A comparison between experiment and theory has indicated an inaccuracy in avalanche breakdown calculations for low voltage silicon p-n junctions. The source of this error has been located. It is shown that the published values of effective carrier ionization rate, at avalanche breakdown, do not apply for low values of avalanche breakdown voltage. This error results from an inadequate mathematical approximation used during the empirical determination of effective carrier ionization rates for both silicon and germanium p-n junctions.