  Conditions required for the streaming effect and the optical-phonon
transit-time resonance to take place in a compensated bulk GaN are analyzed in
detail. Monte Carlo calculations of the high-frequency differential electron
mobility are carried out. It is shown that the negative dynamic differential
mobility can be realized in the terahertz frequency range, at low lattice
temperatures of 30--77 K, and applied electric fields of 3--10 kV/cm. New
manifestations of the streaming effect are revealed, namely, the anisotropy of
the dynamic differential mobility and a specific behavior of the diffusion
coefficient in the direction perpendicular to the applied electric field. The
theory of terahertz radiation transmission through the structure with an
epitaxial GaN layer is developed. Conditions for the amplification of
electromagnetic waves in the frequency range of 0.5--2 THz are obtained. The
polarization dependence of the radiation transmission coefficient through the
structure in electric fields above 1 kV/cm is found.
