Why Pure silicon or germanium is doped with impurity atoms to change its conduction. Adding a donor impurity with five valence electrons (phosphorus, arsenic, antimony) leaves one electron per atom with no bond to fill, so the majority carriers are free electrons. That material is called N-type because the mobile charge carriers are Negative.
Watch out P-type is the opposite: it is doped with a trivalent acceptor such as boron or indium, leaving holes (missing electrons) as the majority carrier. Bipolar and insulated gate are transistor construction terms, not semiconductor material types.
N = Negative carriers = free electrons; P = Positive carriers = holes.