The correct answer is A. Silicon crystal.
A bulk substrate is a piece of semiconductor material that is used as the foundation for a semiconductor device. The most common type of bulk substrate is silicon crystal, which is a single crystal of silicon. Silicon crystals are grown by the Czochralski process, which involves melting silicon and then slowly pulling a single crystal out of the melt.
Silicon-on-sapphire (SOS) is a type of semiconductor material that is made by growing a thin layer of silicon on a sapphire substrate. SOS is used in some high-performance applications because it has better thermal conductivity than silicon crystal.
Phosphorus is a chemical element that is used as a dopant in silicon to create n-type silicon. Doping is the process of adding impurities to a semiconductor material to change its electrical properties.
Silicon dioxide (SiO2) is a type of semiconductor material that is used as an insulator in semiconductor devices. SiO2 is also used as a passivation layer on the surface of silicon wafers.
In conclusion, the most common type of bulk substrate used in nMOS fabrication is silicon crystal.