  We study the phase diagram of the Gross-Neveu model in d=2+1 space-time
dimensions in the plane spanned by temperature and the number of massless
fermion flavors. We use a functional renormalization group approach based on a
nonperturbative derivative expansion that accounts for fermionic as well as
composite bosonic fluctuations. We map out the phase boundary separating the
ordered massive low-temperature phase from the disordered high-temperature
phase. The phases are separated by a second-order phase transition in the 2d
Ising universality class. We determine the size of the Ginzburg region and show
that it scales to zero for large $\Nf$ following a powerlaw, in agreement with
large-$\Nf$ and lattice studies. We also study the regimes of local order above
as well as the classical regime below the critical temperature.
