1.07 Extrinsic semiconductor

To produce extrinsic semiconductor material specific amounts of impurity 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.

N type semiconductor

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 is not part of any covalent bond and is therefore a free electron. Every 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 large number of free electrons created by the doping process.

N type semiconductor

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).

Energy band diagram

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 free electrons from the donor atoms.

Energy band diagram for N type semiconductor