We describe experiments on Si doping in Alx Ga1-x N grown by gas source molecular beam epitaxy with ammonia and silane. Growth conditions that minimize self-compensation were used to assure Si incorporation at a level of 2× 1020 cm-3 for the entire range of compositions investigated, from x=0.56 to 1.0. These conditions resulted in donor concentrations of ∼1× 1019 cm-3 up to x=0.85. Layers of Alx Ga1-x N up to x=0.85 show good mobility and low resistivity. In these layers, the activation energy, Ea, of Si stays below ∼25 meV and Si can be considered a shallow donor. For AlN content above x=0.85 the donor activation energy increases to Ea ∼250 meV in AlN. The change in donor activation energy correlates with increased incorporation of oxygen and carbon.