  This work reports about electronic structure calculations for the Heusler
compound Co$_2$FeAl$_{1-x}$Si$_x$. Particular emphasis was put on the role of
the main group element in this compound. The substitution of Al by Si leads to
an increase of the number of valence electrons with increasing Si content and
may be seen as electron-doping. Self-consistent electronic structure
calculations were performed to investigate the consequences of the electron
doping for the magnetic properties. The series Co$_2$FeAl$_{1-x}$Si$_x$ is
found to exhibit half-metallic ferromagnetism and the magnetic moment follows
the Slater-Pauling rule. It is shown that the electron-doping stabilises the
gap in the minority states for $x=0.5$.
