A model is presented which accounts for the anomalous diffusion of thep‐type base dopant during the growth of AlGaAs‐GaAsnpnbipolar transistors. The model is based upon Fermi level pinning at the crystal surface during epitaxial growth which leads to an increased concentration of column III interstitial defects in heavilyn‐type AlGaAs or GaAs. The excess column III interstitials generated in then‐type crystal flow into thep+base region causing a transfer of thep‐type impurity atoms from column III lattice sites to interstitial positions through a ‘‘kick‐out’’ mechanism. Once in interstitial positions the impurity atoms are known to diffuse rapidly. The model is consistent with previously proposed mechanisms for both impurity diffusion and column III self‐diffusion.