The ground state binding energy of a donor centered in a GaAs/Ga0.7Al0.3As spherical quantum dot is calculated, by using a numerical procedure based on trigonometric sweep method. It is considered a model that takes into account the effects produced by (i) a graded variation of Al concentration at the interface, (ii) the dependence of the material parameters on the Al concentration and (iii) the non-parabolicity of the GaAs conduction band. Comparing to previous results, obtained for a model with a spherical rectangular potential, a considerable binding energy enhancement, ranging from 20 to 50% for different quantum dot radii, has been found.