To produce extrinsic semiconductor material, specific amounts of impurities are added to the pure intrinsic semiconductor. This process is called doping and the impurity atoms are called donor atoms. There are two types of extrinsic semiconductor that are manufactured, P type semiconductor and N type semiconductor. The production of extrinsic semiconductor is described below for silicon but the process is identical for germanium.
The pure silicon is doped with a group 5 element, such as phosphorus, antimony or arsenic. These materials have atoms with five valence electrons (pentavalent atoms ). Four of these electrons will form covalent bonds with neighbouring silicon atoms. The fifth electron has no electron to bond with and is therefore a free electron within the material. Each impurity atom will contribute a free electron to the conduction band. These free electrons will drift to produce an electrical current, if a voltage is applied to the material. The N type semiconductor is much more conductive than the pure silicon due to the free electrons created by the doping process.
Note it is important to point out that the material is called N type semiconductor because the mobile charge carriers are negatively charged electrons. The N type material itself is of course not negatively charged. (The negative charge of the electrons of the donor atoms, is balanced by the positive charge of their nucleus).
The diagram below shows an energy band diagram for N type semiconductor. The valence band is completely full, as all of the covalent bonds are complete. The conduction band contains the free electrons due to the donor atoms.