  We have performed high-resolution angle-resolved photoemission spectroscopy
on heavily electron-doped non-superconducting (SC)
BaFe$_{1.7}$Co$_{0.3}$As$_2$. We find that the two hole Fermi surface pockets
at the zone center observed in the hole-doped superconducting
Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ are absent or very small in this compound,
while the two electron pockets at the M point significantly expand due to
electron doping by the Co substitution. Comparison of the Fermi surface between
non-SC and SC samples indicates that the coexistence of hole and electron
pockets connected via the antiferromagnetic wave vector is essential in
realizing the mechanism of superconductivity in the iron-based superconductors.
