  A simple two-well design for terahertz quantum cascade lasers is proposed
which is based on scattering injection and the efficient extraction of
electrons from the lower laser level by resonant tunneling. In contrast to
existing designs this extraction also controls the positive differential
conductivity. The device is analyzed by calculations based on nonequilibrium
Green functions, which predict lasing operation well above 200 K at a frequency
of 2.8 THz.
