Assuming that current-density/electric-field curves are provided and that the Hall mobility does not change appreciably with the electric field, the small-signal tensor conductivity of n-GaAs in the presence of d.c. electric and magnetic fields is derived. Using this tensor conductivity, carrier-wave equations are solved for a long slab of n-GaAs sandwiched between dielectrics. It is found that wave vectors are generally skewed against the direction of the direct current. The gain factor of the propagation coefficient is found to be controllable by the magnetic field, especially when the slab is thin and the d.c. electric field is near the threshold value.