A nonisothermal drift‐diffusion model of avalanche diodes is described. The novel feature of the model is the inclusion of the heat conduction equation in the basic set of semiconductor equations. The influence of a nonuniform temperature distribution across the active region on the diode electrical characteristics is investigated and the usual assumption of constant diode junction temperature is evaluated. Both single‐drift and double‐drift diodes are simulated. The temperature distribution is calculated as a function of the dc bias current density and the amplitude of the rf driving voltage. The diode impedance is also determined and compared with results from an isothermal simulation. Numerical results are presented atJ‐(10–20 GHz) andW‐band frequencies (75–110 GHz).