We performed a theoretical investigation on the effects of extended defects on the structural and electronic properties of dopant atoms in gallium arsenide. We observed that silicon impurities segregate at GaAs stacking faults. A Si atom at a Ga site in a stacking fault in either a neutral or a negatively charged state is energetically favourable as compared to a Si atom at a Ga site in a crystalline environment by as much as 0.2 eV. Additionally, a substitutional Si impurity in the negative charge state in a stacking fault has a distinct structure as compared to the same impurity in a crystal. The results suggest that the stacking fault may prevent the formation of metastable defects, such as DX centres.