The gap equation at finite temperature in the top-quark condensate scheme of electroweak symmetry breaking is proved to have the identical form in both the imaginary and the real time formalism of thermal field theory. By means of the gap equation, combined with the basic relation to define the vacuum expectation value v of the effective Higgs field, we analyse the dependence on temperature T and chemical potential μ of the dynamical top-quark mass as the order parameter characteristic of

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... ry breaking, and obtain the μ-T criticality curve for symmetry restoration. We find out that the critical temperature T_c=2v for μ=0 and the critical chemical potential μ_c=2π v/√(3) for T=0. When μ=0, the top-quark mass near T_c has the leading (T_c^2-T^2)^1/2 behavior with an extra factor dependent on temperature T and the momentum cut-off Λ. However, it is generally argued that the symmetry restoration at T≥ T_c is still a second-order phase transition.