Pure silicon does not conduct electricity. Doping is the replacement of some silicon atoms in the crystal lattice by adding trace amounts of impurities from either Group VA or Group lllA elements, enabling the silicon to conduct electricity. When trace impurities from Group VA are added, such as phosphorus, extra electrons are available to conduct electricity. This is referred to as n-type doping because of the negative charge on the electrons. When impurities from Group lllA are added, such as boron, extra "holes" are available to allow the flow of electrons. This is referred to as p-type doping because of the positive charge from the lack of electrons, or "holes".
Since electrical current is used to heat the rods for the thermal deposition process, the rods must be doped so they can conduct current. The filaments are doped as they are "pulled" in the manufacturing process. A controlled amount of phosphine (PH3) is blown on the molten silicon in the melt zone. The phosphorus atoms replace silicon atoms in the crystal lattice, changing the resistivity of the filament. The hydrogen atoms don't enter the lattice, and are carried off in the argon gas flowing through the growth chamber. The result is a filament which is able to conduct electricity.
The dopant is an undesirable Impurity in the final product, so the amount is minimized. The filaments are only doped enough to allow them to "start" in the reactors. The following table shows the end to end resistance of the various types of filaments.