Using deep level transient spectroscopy (DLTS), DX center has been characterized in GaAs‐AlAs superlattices grown by molecular beam epitaxy and selectively Si‐doped either in the AlAs layers or in the middle of the GaAs layers. The activation energy for thermal emission, which is the summation of the binding energy Et and the thermal capture energy Ec, is Ea=0.42 eV in both superlattices. The lowest DX concentration is obtained for the case where the only GaAs layers are doped. For the first time, a study of the capture reveals a capture activation energy Ec=0.36 eV, which locates the DX at Et≊60 meV below the conduction miniband. Taking into account the measured energies and trap concentrations, we show that the only observed DX on such structures is due to the silicon diffusion into AlAs layers.