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A simplified microscopic model for investigating energy relaxation effects in millimeter-wave IMPATT devices is presented. A statistical process is used to describe electron-hole multiplication by impact ionization from knowledge of the ionization coefficients. These coefficients are assumed to be functions of the individual energy of carriers (holes and electrons). A relaxation time formulation is used to calculate the energy of each carrier. Drift in the electric field and diffusion are modeled using the diffusive model proposed by Hockney. Simulations are carried out for silicon diodes. It is found that inclusion of the energy relaxation mechanisms modifies mainly the avalanche process for such material. The implications of these mechanisms on device performances are then discussed by calculating the large signal level dependence of the conversion efficiency and admittance for a typical double-drift structure at 100 GHz. The resulting calculations show good agreement with existing experimental data on these structures.