Intrinsic semiconductor materials

1.03 Intrinsic semiconductor materials

The naturally occurring semiconductor materials that are used to manufacture electronic devices are Silicon and Germanium (Germanium is an older choice of material which is less used today). The following text refers to silicon but is equally applicable to germanium.

Pure Silicon
First a very pure crystal of silicon must be produced. The atomic structure of the silicon can be represented by the diagram below. Silicon like all semiconductors is a group 4 element and its atoms have only four electrons in the outer shell ( 4 valence electrons). It takes eight electrons to fill the outer shell and make it stable. The atoms share their valence electrons with neighbouring atoms so that each atom effectively contains eight electrons in the outer shell. This sharing of valence electrons with neighbouring atoms forms covalent bonds. It is these covalent bonds that bind the atoms together.

  • The silicon atoms form a square lattice
  • Each silicon nucleus has four electrons in its outer shell
  • These electrons are paired with the corresponding electrons in adjacent atoms.
  • These are called covalent bonds. Covalent bonds are what binds the material together
  • The net result is that each nuclei (along with the electrons in the inner shells) are surrounded by eight outer electrons tightly bound in the atomic structure.

Note this is a simplified diagram showing a 2 dimensional representation of the structure of silicon. Obviously silicon has a 3 dimensional structure and the covalent bonds do not really lie in a single plane as shown in the diagram. The actual arrangement of covalent bonds forms a shape called a tetrahedron.

This diagram does give a good representation of how the electrons are bound to the atoms. This reflects the fact that there are no free electrons to produce an electrical current if a voltage is applied to the material. However an energy level diagram is better for explaining more about the electrical properties of silicon.