Experimental information is presented which shows that abruptp+njunctions break down by the Zener internal field emission effect when the base resistivity is low and by ionization avalanche when the base resistivity is high. In intermediate regions of base resistivity both mechanisms are observed in the same junction. The onset of Zener current is accompanied by a drop, of unexplained origin, in the junction collection efficiency for minority carriers photoinjected into the base. Applicability of the Zener theory to abruptp+njunctions is discussed.