  In a superlattice placed in crossed electric and magnetic fields, under
certain conditions, the inversion of electron population can appear at which
the average energy of electrons is above the middle of the miniband and the
effective mass of the electron is negative. This is the implementation of the
negative effective mass amplifier and generator (NEMAG) in the superlattice. It
can result in the amplification and generation of terahertz radiation even in
the absence of negative differential conductivity.
