The simulations of recently discovered effect of subnanosecond avalanche switching of Si n ^+− n − n ^+-structures have been performed. The electric field in n ^+− n − n ^+-structure is shown to remain quasi-uniform along the current flow direction during the voltage rise stage and it reaches the effective threshold of impact ionization of ~200 kV/cm in the whole n-base. Comparing simulation results with experiments we argue that the field distribution is as well uniform in the transverse direction. Hense, the ultrafast avalanche transient develops quasi-uniformly in the whole n-base volume. The switching time is about ~150 ps. We compare numerical results obtained for various impact ionization models and estimate parameters of the initial voltage pulse that are required for ultrafast avalanche switching of n ^+− n − n ^+-structures.