  We study, within the fluctuation exchange approximation, the
spin-fluctuation-mediated superconductivity in Hubbard-type models possessing
electron and hole bands, and compare them with a model on a square lattice with
a large Fermi surface. In the square lattice model, superconductivity is more
enhanced for better nesting for a fixed band filling. By contrast, in the
models with electron and hole bands, superconductivity is optimized when the
Fermi surface nesting is degraded to some extent, where finite energy spin
fluctuation around the nesting vector develops. The difference lies in the
robustness of the nesting vector, namely, in models with electron and hole
bands, the wave vector at which the spin susceptibility is maximized is fixed
even when the nesting is degraded, whereas when the Fermi surface is large, the
nesting vector varies with the deformation of the Fermi surface. We also
discuss the possibility of realizing in actual materials the bilayer Hubbard
model, which is a simple model with electron and hole bands, and is expected to
have a very high T_c.
