A multilayered Si doped MBE structure was used to study the effective diffusivity of Si and the results are modeled with an (n/ni)2 dependence over the temperature range of 750°C – 950°C. An activation energy, Ea′, of 4.0 eV is obtained which is higher than normally reported in the literature. This higher Ea′ value results from appropriate accounting of the temperature dependence of ni, which is often neglected in the expression for Deff. Si diffusion at a buried n+/n++junction of a MOCVD grown structure also follows an (n/ni)2 dependence. These results support a Fermi-level model of Si diffusion in GaAs and suggest that the local point defect chemistry of the GaAs, through Si doping, is responsible for this diffusion behavior, regardless of the proximity to the surface.