  To realize first-order electroweak phase transition, it is necessary to
generate a barrier in the thermal Higgs potential, which is usually triggered
by scalar degree of freedom. We instead investigate phase transition patterns
in pure fermion extensions of the standard model, and find that additional
fermions with mass hierarchy and mixing could develop such barrier and realize
strongly first-order phase transition in such models. In the Higgs potential
with polynomial parametrization, the barrier can be generated in the following
two patterns: (I) positive quadratic term, negative cubic term and positive
quartic term or (II) positive quadratic term, negative quartic term and
positive higher dimensional term, such as dimensional 6 operator.
