  We investigate the phase transition in cold and dense quark matter in an
intuitive way that shares common features of the effective model approaches. We
first express the quasi-particle contribution to the thermodynamic potential
with the dynamical mass M and then discuss how we can understand the possible
first-order phase transition with and without the vector interaction from the
saturation curve on the plane of the energy per particle and the density. We
next extend our analysis including inhomogeneity and discuss the relation
between the order of the phase transition and the saturation curve. We
emphasize that the saturation curve is useful to infer qualitative nature of
the phase transition even without knowing the explicit solution of the ground
state.
