  Magnetic semiconductors are a powerful platform for understanding, utilizing
and tuning the interplay between magnetic order and electronic transport.
Compared to bulk crystals, two-dimensional magnetic semiconductors have greater
tunability, as illustrated by the gate modulation of magnetism in exfoliated
CrI$_3$ and Cr$_2$Ge$_2$Te$_6$, but their electrically insulating properties
limit their utility in devices. Here we report the simultaneous electrostatic
and magnetic control of electronic transport in atomically-thin CrSBr, an
A-type antiferromagnetic semiconductor. Through magnetotransport measurements,
we find that spin-flip scattering from the interlayer antiferromagnetic
configuration of multilayer flakes results in giant negative magnetoresistance.
Conversely, magnetoresistance of the ferromagnetic monolayer CrSBr vanishes
below the Curie temperature. A second transition ascribed to the ferromagnetic
ordering of magnetic defects manifests in a large positive magnetoresistance in
the monolayer and a sudden increase of the bulk magnetic susceptibility. We
demonstrate this magnetoresistance is tunable with an electrostatic gate,
revealing that the ferromagnetic coupling of defects is carrier mediated.
