  Linear and nonlinear ion-acoustic waves are studied in a fluid model for
non-relativistic, unmagnetized quantum plasma with electrons with an arbitrary
degeneracy degree. The equation of state for electrons follows from a local
Fermi-Dirac distribution function and apply equally well both to fully
degenerate or classical, non-degenerate limits. Ions are assumed to be cold.
Quantum diffraction effects through the Bohm potential are also taken into
account. A general coupling parameter valid for dilute and dense plasmas is
proposed. The linear dispersion relation of the ion-acoustic waves is obtained
and the ion-acoustic speed is discussed for the limiting cases of extremely
dense or dilute systems. In the long wavelength limit the results agree with
quantum kinetic theory. Using the reductive perturbation method, the
appropriate Korteweg-de Vries equation for weakly nonlinear solutions is
obtained and the corresponding soliton propagation is analyzed. It is found
that soliton hump and dip structures are formed depending on the value of the
quantum parameter for the degenerate electrons, which affect the phase
velocities in the dispersive medium.
