  We propose and analyze the heterostructure comprising a
black-arsenic-phosphorus layer (b-As$_{1-x}$P$_x$L) and a graphene layer (GL)
integrated with a light-emitting diode (LED). The integrated
b-As$_{1-x}$P$_x$L-GL-LED heterostructure can serve as an active part of the
terahertz (THz) laser using the interband radiative transitions in the GL. The
feasibility of the proposed concept is enabled by the combination of relatively
narrow energy gap in the b-As$_{1-x}$P$_x$L and the proper band alignment with
the GL. The operation of the device in question is associated with the
generation of the electron-hole pairs by the LED emitted near-infrared
radiation in the b-As$_{1-x}$P$_x$L, cooling of the photogenerated electrons
and holes in this layer, and their injection into the GL. Since the minimum
b-As$_{1-x}$PL energy gap is smaller than the energy of optical phonons in the
GL, , the injection into the GL can lead to a relatively weak heating of the
two-dimensional electron-hole plasma (2D-EHP) in the GL. At the temperatures
somewhat lower than the room temperature, the injection can cool the 2D-EHP.
This is beneficial for the interband population inversion in the GL,
reinforcement of its negative dynamic conductivity, %in the THz range and the
realization of the optical and plasmonic modes lasing supporting the new types
of the THz radiation sources.
