  Metal-insulator transition (MIT) is one of the most conspicuous phenomena in
correlated electron systems. However such transition has rarely been induced by
an external magnetic field as the field scale is normally too small compared
with the charge gap. In this paper we present the observation of a
magnetic-field-driven MIT in a magnetic semiconductor $\beta $-EuP$_3$.
Concomitantly, we found a colossal magnetoresistance (CMR) in an extreme way:
the resistance drops billionfold at 2 kelvins in a magnetic field less than 3
teslas. We ascribe this striking MIT as a field-driven transition from an
antiferromagnetic and paramagnetic insulator to a spin-polarized topological
semimetal, in which the spin configuration of $\mathrm{Eu^{2+}}$ cations and
spin-orbital coupling (SOC) play a crucial role. As a phosphorene-bearing
compound whose electrical properties can be controlled by the application of
field, $\beta $-EuP$_3$ may serve as a tantalizing material in the basic
research and even future electronics.
