended semiconductor with negative differential mobility shows that perturbations of the total-current density always lead to an absolute instability. Therefore the experimentally observed switching behaviour of highly doped Gunn diodes cannot simply be interpreted as a consequence of the diffusion-induced absolute instability as was done by Gueret in 1971. This interpretation to apply rather requires the introduction of a zero-field boundary condition which is shown to cancel the total-current-induced instability.