A trace of phosphorus, which carries five valence electrons, is added to the silicon crystal. Four of them go into the four bonds; the fifth finds no partner and is left over.
That spare electron leaves its atom with only a little heat and moves freely through the crystal. No bond has to be broken, as one must in pure silicon, so far less energy is needed before charge can be carried. How much charge is available is set by how much phosphorus is added.
The phosphorus that let the electron go stays fixed in the lattice as a positive ion. Only the electron moves; the ion never leaves its site. Because the charge being carried is negative, this material is called n-type.