A nonisothermal analysis of dc conduction inn‐ andp‐type indium antimonide subjected to orthogonal electric and magnetic fields has been developed including the effects of energy‐dependent acoustic phonon and polar optical phonon scattering as well as minority carrier drift. Also, included are carrier generation, based on an equilibrium between carrier densities and particle temperatures, magnetoresistive effects and carrier injection. Threshold microwave emission curves are calculated assuming that a minimum minority carrier density is required to observe shot noise. The threshold curves give general agreement with the experimental results. When the distortion of the field due to the Hall effect is included, excellent agreement between theoretical and experimental curves is obtained.