The ionization rates of GaAs are first empirically determined from breakdown voltage measurements ofp+‐njunctions with thicknlayers. The theoretical analysis is then extended top+‐n‐n+structures in which space‐charge punch‐through occurs before breakdown takes place. The avalanche breakdown voltages of GaAsp+‐n‐n+diode structures are calculated as a function of thenlayer thicknessWas well as the doping densityn. Comparison of the theoretical calculation with experimental data shows very good agreement.