  We suggest to use InAs/GaSb coupled quantum wells for THz lasing. In these
heterostructures THz lasing is based not on intersubband but on interband
transitions. Crucial advantages of this design in comparison with intersubband
lasers are (i) a large value of the interband dipole matrix element and (ii)
easier maintaining of population inversion. These advantages lead to a gain of
two orders of magnitude higher than for intersubband lasing. Even higher gain
can be obtained in special design InAs/GaSb W-structures where a hybridization
gap of 1-3THz is formed and optical density of states is singular.
