Al content dependence of GaAs/AlxGa1−xAs superlattice ionization rates was studied. The electron ionization rate is enhanced when the AlGaAs of the barrier is a direct transition type. It is drastically reduced at the &Ggr;‐Xband crossover in the AlGaAs layer. The ionization rate ratio (hole to electron) as determined by excess multiplication noise measurement is reduced from a value of 0.5 atx=0.3 to 0.14 atx=0.45. At higher values ofx, corresponding to the onset of indirect electron transitions, the noise is increased.