  The restoration of chiral symmetry at finite density and/or temperature is
investigated in a wide class of one-gluon exchange models in the instantaneous
approximation. If the effective quark interaction is less divergent than
$1/k^2$ for small momentum transfer $k$, we obtain Gaussian critical exponents
for the chiral phase transitions at finite temperature and density,
respectively. In the opposite case, for an interaction diverging faster than
$1/k^2$ in the infrared region, a qualitative different behavior of the quark
self-energy near the critical Fermi momentum $k_c$ and the critical temperature
$T_c$, respectively, is observed. In the first scenario, we find $k_c \approx 2
\ln 2 \; T_c$, which compares well with recent data from QCD lattice
simulations.
