The diffusion equation for electrons in a nonuniform field is solved and the breakdown condition derived. The breakdown condition is expressed in such a manner that an effective characteristic diffusion length Λeis determined; the meaning of Λeexpresses the equivalent characteristic diffusion length for uniform electric fields. From the experimental breakdown fields, in which the electric field is uniform, Λeis determined and used to predict theoretical breakdown curves for the nonuniform cases. Theory and experiment are compared, the agreement verifying the correctness of the approach.