We demonstrate the validity of the dead space approximation for impact ionization in silicon. Monte Carlo simulations are used to obtain realistic ionization probabilities, and the corresponding avalanche gain in constant field structures is computed. We show that the hard‐threshold dead space model is in good agreement with a more refined model taking into account soft‐threshold effects, if an effective threshold energy of 3 eV is adopted for electrons. We also show that hole nonlocal effects do not significantly affect the result. ©1996 American Institute of Physics.