1.03 Intrinsic semiconductor materials.

The naturally occurring semiconductor materials that are used to manufacture electronic devices, are Silicon and Germanium (Silicon is more widely used, due to its lower leakage current (this is explained later)). The following text refers to silicon but is equally applicable to germanium.

Pure Silicon.

To produce silicon suitable for use in the manufacture of semiconductor devices, first a very pure crystal of silicon must be produced. The atomic structure of pure silicon can be represented by the diagram below. Silicon, like all semiconductors, is a group 4 element and so its atoms have only four electrons in the outer shell ( i.e. it has four 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.

intrinsic semiconductor

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 arrangement of covalent bonds forms a shape called a tetrahedron.
It is clear from the diagram that there are no free electrons, (they are either bound in the inner electron shells or bound in covalent bonds between atoms).