  The gap equations for the surface quasiparticle propagators in a slab of
three-dimensional topological insulator in external electric and magnetic
fields perpendicular to the slab surfaces are analyzed and solved. A different
type of magnetic catalysis is revealed with the dynamical generation of both
Haldane and Dirac gaps. Its characteristic feature manifests itself in the
crucial role that the electric field plays in dynamical symmetry breaking and
the generation of a Dirac gap in the slab. It is argued that, for a
sufficiently large external electric field, the ground state of the system is a
phase with a homogeneous surface charge density.
