The enhancement in superlattices of α/β (electron to hole ionization coefficient ratio) in AIGaAs/GaAs and in AllnAs/GalnAs, and of β/α in InP/GalnAs, is modelled using lucky-drift theory. Agreement with observation in the case of AIGaAs/GaAs is obtained only if the old 85 :15 band-edge discontinuity rule is assumed and geometrical reduction factors are ignored. The theory of the ionization rate for nearly parabolic bands is recalled, and the crucial role of overlap integrals is discussed. Simple theory predicts that ionization processes involving light holes are much more rapid than for heavy holes. An estimate of the rate near threshold for GaAs is made and it is shown to be over a hundred times smaller than the phonon-scattering rate. The impact ionization threshold for GaAs is therefore ‘soft’, in contrast to what is normally assumed. A soft threshold has important implications for the efficient working of staircase avalanche photodiodes (APDs), and for the modelling of the impact ionization process.