  We extend some two Higgs doublet models, where the Yukawa couplings for the
charged fermion mass generation only involve one Higgs doublet, by two singlet
scalars respectively carrying a singly electric charge and a doubly electric
charge. The doublet and singlet scalars together can mediate a two-loop diagram
to generate a tiny Majorana mass matrix of the standard model neutrinos.
Remarkably, the structure of the neutrino mass matrix is fully determined by
the symmetric Yukawa couplings of the doubly charged scalar to the right-handed
leptons. Meanwhile, a one-loop induced neutrinoless double beta decay can
arrive at a testable level even if the electron neutrino has an extremely small
Majorana mass. We also study other experimental constraints and implications
including some rare processes and Higgs phenomenology.
