  Electrically tuning long-range magnetic orders has been realized in
two-dimensional (2D) semiconductors via electrostatic doping. On the other
hand, the observations are highly diverse: the transition can be realized by
either electrons or holes or both depending on specific materials. Moreover,
doped carriers seem to always favor the ferromagnetic (FM) ground state. The
mechanism behind those diverse observations remains uncovered. Combining
first-principles simulations, we analyze the spin superexchange paths of the
correlated d/f orbitals around band edges and assign 2D magnetic semiconductors
into three types by their projected density of states (PDOS). We find that each
type of PDOS corresponds to a specific carrier-driven magnetic phase transition
and the critical doping density and type of carriers can be quantitatively
obtained by calculating the superexchange coupling strength. The model results
are in good agreements with first-principles calculations and available
measurements. After understanding the mechanism, we can design heterostructures
to realize the FM to antiferromagnetic transition, which has not been realized
before. This model is helpful to understand diverse measurements and expand the
degrees of freedom to control long-range magnetic orders in 2D semiconductors.
