Semiconductor superlattice in a biharmonic field: Absolute negative conductivity and static electric-field generation

We analyze the transport properties of a semiconductor superlattice in the presence of a biharmonic electric field. The modification of current-voltage characteristics induced by the biharmonic radiation is obtained. The conditions for absolute negative conductivity and for the spontaneous generation of a significant static electric field are determined. We also show that a simple harmonic field can experience nonlinear amplification even when the differential superlattice dc conductivity is positive, and we determine the corresponding range of parameters.