1.08 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 which are manufactured, P type semiconductor and N type semiconductor. The production of extrinsic semiconductor will be described for the more common silicon semiconductor material 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. As their are only four covalent bonds binding the donor atom to the neighbouring silicon atoms the fifth electron is not part of a covalent bond, and is therefore a free electron. Every impurity atom will produce a free electron in the conduction band. These electrons will drift to produce an electrical current if a voltage is applied to the material and the N type semiconductor is a much better conductor than the intrinsic pure silicon material.

N type semiconductor
  • The silicon atoms form a square lattice
  • The green atoms represent the donor atoms
  • Four of the five valence electrons form covalent bonds with neighbouring silicon atoms
  • The fifth electron has no neighbouring electron to pair with and is a free electron
  • Each donor atom produces a free electron

Note it is important to point out that the material is called N type semiconductor because the majority of charge carriers which will contribute to an electrical current through the material are negatively charged free electrons produced by the doping process. There will be some contribution to the current flow from positively charge holes due to electron hole pair generation but these holes are the minority charge carriers in this material. The N type material itself is not negatively charged. The negative charge of the electrons of the donor atoms is balanced by the positive charge in the 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 fifth valence electrons in the donor atoms.

Energy band diagram for N type semiconductor

Note this diagram does not show the electron hole pairs that would be present due to thermal energy. The electron hole pairs are minority charge carriers in N type semiconductors, the majority being the free electrons produced by the doping process.