Dynamic bulk negative differential conductivity in semiconductors

The Gunn effect results from the existence of a static negative differential conductivity that is, the real part of the differential conductivity σr′(ω) is negative at zero frequency for a certain range of bias fields; at some high frequency, σr′(ω) passes through zero, and eventually approaches zero from above the axis as ω → ∞. We have investigated the possibility of the existence of a dynamic negative differential conductivity in bulk semiconductors that is, the existence of cases where σr′(ω) is positive at low frequencies, but becomes negative at high frequencies, finally approaching zero from below the axis. The relation of the structure of σr′(ω) to the impulse response of the semiconductor has been analyzed, and two model calculations which show the desired effect are presented. © 1971.