  Transport mechanism in the normal state of high-$T_c$ superconductors is
described using the well known Fermi-Dirac statistics in which an additional
restrictive constraint is introduced so as to capture the variation of
resistivity with temperature and doping. The additional restrictive condition
is the ionization energy that will eventually determine the properties of
charge carriers' in the normal state of high-$T_c$ superconductors. The
magnitude and the variation of charge carriers concentration and resistivities
(polycrystalline, c-axis and $ab$-planes) with temperature and doping are very
well described by the ionization energy based Fermi-Dirac statistics. However,
these transport models are not appropriate for cuprates below the
characteristics ($T^*$) and critical temperatures ($T_c$), metals with free
electrons and strong electron-phonon scattering. Ionization energy is found to
be an essential parameter to accurately predict variations of resistivity's
magnitude with doping, charge carriers' concentrations, scattering rate
constants as well as the effective mass. Apart from that, iFDS based
resistivity models provide the comprehensive information on
crossover-temperature (metallic $\to$ insulating transition temperature) in the
normal state of high-$T_c$ superconductors.
