Shockley's positive conductance theorem for Gunn materials with field-dependent diffusion

With the field of directions governing the field distribution in Gunn effect materials, it is shown that Shockley's positive conductance theorem is not in contradiction with the static negative resistance (SNR) observed recently when the diffusion coefficient increases with electric field. Although an SNR can be observed under certain boundary conditions, it can only occur if a time-dependent transition from one type of stationary solution to another is allowed (necessity of an instability).